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Methods for accurately estimating within-field yield are essential to improve site-specific crop management and resource use efficiencies, which would be a major step toward sustainable intensification of agricultural systems. We set out to assess the accuracy of within-field soybean yields predicted by two data assimilation methods and to assess these methods’ assimilation efficiency (AE). Yields were estimated by assimilating remotely sensed leaf area index (LAI) data from Sentinel-2 into a soybean crop growth model on a pixel basis. The LAI data was integrated into the model by Ensemble Kalman Filtering (EnKF) or by recalibrating with the Subplex algorithm (recalibration-based). An open-loop setting which only integrates information on the soil layers was used as a baseline scenario for quantifying the AE. We assessed both data assimilation techniques on eight fields (3067 pixels) in the Corn Belt region (Nebraska, Kansas and Kentucky) in the United States. The data set encompassed substantial variation in crop growth conditions: three growing seasons (2018, 2019 and 2020), rainfed and irrigated fields, and early and late planting dates. Ground truth yield acquired from combine monitors was used to validate the yield estimations. Agreement between predicted and observed yield at pixel level was two times higher for both data assimilation methods compared to the open-loop. The root mean square error (RMSE) was 476 kg.ha−1 (RRMSE of 10 %) in the recalibration-based method and 573 kg.ha−1 (RRMSE of 12 %) in the EnKF-based method. For both data assimilation methods, assimilating the LAI improved predictions for 68 % of the pixels. For a further 12 % of pixels, there was no accuracy improvement. For the remaining 20 %, AE was positive for one of the two assimilation methods. The high proportion of pixels with positive AE indicates the potential for overcoming the limitations in applying crop models at high spatial resolution by integrating a crop growth indicator. Ass

We aimed at studying the mechanisms underneath the ascending and descending sections of the thermal performance curves in marine protozoans. To do so, we exposed Gyrodinium dominans and two strains of Oxyrrhis marina from different origins to three temperatures representative of each section of the thermal response curve (12ºC, ascending section; 18ºC, top; 25ºC, descending section). As variables, we measured growth, ingestion, and respiration rates (this latter with and without food). The growth rates of O. marina strains plotted as a function of temperature showed a triangular response with maximum values at the intermediate temperature. However, G. dominans showed similar growth rates at 12 and 18ºC, and even if showed a marked decrease in growth rates at 25ºC, this was not significant. Ingestion rates were higher at 18ºC for all the strains. The respiration rates of G. dominans were unaffected by temperature, but the respiration rates of both O. marina strains increased with temperature. The specific dynamic action produced by feeding activity ranged from 2 to 20% of the daily carbon ingestion for all organisms investigated. The calculated energetic budget indicated that the responses to temperature were diverse, even within strains of the same species. G. dominans maintained similar growth at all temperatures by balancing anabolism and catabolism functions. In O. marina strains, on the other hand, the decrease in growth rates at the lowest temperature was driven mainly by reduced ingestion rates. However, increased respiration seemed the primary factor affecting the decrease in growth rates at the highest temperature. These results are discussed in the light of previous studies and on its suitability to understand the response of wild organisms to fluctuations in temperature

Food quantity-quality interactions determine growth rates and reproductive success of consumers and thereby regulate community dynamics and food web structure. Predator-prey models that shape our conceptual understanding of foraging ecology typically rely on the parametrization of fixed consumer responses to either food quantity or food quality. In nature, however, consumers optimize their fitness by responding simultaneously to changes in food quantity and quality. Therefore, we assessed consumer responses to changing food environments using a new fitness optimization model that accounted for food quality-quantity interactions to better capture the regulatory flexibility of consumers. Our simulations demonstrated that the impact of food quality on important consumer traits can be altered or even reversed by changes in food quality. Low food quality, for example, affected feeding rates negatively at low food concentrations but triggered surplus feeding at high food concentrations. The scope of surplus feeding was thereby mainly dependent on dynamics of nutrient digestion and in contrast to previous assumptions, energy costs of feeding played a minor role. Further, the regulation of digestive enzyme production, a crucial factor determining assimilation efficiencies, was strongly dependent on whether nonessential or essential nutrients were limiting growth. Consequently, not only the degree but also the type of nutrient limitation mediated the impact of the food environment on consumers' fitness. At the community level, food quality was key in shaping predator-prey biomass ratios. High food qualities resulted in top-heavy systems with larger consumer than prey biomass. Decreases of prey digestibility or the availability of essential nutrients, however, triggered a switch from inverted to classical pyramid shapes of bi-trophic systems. The impact of food quantity on trophic transfer and emerging structural ecosystem properties thus critically hinges on behavioral and p

Carbon use efficiency (CUE), the proportion of carbon (C) consumed by microbes that is converted into biomass, is an important parameter for soil C models with explicit microbial controls. While often considered as a single parameter, CUE is an emergent property of multiple microbial processes, including assimilation efficiency, biomass-specific respiration, enzyme production, and respiratory costs of enzyme production. These processes occur over variable time scales and imply different fates for C, and the same emergent CUE value can result when C is allocated in fundamentally different ways (e.g. a high investment in enzyme production vs. a high assimilation cost). We developed a model that represents the individual processes underlying emergent CUE to test how shifts in microbial allocation alter equilibrium soil C pool sizes. We found that an increase in emergent CUE that results from a change in assimilation efficiency, biomass specific respiration, or respiration costs from enzyme production causes soil organic C (SOC) to decline, while the same change in emergent CUE resulting from a change in enzyme production causes SOC to increase. We also used the model to test the sensitivity of CUE from isotopic C tracer estimates to changes in microbial allocation processes. We found that these estimates do not account for the same microbial processes represented by emergent CUE in models. We propose that considering microbial processes explicitly rather than representing CUE as a single parameter can improve data-model integration. In addition, modeling microbial processes explicitly will account for a wider range of possible outcomes from shifts in microbial C allocation, such as when increased SOC results from increasing CUE.

Carbon use efficiency (CUE), the proportion of carbon (C) consumed by microbes that is converted into biomass, is an important parameter for soil C models with explicit microbial controls. While often considered as a single parameter, CUE is an emergent property of multiple microbial processes, including assimilation efficiency, biomass-specific respiration, enzyme production, and respiratory costs of enzyme production. These processes occur over variable time scales and imply different fates for C, and the same emergent CUE value can result when C is allocated in fundamentally different ways (e.g. a high investment in enzyme production vs. a high assimilation cost). We developed a model that represents the individual processes underlying emergent CUE to test how shifts in microbial allocation alter equilibrium soil C pool sizes. We found that an increase in emergent CUE that results from a change in assimilation efficiency, biomass specific respiration, or respiration costs from enzyme production causes soil organic C (SOC) to decline, while the same change in emergent CUE resulting from a change in enzyme production causes SOC to increase. We also used the model to test the sensitivity of CUE from isotopic C tracer estimates to changes in microbial allocation processes. We found that these estimates do not account for the same microbial processes represented by emergent CUE in models. We propose that considering microbial processes explicitly rather than representing CUE as a single parameter can improve data-model integration. In addition, modeling microbial processes explicitly will account for a wider range of possible outcomes from shifts in microbial C allocation, such as when increased SOC results from increasing CUE.

Endothermic organisms can adapt to short growing seasons, low temperatures and nutrient limitation by developing high growth rates and high gross growth efficiencies (GGEs). Animals with high GGEs are better at assimilating limiting nutrients and thus should recycle (or lose) fewer nutrients. Longer guts in relation to body mass may facilitate higher GGE under resource limitation. Within the context of ecological stoichiometry theory, this study combines ecology with evolution by relating latitudinal life-history adaptations in GGE, mediated by gut length, to its ecosystem consequences, such as consumer-mediated nutrient recycling. In common garden experiments, we raised Rana temporaria tadpoles from two regions (Arctic/Boreal) under two temperature regimes (18/23 degrees C) crossed with two food quality treatments (high/low-nitrogen content). We measured tadpole GGEs, total nutrient loss (excretion+egestion) rates and gut length during ontogeny. In order to maintain their elemental balance, tadpoles fed low-nitrogen (N) food had lower N excretion rates and higher total phosphorous (P) loss rates than tadpoles fed high-quality food. In accordance with expectations, Arctic tadpoles had higher GGEs and lower N loss rates than their low-latitude conspecifics, especially when fed low-N food, but only in ambient temperature treatments. Arctic tadpoles also had relatively longer guts than Boreal tadpoles during early development. That temperature and food quality interacted with tadpole region of origin in affecting tadpole GGEs, nutrient loss rates and relative gut length, suggests evolved adaptation to temperature and resource differences. With future climate change, mean annual temperatures will increase. Additionally, species and genotypes will migrate north. This will change the functioning of Boreal and Arctic ecosystems by affecting consumer-mediated nutrient recycling and thus affect nutrient dynamics in general. Our study shows that evolved latitudinal adaption c

Endothermic organisms can adapt to short growing seasons, low temperatures and nutrient limitation by developing high growth rates and high gross growth efficiencies (GGEs). Animals with high GGEs are better at assimilating limiting nutrients and thus should recycle (or lose) fewer nutrients. Longer guts in relation to body mass may facilitate higher GGE under resource limitation. Within the context of ecological stoichiometry theory, this study combines ecology with evolution by relating latitudinal life-history adaptations in GGE, mediated by gut length, to its ecosystem consequences, such as consumer-mediated nutrient recycling. In common garden experiments, we raised Rana temporaria tadpoles from two regions (Arctic/Boreal) under two temperature regimes (18/23 degrees C) crossed with two food quality treatments (high/low-nitrogen content). We measured tadpole GGEs, total nutrient loss (excretion+egestion) rates and gut length during ontogeny. In order to maintain their elemental balance, tadpoles fed low-nitrogen (N) food had lower N excretion rates and higher total phosphorous (P) loss rates than tadpoles fed high-quality food. In accordance with expectations, Arctic tadpoles had higher GGEs and lower N loss rates than their low-latitude conspecifics, especially when fed low-N food, but only in ambient temperature treatments. Arctic tadpoles also had relatively longer guts than Boreal tadpoles during early development. That temperature and food quality interacted with tadpole region of origin in affecting tadpole GGEs, nutrient loss rates and relative gut length, suggests evolved adaptation to temperature and resource differences. With future climate change, mean annual temperatures will increase. Additionally, species and genotypes will migrate north. This will change the functioning of Boreal and Arctic ecosystems by affecting consumer-mediated nutrient recycling and thus affect nutrient dynamics in general. Our study shows that evolved latitudinal adaption c

We aimed to evaluate three defined monoalgal diets, Isochrysis galbana clone T-ISO, Tetraselmis suecica and Rhodomonas lens, as a food source for Mytilus galloprovincialis to ascertain which of the diets maximized the feeding, digestion and the assimilatory balance of nutrients and energy. Mussels fed with Rhodomonas yielded the highest clearance and ingestion rates (CR and IR), suggesting that the dimensions of Tetraselmis and T-ISO might have restricted their capture by the mussels’ gill. Absorption efficiency (AE), an indicator of digestibility, was significantly higher for Rhodomonas (69.6%) than for Tetraselmis (38.4%) or T-ISO (23.6%) diets. This could be explained by the greater proportion of refractive non-digestible material contained in Tetraselmis and T-ISO diets, together with the low digestibility of the cell wall of Tetraselmis. The Rhodomonas diet showed the highest protein content, which was reflected in the highest ingestion and absorption of proteins compared with the other diets. However, the amount of carbohydrates and lipids ingested did not match the amount absorbed, probably owing to inefficient carbohydrate digestion and lipids lost through metabolic fecal losses. The total energy absorbed was higher for Rhodomonas (34.5 Jh− 1) than for T-ISO (20.1 J h− 1) or Tetraselmis (13.9 J h− 1) diet. The optimal feeding and digestive behaviour obtained for mussels fed with Rhodomonas diet, coupled with its ideal size, volume, weight and biochemical composition, might provide a better coverage for the anabolic demands of proteins during the seasonal growth cycle, especially during shell formation, gametogenesis and byssogenesis. Statement of relevance We state that this manuscript is relevant for the field of bivalve aquaculture and hatchery. We recommend that the main criteria in selecting algae for culturing mussels should be based on choosing the suitable dimension (size, volume, weight) of algal cell depending on the feeding capacity of mussels, whi

Gammarids are aquatic amphipods widely used for water quality monitoring. To investigate the copper and cadmium diet-borne metal uptake in Gammarus pulex, we adapted the pulse-chase stable isotopes-based approach to determine the food ingestion rate (IR), the gut retention time (GRT) and the metal assimilation efficiencies (AE). G. pulex were fed with 65Cu-, 106Cd-, and 53Cr-labeled alder leaves for 7.5 h and then with unlabeled leaves for 5 d. The metal stable isotope contents in the gammarids, leaves, filtered water and periodically collected feces were determined. Chromium was poorly assimilated by the gammarids; thus, Cr was used as an unassimilated tracer. The first tracer defecation occurred before the first feces harvest, indicating a gut passage time of less than 9 h. A 24-h GRT and a 0.69 g g−1 d−1 IR were estimated. The Cd AE value was estimated as 5–47%, depending on the assimilation determination method applied. The Cu AE value could not be evaluated regardless of the determination method used, most likely because of the rapid Cu regulation in gammarids in addition to analytical uncertainties when determining the Cu content in leaves. Application of the Cd AE value in the framework of the biodynamic bioaccumulation model shows that the diet-borne uptake of Cd significantly contributes (66–95%) to the metal bioaccumulation in G. pulex fed with alder leaves.

Gammarids are aquatic amphipods widely used for water quality monitoring. To investigate the copper and cadmium diet-borne metal uptake in Gammarus pulex, we adapted the pulse-chase stable isotopes-based approach to determine the food ingestion rate (IR), the gut retention time (GRT) and the metal assimilation efficiencies (AE). G. pulex were fed with 65Cu-, 106Cd-, and 53Cr-labeled alder leaves for 7.5 h and then with unlabeled leaves for 5 d. The metal stable isotope contents in the gammarids, leaves, filtered water and periodically collected feces were determined. Chromium was poorly assimilated by the gammarids; thus, Cr was used as an unassimilated tracer. The first tracer defecation occurred before the first feces harvest, indicating a gut passage time of less than 9 h. A 24-h GRT and a 0.69 g g−1 d−1 IR were estimated. The Cd AE value was estimated as 5–47%, depending on the assimilation determination method applied. The Cu AE value could not be evaluated regardless of the determination method used, most likely because of the rapid Cu regulation in gammarids in addition to analytical uncertainties when determining the Cu content in leaves. Application of the Cd AE value in the framework of the biodynamic bioaccumulation model shows that the diet-borne uptake of Cd significantly contributes (66–95%) to the metal bioaccumulation in G. pulex fed with alder leaves.

Gammarids are aquatic amphipods widely used for water quality monitoring. To investigate the copper and cadmium diet-borne metal uptake in Gammarus pulex, we adapted the pulse-chase stable isotopes-based approach to determine the food ingestion rate (IR), the gut retention time (GRT) and the metal assimilation efficiencies (AE). G. pulex were fed with 65Cu-, 106Cd-, and 53Cr-labeled alder leaves for 7.5 h and then with unlabeled leaves for 5 d. The metal stable isotope contents in the gammarids, leaves, filtered water and periodically collected feces were determined. Chromium was poorly assimilated by the gammarids; thus, Cr was used as an unassimilated tracer. The first tracer defecation occurred before the first feces harvest, indicating a gut passage time of less than 9 h. A 24-h GRT and a 0.69 g g−1 d−1 IR were estimated. The Cd AE value was estimated as 5–47%, depending on the assimilation determination method applied. The Cu AE value could not be evaluated regardless of the determination method used, most likely because of the rapid Cu regulation in gammarids in addition to analytical uncertainties when determining the Cu content in leaves. Application of the Cd AE value in the framework of the biodynamic bioaccumulation model shows that the diet-borne uptake of Cd significantly contributes (66–95%) to the metal bioaccumulation in G. pulex fed with alder leaves.

Understanding how water use efficiency (WUE) changes under drought is crucial for interpreting adaptive responses of species and cultivars to such abiotic stress. Several recent papers have concluded that in grapevine these responses and the guidelines stemming therefrom can differ, depending upon the parameter chosen to express WUE. In the present paper a complete set of WUE expressions, including the physiological and agronomical, were compared in potted, fruiting cv. Montepulciano (Vitis vinifera L.) grapevines which were either well watered (WW) or subjected to progressive pre-veraison drought (WS) by supplying decreasing fractions (i.e. 70%, 50% and 30% of daily vine transpiration, Tg) determined gravimetrically before vines were fully rewatered. While single-leaf intrinsic water-use efficiency (WUEi) increased with water stress severity, seasonal and diurnal whole-canopy WUE were similar at pre-stress, 70% Tg, and upon rewatering but dropped in WS during severe water stress. Agronomic WUE calculated as mass of dry weight stored in annual biomass (leaves, canes and bunches) per L of water used, was also lower in WS, whereas WS had similar must composition with WW despite a 37% reduction in the yield per vine. Results warn that whole-canopy WUE is a much better index than any single-leaf based WUE parameter for extrapolation to agronomic WUE and actual grape composition. Under our specific case study, it can be recommended that, to avoid significant profit loss, water supply to drought stressed Montepulciano at pre-veraison should not be lower than 70% of daily vine water use.

The assimilation efficiency of zooplankton is an essential parameter required to estimate energy transfer to higher trophic levels in marine ecosystems. However, little information is available for large oceanic copepods, especially the Neocalanus and Eucalanus species dominant in the subarctic Pacific. In this study, the assimilation efficiencies of the C5 stages of Neocalanus cristatus, Neocalanus flemingeri and Eucalanus bungii were evaluated using eight phytoplankton species as food. The average assimilation efficiencies of N. cristatus, N. flemingeri and E. bungii ranged between 45 and 66%, 44 and 66% and 34 and 65%, respectively. The assimilation efficiency was highly variable depending on the food phytoplankton species. In all species, the assimilation efficiency showed a significant negative relationship with the ash content of the phytoplankton (r2 = 0.79–0.87, p < 0.001). The assimilation efficiency of large-body sized N. cristatus for large-sized diatoms was higher than for the other copepod species. In population models of N. cristatus, changes in assimilation efficiency affect the growth and survival rates of the population. The Lagrangian ensemble model (LEM) for N. cristatus showed that, for assimilation efficiencies less than 57%, the population could not be maintained. Because variations in assimilation efficiency may have significant effects on the copepod population, their variability should be incorporated into marine ecosystem models in the future.

The assimilation efficiency of zooplankton is an essential parameter required to estimate energy transfer to higher trophic levels in marine ecosystems. However, little information is available for large oceanic copepods, especially the Neocalanus and Eucalanus species dominant in the subarctic Pacific. In this study, the assimilation efficiencies of the C5 stages of Neocalanus cristatus, Neocalanus flemingeri and Eucalanus bungii were evaluated using eight phytoplankton species as food. The average assimilation efficiencies of N. cristatus, N. flemingeri and E. bungii ranged between 45 and 66%, 44 and 66% and 34 and 65%, respectively. The assimilation efficiency was highly variable depending on the food phytoplankton species. In all species, the assimilation efficiency showed a significant negative relationship with the ash content of the phytoplankton (r2 = 0.79–0.87, p < 0.001). The assimilation efficiency of large-body sized N. cristatus for large-sized diatoms was higher than for the other copepod species. In population models of N. cristatus, changes in assimilation efficiency affect the growth and survival rates of the population. The Lagrangian ensemble model (LEM) for N. cristatus showed that, for assimilation efficiencies less than 57%, the population could not be maintained. Because variations in assimilation efficiency may have significant effects on the copepod population, their variability should be incorporated into marine ecosystem models in the future.

The assimilation efficiency of zooplankton is an essential parameter required to estimate energy transfer to higher trophic levels in marine ecosystems. However, little information is available for large oceanic copepods, especially the Neocalanus and Eucalanus species dominant in the subarctic Pacific. In this study, the assimilation efficiencies of the C5 stages of Neocalanus cristatus, Neocalanus flemingeri and Eucalanus bungii were evaluated using eight phytoplankton species as food. The average assimilation efficiencies of N. cristatus, N. flemingeri and E. bungii ranged between 45 and 66%, 44 and 66% and 34 and 65%, respectively. The assimilation efficiency was highly variable depending on the food phytoplankton species. In all species, the assimilation efficiency showed a significant negative relationship with the ash content of the phytoplankton (r2 = 0.79–0.87, p < 0.001). The assimilation efficiency of large-body sized N. cristatus for large-sized diatoms was higher than for the other copepod species. In population models of N. cristatus, changes in assimilation efficiency affect the growth and survival rates of the population. The Lagrangian ensemble model (LEM) for N. cristatus showed that, for assimilation efficiencies less than 57%, the population could not be maintained. Because variations in assimilation efficiency may have significant effects on the copepod population, their variability should be incorporated into marine ecosystem models in the future.

1. Bacteria can be an important resource for zooplankton production in aquatic food webs, although the degree to which bacteria sustain zooplankton growth and reproduction is not clear. We performed a growth experiment with Daphnia galeata feeding on different ratios of P-replete Pseudomonas and Rhodomonas, ranging from a 100% bacterial to a 100% algal diet. 2. A pure bacterial diet did not support survival, growth or reproduction of D. galeata. While a 20% share of Rhodomonas in the food allowed survival of daphniids, the occurrence of offspring on a 50% algal diet indicated that the threshold for successful reproduction was between those two proportions of algal food. Increasing the proportion of the alga further increased growth and reproductive output, indicating that Rhodomonas was a higher-quality food than Pseudomonas. 3. A subsequent labelling experiment demonstrated that D. galeata incorporated phosphorus from Pseudomonas and Rhodomonas with similar efficiency, whereas carbon was incorporated more efficiently from Pseudomonas than from Rhodomonas. 4. Hence, we hypothesise that inadequate levels of essential biochemicals in pure bacterial diets led to decreased Daphnia performance. Concentrations of fatty acids in general, and especially of polyunsaturated fatty acids, were much lower in Pseudomonas than in Rhodomonas. This difference could explain the different growth and reproduction responses, although limitation by other essential biochemicals (e.g. sterols) cannot be ruled out. 5. Hence, where they dominate, bacteria may provide a significant part of the elemental flux to species feeding higher in the food web on the short term. However, the performance of consumers may be constrained by essential biochemicals.

This study investigates the ability of the biodynamic model to predict the trophic bioaccumulation of cadmium (Cd), chromium (Cr), copper (Cu), nickel (Ni) and zinc (Zn) in a freshwater bivalve. Zebra mussels were transplanted to three sites along the Seine River (France) and collected monthly for 11 months. Measurements of the metal body burdens in mussels were compared with the predictions from the biodynamic model. The exchangeable fraction of metal particles did not account for the bioavailability of particulate metals, since it did not capture the differences between sites. The assimilation efficiency (AE) parameter is necessary to take into account biotic factors influencing particulate metal bioavailability. The biodynamic model, applied with AEs from the literature, overestimated the measured concentrations in zebra mussels, the extent of overestimation being site-specific. Therefore, an original methodology was proposed for in situ AE measurements for each site and metal. (C) 2011 Elsevier Ltd. All rights reserved.

This study investigates the ability of the biodynamic model to predict the trophic bioaccumulation of cadmium (Cd), chromium (Cr), copper (Cu), nickel (Ni) and zinc (Zn) in a freshwater bivalve. Zebra mussels were transplanted to three sites along the Seine River (France) and collected monthly for 11 months. Measurements of the metal body burdens in mussels were compared with the predictions from the biodynamic model. The exchangeable fraction of metal particles did not account for the bioavailability of particulate metals, since it did not capture the differences between sites. The assimilation efficiency (AE) parameter is necessary to take into account biotic factors influencing particulate metal bioavailability. The biodynamic model, applied with AEs from the literature, overestimated the measured concentrations in zebra mussels, the extent of overestimation being site-specific. Therefore, an original methodology was proposed for in situ AE measurements for each site and metal. (C) 2011 Elsevier Ltd. All rights reserved.

This study investigates the ability of the biodynamic model to predict the trophic bioaccumulation of cadmium (Cd), chromium (Cr), copper (Cu), nickel (Ni) and zinc (Zn) in a freshwater bivalve. Zebra mussels were transplanted to three sites along the Seine River (France) and collected monthly for 11 months. Measurements of the metal body burdens in mussels were compared with the predictions from the biodynamic model. The exchangeable fraction of metal particles did not account for the bioavailability of particulate metals, since it did not capture the differences between sites. The assimilation efficiency (AE) parameter is necessary to take into account biotic factors influencing particulate metal bioavailability. The biodynamic model, applied with AEs from the literature, overestimated the measured concentrations in zebra mussels, the extent of overestimation being site-specific. Therefore, an original methodology was proposed for in situ AE measurements for each site and metal. (C) 2011 Elsevier Ltd. All rights reserved.

The uptake of trace metals from the diet is a significant route for their entry into marine animals, and the chemical form of trace metals accumulated by food organisms is one potential factor controlling their assimilation from the diet. Therefore, we investigated relationships between the assimilation efficiencies (AE) of the trace metals Cd, Ag and Zn in the neogastropod mollusc Nassarius festivus and their subcellular compartmentalized fractionation in selected tissues of 4 species of bivalve prey, seeking to identify the relative trophic availabilities of such different fractions. We also sought parallels between the AE of N. festivus and 2 in vitro models of digestion, a generalised invertebrate model and a human digestion model. The bivalves were the scallop Chlamys nobilis, the clam Marcia hiantina, the green mussel Perna viridis and the oyster Saccostrea cucullata, as these show a range of accumulated trace metal concentrations and detoxificatory binding to subcellular compartments. Measured assimilation efficiencies of N. festivus for Cd, Ag and Zn from invertebrate prey tissues are very high in comparison to other marine animals, assimilation occurs from accumulated metal in prey bound in subcellular fractions across the insoluble and soluble spectrum, and assimilation is best modelled in vitro by a human digestion model with low pH (1.3) rather than an invertebrate model (pH 5.6). It is concluded that what is trophically available to one predator (feeding on one prey type) is not necessarily trophically available to another (taxonomically separated) predator even if feeding on the same prey, given the variability between invertebrate digestive systems. Variation in the subcellular distribution of accumulated trace metals within different prey will also add variation in trophic availability even for the same metal.

The uptake of trace metals from the diet is a significant route for their entry into marine animals, and the chemical form of trace metals accumulated by food organisms is one potential factor controlling their assimilation from the diet. Therefore, we investigated relationships between the assimilation efficiencies (AE) of the trace metals Cd, Ag and Zn in the neogastropod mollusc Nassarius festivus and their subcellular compartmentalized fractionation in selected tissues of 4 species of bivalve prey, seeking to identify the relative trophic availabilities of such different fractions. We also sought parallels between the AE of N. festivus and 2 in vitro models of digestion, a generalised invertebrate model and a human digestion model. The bivalves were the scallop Chlamys nobilis, the clam Marcia hiantina, the green mussel Perna viridis and the oyster Saccostrea cucullata, as these show a range of accumulated trace metal concentrations and detoxificatory binding to subcellular compartments. Measured assimilation efficiencies of N. festivus for Cd, Ag and Zn from invertebrate prey tissues are very high in comparison to other marine animals, assimilation occurs from accumulated metal in prey bound in subcellular fractions across the insoluble and soluble spectrum, and assimilation is best modelled in vitro by a human digestion model with low pH (1.3) rather than an invertebrate model (pH 5.6). It is concluded that what is trophically available to one predator (feeding on one prey type) is not necessarily trophically available to another (taxonomically separated) predator even if feeding on the same prey, given the variability between invertebrate digestive systems. Variation in the subcellular distribution of accumulated trace metals within different prey will also add variation in trophic availability even for the same metal.

The uptake of trace metals from the diet is a significant route for their entry into marine animals, and the chemical form of trace metals accumulated by food organisms is one potential factor controlling their assimilation from the diet. Therefore, we investigated relationships between the assimilation efficiencies (AE) of the trace metals Cd, Ag and Zn in the neogastropod mollusc Nassarius festivus and their subcellular compartmentalized fractionation in selected tissues of 4 species of bivalve prey, seeking to identify the relative trophic availabilities of such different fractions. We also sought parallels between the AE of N. festivus and 2 in vitro models of digestion, a generalised invertebrate model and a human digestion model. The bivalves were the scallop Chlamys nobilis, the clam Marcia hiantina, the green mussel Perna viridis and the oyster Saccostrea cucullata, as these show a range of accumulated trace metal concentrations and detoxificatory binding to subcellular compartments. Measured assimilation efficiencies of N. festivus for Cd, Ag and Zn from invertebrate prey tissues are very high in comparison to other marine animals, assimilation occurs from accumulated metal in prey bound in subcellular fractions across the insoluble and soluble spectrum, and assimilation is best modelled in vitro by a human digestion model with low pH (1.3) rather than an invertebrate model (pH 5.6). It is concluded that what is trophically available to one predator (feeding on one prey type) is not necessarily trophically available to another (taxonomically separated) predator even if feeding on the same prey, given the variability between invertebrate digestive systems. Variation in the subcellular distribution of accumulated trace metals within different prey will also add variation in trophic availability even for the same metal.

1. The carbon budgets and assimilation efficiencies (AEs) of adults and juveniles of Daphnia magna were quantified using C-14 as a tracer. Animals were fed pure Chlamydomonas reinhardtii or Scenedesmus obliquus at different food concentrations. Carbon AEs (46-70\%) were comparable at food concentrations of 0.03-0.30 mg C L-1 for both algal species, but decreased to 34-49\% when the food concentration further increased by 10-fold. The carbon AEs were significantly and negatively correlated with the food level. 2. During the postdigestive period, partitioning of ingested carbon into different compartments including excretion, respiration and egestion was not influenced by the food species and life stage. There was a negative correlation between respiration (as \% of total loss) and food concentration and a positive correlation between egestion (as \% of total loss) and food concentration. Dissolved organic carbon (DOC) and CO2 accounted for 55-72\% and 9-37\%, respectively, of the total carbon loss from juveniles fed both algal diets. For adults, DOC and CO2 contributed to 44-64\% and 20-47\% of the total carbon loss, respectively. Particulate organic carbon (POC) was a minor pathway for the overall carbon loss. 3. The turnover and release budget of structural carbon (as moults and neonate reproduction) were further evaluated in long-term experiments at different algal concentrations. Food concentration did not affect the carbon efflux or the carbon allocation into different physiological compartments except for respiration. Juveniles had twofold lower carbon turnover rate (0.12-0.16 day(-1)) than those of the adults (0.32-0.35 day(-1)). In adults, comparable carbon was allocated into DOC (35-42\%) and reproduction (27-35\%), which were the dominant routes for carbon loss. For the juveniles, DOC accounted for 42-64\% of the total carbon loss. 4. About 21-38\% of the total DOC released by adults and juveniles was associated with the high molecular weight organic carbon

1. The carbon budgets and assimilation efficiencies (AEs) of adults and juveniles of Daphnia magna were quantified using C-14 as a tracer. Animals were fed pure Chlamydomonas reinhardtii or Scenedesmus obliquus at different food concentrations. Carbon AEs (46-70\%) were comparable at food concentrations of 0.03-0.30 mg C L-1 for both algal species, but decreased to 34-49\% when the food concentration further increased by 10-fold. The carbon AEs were significantly and negatively correlated with the food level. 2. During the postdigestive period, partitioning of ingested carbon into different compartments including excretion, respiration and egestion was not influenced by the food species and life stage. There was a negative correlation between respiration (as \% of total loss) and food concentration and a positive correlation between egestion (as \% of total loss) and food concentration. Dissolved organic carbon (DOC) and CO2 accounted for 55-72\% and 9-37\%, respectively, of the total carbon loss from juveniles fed both algal diets. For adults, DOC and CO2 contributed to 44-64\% and 20-47\% of the total carbon loss, respectively. Particulate organic carbon (POC) was a minor pathway for the overall carbon loss. 3. The turnover and release budget of structural carbon (as moults and neonate reproduction) were further evaluated in long-term experiments at different algal concentrations. Food concentration did not affect the carbon efflux or the carbon allocation into different physiological compartments except for respiration. Juveniles had twofold lower carbon turnover rate (0.12-0.16 day(-1)) than those of the adults (0.32-0.35 day(-1)). In adults, comparable carbon was allocated into DOC (35-42\%) and reproduction (27-35\%), which were the dominant routes for carbon loss. For the juveniles, DOC accounted for 42-64\% of the total carbon loss. 4. About 21-38\% of the total DOC released by adults and juveniles was associated with the high molecular weight organic carbon

We examined the influences of different concentrations and species of Se (selenite, selenate, and seleno-L-methionine) in the ambient environment on the accumulation of inorganic Hg(II) and methylmercury (MeHg) by the diatom Thalassiosira pseudonana and the green mussel Perna viridis. At the experimental concentrations tested (<500 μg l(-1)), selenite and selenate did not significantly affect the uptake of either mercury species by the diatoms and the green mussels. The assimilation efficiency of Hg(II) and MeHg by the mussels from ingested diatoms was also independent of the inorganic Se loadings in the food particles. In contrast, selenomethionine significantly inhibited the uptake of MeHg and enhanced the uptake of Hg(II) by the diatoms and the mussels, but it did not affect the assimilation from the ingested diatoms. The influence of tissue body burden of Se in the green mussels following pre-exposure to selenite and selenomethionine for different periods (1-5 weeks) on the accumulation of Hg(H) and MeHg was further investigated. Our results showed that tissue Se concentrations did not significantly affect the dietary assimilation of mercury, but the influences on the aqueous uptake were variable. Our study thus, strongly highlights the specificity of the Se-Hg interaction in marine mussels for different Se and Hg species. Both dissolved and dietary uptake appeared to be equally important in the accumulation of Hg(II) and MeHg in the green mussels. (C) 2004 Elsevier B.V. All rights reserved.

Rates of uptake from solution and assimilation efficiencies of the trace metals Ag, Cd and Zn were investigated in the barnacle Balanus amphitrite after exposure in the laboratory for 19 days to low and high doses of added Ag and Cd in a diatom (Thalassiosira weissflogii) diet, the major route of metal uptake in barnacles. The hypothesis under test was that acute metal pre-exposure would affect the assimilation efficiency (AE) of that and other metals and their rate of uptake from solution. It was found that pre-exposure of the barnacles to atypically high dietary challenges of Cd and Ag did not cause changes in the rates of uptake of Cd, Ag or Zn from solution. Similarly, there was no clear consistent effect of dietary pre-exposure to Cd or Ag on the assimilation efficiency of Cd, Ag or Zn. The efflux rates of the metals were also comparable following the acute dietary exposure. Subcellular fractionation data indicated that the majority of the three metals were partitioned in the insoluble fraction, with very little in the soluble fraction consisting of metal lothionein-like proteins and other (heat-sensitive) proteins. The lack of induction of increased Cd or Ag AE after pre-exposure in barnacles contrasts with results for mussels; this inconsistency is interpreted to result from differences in physiological accumulation patterns, the barnacles relying to an extreme extent on insoluble detoxification. (C) 2004 Elsevier B.V. All rights reserved.

We examined the influences of different concentrations and species of Se (selenite, selenate, and seleno-L-methionine) in the ambient environment on the accumulation of inorganic Hg(II) and methylmercury (MeHg) by the diatom Thalassiosira pseudonana and the green mussel Perna viridis. At the experimental concentrations tested (<500 μg l(-1)), selenite and selenate did not significantly affect the uptake of either mercury species by the diatoms and the green mussels. The assimilation efficiency of Hg(II) and MeHg by the mussels from ingested diatoms was also independent of the inorganic Se loadings in the food particles. In contrast, selenomethionine significantly inhibited the uptake of MeHg and enhanced the uptake of Hg(II) by the diatoms and the mussels, but it did not affect the assimilation from the ingested diatoms. The influence of tissue body burden of Se in the green mussels following pre-exposure to selenite and selenomethionine for different periods (1-5 weeks) on the accumulation of Hg(H) and MeHg was further investigated. Our results showed that tissue Se concentrations did not significantly affect the dietary assimilation of mercury, but the influences on the aqueous uptake were variable. Our study thus, strongly highlights the specificity of the Se-Hg interaction in marine mussels for different Se and Hg species. Both dissolved and dietary uptake appeared to be equally important in the accumulation of Hg(II) and MeHg in the green mussels. (C) 2004 Elsevier B.V. All rights reserved.

Rates of uptake from solution and assimilation efficiencies of the trace metals Ag, Cd and Zn were investigated in the barnacle Balanus amphitrite after exposure in the laboratory for 19 days to low and high doses of added Ag and Cd in a diatom (Thalassiosira weissflogii) diet, the major route of metal uptake in barnacles. The hypothesis under test was that acute metal pre-exposure would affect the assimilation efficiency (AE) of that and other metals and their rate of uptake from solution. It was found that pre-exposure of the barnacles to atypically high dietary challenges of Cd and Ag did not cause changes in the rates of uptake of Cd, Ag or Zn from solution. Similarly, there was no clear consistent effect of dietary pre-exposure to Cd or Ag on the assimilation efficiency of Cd, Ag or Zn. The efflux rates of the metals were also comparable following the acute dietary exposure. Subcellular fractionation data indicated that the majority of the three metals were partitioned in the insoluble fraction, with very little in the soluble fraction consisting of metal lothionein-like proteins and other (heat-sensitive) proteins. The lack of induction of increased Cd or Ag AE after pre-exposure in barnacles contrasts with results for mussels; this inconsistency is interpreted to result from differences in physiological accumulation patterns, the barnacles relying to an extreme extent on insoluble detoxification. (C) 2004 Elsevier B.V. All rights reserved.

We examined the influences of different concentrations and species of Se (selenite, selenate, and seleno-L-methionine) in the ambient environment on the accumulation of inorganic Hg(II) and methylmercury (MeHg) by the diatom Thalassiosira pseudonana and the green mussel Perna viridis. At the experimental concentrations tested (<500 μg l(-1)), selenite and selenate did not significantly affect the uptake of either mercury species by the diatoms and the green mussels. The assimilation efficiency of Hg(II) and MeHg by the mussels from ingested diatoms was also independent of the inorganic Se loadings in the food particles. In contrast, selenomethionine significantly inhibited the uptake of MeHg and enhanced the uptake of Hg(II) by the diatoms and the mussels, but it did not affect the assimilation from the ingested diatoms. The influence of tissue body burden of Se in the green mussels following pre-exposure to selenite and selenomethionine for different periods (1-5 weeks) on the accumulation of Hg(H) and MeHg was further investigated. Our results showed that tissue Se concentrations did not significantly affect the dietary assimilation of mercury, but the influences on the aqueous uptake were variable. Our study thus, strongly highlights the specificity of the Se-Hg interaction in marine mussels for different Se and Hg species. Both dissolved and dietary uptake appeared to be equally important in the accumulation of Hg(II) and MeHg in the green mussels. (C) 2004 Elsevier B.V. All rights reserved.

Rates of uptake from solution and assimilation efficiencies of the trace metals Ag, Cd and Zn were investigated in the barnacle Balanus amphitrite after exposure in the laboratory for 19 days to low and high doses of added Ag and Cd in a diatom (Thalassiosira weissflogii) diet, the major route of metal uptake in barnacles. The hypothesis under test was that acute metal pre-exposure would affect the assimilation efficiency (AE) of that and other metals and their rate of uptake from solution. It was found that pre-exposure of the barnacles to atypically high dietary challenges of Cd and Ag did not cause changes in the rates of uptake of Cd, Ag or Zn from solution. Similarly, there was no clear consistent effect of dietary pre-exposure to Cd or Ag on the assimilation efficiency of Cd, Ag or Zn. The efflux rates of the metals were also comparable following the acute dietary exposure. Subcellular fractionation data indicated that the majority of the three metals were partitioned in the insoluble fraction, with very little in the soluble fraction consisting of metal lothionein-like proteins and other (heat-sensitive) proteins. The lack of induction of increased Cd or Ag AE after pre-exposure in barnacles contrasts with results for mussels; this inconsistency is interpreted to result from differences in physiological accumulation patterns, the barnacles relying to an extreme extent on insoluble detoxification. (C) 2004 Elsevier B.V. All rights reserved.

We examined the assimilation of Cd, Cr, and Zn by the green mussel Perna viridis under complicated food conditions, including combinations of different compositions and concentrations of food (diatom and sediment), and variable food quantity and quality during particle digestion. At different combinations of food composition and quantity (5 mg l(-1) and 20 mg(-1), below and above the pseudofeces production), the Cd assimilation was significantly dependent on the food composition. The Cd assimilation efficiency (AE) decreased with increasing proportions of sediments in the diets, but its assimilation was not significantly affected by food concentration. In contrast, the assimilation of Cr and Zn decreased significantly with increasing food concentrations, whereas food composition did not significantly affect their AEs. Variations in metal gut passage time accounted partially for the difference in AEs among different combinations of food composition and quantity. By changing the type of particles during metal digestion, their AEs were maintained comparably at a low particle load (1 mg l(-1)), suggesting that variation of food quality during digestion did not affect metal assimilation. At a higher particle load (5 mg l(-1)), variation of food type during digestion affected the AEs of Cr and Zn. An increase in food concentrations from 1 to 15 mg l(-1) during digestion resulted in a significant decrease in the AEs of Cr and Zn bound with either sediments or diatoms. Conversely, decreasing the food concentrations from 15 to 1 mg l(-1) did not affect the AEs of metals, except for Zn bound with diatoms. Overall, our results highlighted the metal-specificity in their assimilation as influenced by complicated food environments, probably caused by different metal geochemical and biological behavior in the mussels. Feeding selectivity may have a greater control on the influx rate into the mussels than metal assimilation. (C) 2003 Elsevier Science B.V All rights reserved.

Trace metal (Cd, Cr, Se, and Zn) uptake from the dissolved phase, assimilation efficiency from the dietary phase, and metal body burden, as well as the clearance rate, apparent water permeability and gill surface area were measured in the green mussel Perna viridis collected from two contrasting salinity sites in Hong Kong coastal waters with following acclimation in the laboratory at different salinities. The concentrations of metals were 1.2-6.4 x greater in mussels collected from the low salinity site, i.e. Tai O, as compared with Tap Mun (high salinity) mussels. Influx of Cd and Zn from the dissolved phase increased with decreased salinity. Furthermore, Cr(VI) and Se (selenite) influx were also affected by decreased salinity, in particular below 17 psu, suggesting that speciation was not the only factor to effect metal uptake. Long-term acclimation to different salinity also had an effect on metal uptake. Mussels collected from Tap Mun (high salinity) accumulated metals 1.2-2.2 x faster than mussels from Tai O (low salinity) at intermediate and high experimental salinities > 17 psu. Metal assimilation efficiencies were unaffected by salinity variation, but gut passage times were significantly longer at low (10 psu) salinities, indicating some effect of lowered salinity on mussels gut physiology. Gill surface area, which was similar in both populations of mussels, and filtration rate, which was generally similar or higher in those groups with low metal uptake, could not explain the inter-population difference in metal accumulation. Although there was no significant difference, due to high inter-individual variability, the apparent water permeability of the Tap Mun population was on average about 1.6 x greater compared with Tai 0 mussels, and may partially account for the difference in metal uptake between these two populations acclimated at salinity > 17 psu. Thus, the effect of salinity on metal uptake is dependent on metal biogeochemistry as well as a range of

Rates of uptake from solution and assimilation efficiencies of trace metals were investigated in populations of the barnacle Balanus amphitrite with different metal contamination histories in Hong Kong coastal waters. The hypothesis under examination was that differential metal pre-exposure in the field would affect the assimilation efficiency (AE) of that metal and its rate of uptake from solution. There were no significant differences between the AE of Ag, Cd, and Zn of the different populations under the same experimental conditions, showing that previous field contamination history had no effect on the assimilation of the 3 trace metals examined. On the other hand, dissolved Cd and Zn uptake-rate constants did vary significantly between populations under the same experimental conditions, but for neither metal was there a significant correlation between metal uptake rate from solution and accumulated metal concentration, which is a surrogate measure of previous contaminant history. Thus, previous field-exposure history had no significant effect on the subsequent uptake of Zn or Cd from solution. Efflux-rate constants of accumulated Ag, Cd, and Zn in the barnacle are low in comparison to those of other benthic invertebrates, in a possible correlation with the large percentage of metal incorporated in insoluble form. A change in salinity from 33 to 15 caused an increase in the dissolved uptake of Zn and Cd in barnacles collected from a fully saline site, expected from physicochemical changes in the complexation of free Zn and Cd ions by chloride. The same salinity change caused an increase in dissolved Cd (but not Zn) uptake in barnacles from a low salinity site, suggesting some physiological acclimation. AE of Ag, Cd, and Zn did not differ at salinities of 15 and 33 in barnacles from the low salinity site. Increased exposure to metals in the laboratory or in extreme field conditions may nevertheless bring about changes in metal uptake rates in coastal invertebrate

Rates of uptake from solution and assimilation efficiencies of trace metals were investigated in populations of the barnacle Balanus amphitrite with different metal contamination histories in Hong Kong coastal waters. The hypothesis under examination was that differential metal pre-exposure in the field would affect the assimilation efficiency (AE) of that metal and its rate of uptake from solution. There were no significant differences between the AE of Ag, Cd, and Zn of the different populations under the same experimental conditions, showing that previous field contamination history had no effect on the assimilation of the 3 trace metals examined. On the other hand, dissolved Cd and Zn uptake-rate constants did vary significantly between populations under the same experimental conditions, but for neither metal was there a significant correlation between metal uptake rate from solution and accumulated metal concentration, which is a surrogate measure of previous contaminant history. Thus, previous field-exposure history had no significant effect on the subsequent uptake of Zn or Cd from solution. Efflux-rate constants of accumulated Ag, Cd, and Zn in the barnacle are low in comparison to those of other benthic invertebrates, in a possible correlation with the large percentage of metal incorporated in insoluble form. A change in salinity from 33 to 15 caused an increase in the dissolved uptake of Zn and Cd in barnacles collected from a fully saline site, expected from physicochemical changes in the complexation of free Zn and Cd ions by chloride. The same salinity change caused an increase in dissolved Cd (but not Zn) uptake in barnacles from a low salinity site, suggesting some physiological acclimation. AE of Ag, Cd, and Zn did not differ at salinities of 15 and 33 in barnacles from the low salinity site. Increased exposure to metals in the laboratory or in extreme field conditions may nevertheless bring about changes in metal uptake rates in coastal invertebrate

Trace metal (Cd, Cr, Se, and Zn) uptake from the dissolved phase, assimilation efficiency from the dietary phase, and metal body burden, as well as the clearance rate, apparent water permeability and gill surface area were measured in the green mussel Perna viridis collected from two contrasting salinity sites in Hong Kong coastal waters with following acclimation in the laboratory at different salinities. The concentrations of metals were 1.2-6.4 x greater in mussels collected from the low salinity site, i.e. Tai O, as compared with Tap Mun (high salinity) mussels. Influx of Cd and Zn from the dissolved phase increased with decreased salinity. Furthermore, Cr(VI) and Se (selenite) influx were also affected by decreased salinity, in particular below 17 psu, suggesting that speciation was not the only factor to effect metal uptake. Long-term acclimation to different salinity also had an effect on metal uptake. Mussels collected from Tap Mun (high salinity) accumulated metals 1.2-2.2 x faster than mussels from Tai O (low salinity) at intermediate and high experimental salinities > 17 psu. Metal assimilation efficiencies were unaffected by salinity variation, but gut passage times were significantly longer at low (10 psu) salinities, indicating some effect of lowered salinity on mussels gut physiology. Gill surface area, which was similar in both populations of mussels, and filtration rate, which was generally similar or higher in those groups with low metal uptake, could not explain the inter-population difference in metal accumulation. Although there was no significant difference, due to high inter-individual variability, the apparent water permeability of the Tap Mun population was on average about 1.6 x greater compared with Tai 0 mussels, and may partially account for the difference in metal uptake between these two populations acclimated at salinity > 17 psu. Thus, the effect of salinity on metal uptake is dependent on metal biogeochemistry as well as a range of

Rates of uptake from solution and assimilation efficiencies of trace metals were investigated in populations of the barnacle Balanus amphitrite with different metal contamination histories in Hong Kong coastal waters. The hypothesis under examination was that differential metal pre-exposure in the field would affect the assimilation efficiency (AE) of that metal and its rate of uptake from solution. There were no significant differences between the AE of Ag, Cd, and Zn of the different populations under the same experimental conditions, showing that previous field contamination history had no effect on the assimilation of the 3 trace metals examined. On the other hand, dissolved Cd and Zn uptake-rate constants did vary significantly between populations under the same experimental conditions, but for neither metal was there a significant correlation between metal uptake rate from solution and accumulated metal concentration, which is a surrogate measure of previous contaminant history. Thus, previous field-exposure history had no significant effect on the subsequent uptake of Zn or Cd from solution. Efflux-rate constants of accumulated Ag, Cd, and Zn in the barnacle are low in comparison to those of other benthic invertebrates, in a possible correlation with the large percentage of metal incorporated in insoluble form. A change in salinity from 33 to 15 caused an increase in the dissolved uptake of Zn and Cd in barnacles collected from a fully saline site, expected from physicochemical changes in the complexation of free Zn and Cd ions by chloride. The same salinity change caused an increase in dissolved Cd (but not Zn) uptake in barnacles from a low salinity site, suggesting some physiological acclimation. AE of Ag, Cd, and Zn did not differ at salinities of 15 and 33 in barnacles from the low salinity site. Increased exposure to metals in the laboratory or in extreme field conditions may nevertheless bring about changes in metal uptake rates in coastal invertebrate

We examined the assimilation of Cd, Cr, and Zn by the green mussel Perna viridis under complicated food conditions, including combinations of different compositions and concentrations of food (diatom and sediment), and variable food quantity and quality during particle digestion. At different combinations of food composition and quantity (5 mg l(-1) and 20 mg(-1), below and above the pseudofeces production), the Cd assimilation was significantly dependent on the food composition. The Cd assimilation efficiency (AE) decreased with increasing proportions of sediments in the diets, but its assimilation was not significantly affected by food concentration. In contrast, the assimilation of Cr and Zn decreased significantly with increasing food concentrations, whereas food composition did not significantly affect their AEs. Variations in metal gut passage time accounted partially for the difference in AEs among different combinations of food composition and quantity. By changing the type of particles during metal digestion, their AEs were maintained comparably at a low particle load (1 mg l(-1)), suggesting that variation of food quality during digestion did not affect metal assimilation. At a higher particle load (5 mg l(-1)), variation of food type during digestion affected the AEs of Cr and Zn. An increase in food concentrations from 1 to 15 mg l(-1) during digestion resulted in a significant decrease in the AEs of Cr and Zn bound with either sediments or diatoms. Conversely, decreasing the food concentrations from 15 to 1 mg l(-1) did not affect the AEs of metals, except for Zn bound with diatoms. Overall, our results highlighted the metal-specificity in their assimilation as influenced by complicated food environments, probably caused by different metal geochemical and biological behavior in the mussels. Feeding selectivity may have a greater control on the influx rate into the mussels than metal assimilation. (C) 2003 Elsevier Science B.V All rights reserved.

Trace metal (Cd, Cr, Se, and Zn) uptake from the dissolved phase, assimilation efficiency from the dietary phase, and metal body burden, as well as the clearance rate, apparent water permeability and gill surface area were measured in the green mussel Perna viridis collected from two contrasting salinity sites in Hong Kong coastal waters with following acclimation in the laboratory at different salinities. The concentrations of metals were 1.2-6.4 x greater in mussels collected from the low salinity site, i.e. Tai O, as compared with Tap Mun (high salinity) mussels. Influx of Cd and Zn from the dissolved phase increased with decreased salinity. Furthermore, Cr(VI) and Se (selenite) influx were also affected by decreased salinity, in particular below 17 psu, suggesting that speciation was not the only factor to effect metal uptake. Long-term acclimation to different salinity also had an effect on metal uptake. Mussels collected from Tap Mun (high salinity) accumulated metals 1.2-2.2 x faster than mussels from Tai O (low salinity) at intermediate and high experimental salinities > 17 psu. Metal assimilation efficiencies were unaffected by salinity variation, but gut passage times were significantly longer at low (10 psu) salinities, indicating some effect of lowered salinity on mussels gut physiology. Gill surface area, which was similar in both populations of mussels, and filtration rate, which was generally similar or higher in those groups with low metal uptake, could not explain the inter-population difference in metal accumulation. Although there was no significant difference, due to high inter-individual variability, the apparent water permeability of the Tap Mun population was on average about 1.6 x greater compared with Tai 0 mussels, and may partially account for the difference in metal uptake between these two populations acclimated at salinity > 17 psu. Thus, the effect of salinity on metal uptake is dependent on metal biogeochemistry as well as a range of

The trophic transfer of metals along the food chain has been recognized as an important issue in the study of water quality in recent years. Feeding experiments were conducted to examine the assimilation of three metals (Cd, Cr and Zn) by the zebrafish Danio reiro feeding on the freshwater zooplankton Daphnia magna. The zooplankton were exposed to radiotracers from both the aqueous and dietary phases for different duration, and then pulse-fed to the zebrafish for measurements of metal assimilation efficiency (AE). The calculated AEs were 3-8\% for Cd, 2-39\% for Cr, and 17-36\% for Zn in the zebrafish. For Cd and Zn, there was no statistically significant difference between the two different radiolabeling routes (aqueous and dietary exposure). For Cr, the AEs were higher when it was accumulated by D. magna from the dietary source than when it was accumulated from the aqueous phase. The gut passage time (GPT) was 6-10 h for all metals, with less variation for Zn among the different treatments. There was no obvious relationship between metal GPT and metal AE, presumably due to the narrow range of variation of metal gut passage. About 5-36\%, 20-31\%, and 8-30\% of the total Cd, Cr and Zn was found in the soft tissue of D. magna after the radiolabeling. A much higher fraction of Cd and Zn was found in the soft tissue of D. magna when the metals were accumulated from the dietary phase. No significant relationship between the metal AE and the metal distribution in the soft tissue of D. magna was however documented in this study. Our results demonstrated that there was major difference in metal AE in freshwater fish among different metals. Metal localization in prey organisms and GPT appear to have little influence on metal assimilation by the zebrafish. (C) 2002 Elsevier Science Ltd. All rights reserved.

The trophic transfer of metals along the food chain has been recognized as an important issue in the study of water quality in recent years. Feeding experiments were conducted to examine the assimilation of three metals (Cd, Cr and Zn) by the zebrafish Danio reiro feeding on the freshwater zooplankton Daphnia magna. The zooplankton were exposed to radiotracers from both the aqueous and dietary phases for different duration, and then pulse-fed to the zebrafish for measurements of metal assimilation efficiency (AE). The calculated AEs were 3-8\% for Cd, 2-39\% for Cr, and 17-36\% for Zn in the zebrafish. For Cd and Zn, there was no statistically significant difference between the two different radiolabeling routes (aqueous and dietary exposure). For Cr, the AEs were higher when it was accumulated by D. magna from the dietary source than when it was accumulated from the aqueous phase. The gut passage time (GPT) was 6-10 h for all metals, with less variation for Zn among the different treatments. There was no obvious relationship between metal GPT and metal AE, presumably due to the narrow range of variation of metal gut passage. About 5-36\%, 20-31\%, and 8-30\% of the total Cd, Cr and Zn was found in the soft tissue of D. magna after the radiolabeling. A much higher fraction of Cd and Zn was found in the soft tissue of D. magna when the metals were accumulated from the dietary phase. No significant relationship between the metal AE and the metal distribution in the soft tissue of D. magna was however documented in this study. Our results demonstrated that there was major difference in metal AE in freshwater fish among different metals. Metal localization in prey organisms and GPT appear to have little influence on metal assimilation by the zebrafish. (C) 2002 Elsevier Science Ltd. All rights reserved.

The trophic transfer of metals along the food chain has been recognized as an important issue in the study of water quality in recent years. Feeding experiments were conducted to examine the assimilation of three metals (Cd, Cr and Zn) by the zebrafish Danio reiro feeding on the freshwater zooplankton Daphnia magna. The zooplankton were exposed to radiotracers from both the aqueous and dietary phases for different duration, and then pulse-fed to the zebrafish for measurements of metal assimilation efficiency (AE). The calculated AEs were 3-8\% for Cd, 2-39\% for Cr, and 17-36\% for Zn in the zebrafish. For Cd and Zn, there was no statistically significant difference between the two different radiolabeling routes (aqueous and dietary exposure). For Cr, the AEs were higher when it was accumulated by D. magna from the dietary source than when it was accumulated from the aqueous phase. The gut passage time (GPT) was 6-10 h for all metals, with less variation for Zn among the different treatments. There was no obvious relationship between metal GPT and metal AE, presumably due to the narrow range of variation of metal gut passage. About 5-36\%, 20-31\%, and 8-30\% of the total Cd, Cr and Zn was found in the soft tissue of D. magna after the radiolabeling. A much higher fraction of Cd and Zn was found in the soft tissue of D. magna when the metals were accumulated from the dietary phase. No significant relationship between the metal AE and the metal distribution in the soft tissue of D. magna was however documented in this study. Our results demonstrated that there was major difference in metal AE in freshwater fish among different metals. Metal localization in prey organisms and GPT appear to have little influence on metal assimilation by the zebrafish. (C) 2002 Elsevier Science Ltd. All rights reserved.

The assimilation efficiencies (AEs) of 3 trace elements (Cd, Se, and Zn) from ingested algal food (the diatom Thalassiosira weissflogii and the dinoflagellate Prorocentrum minimum) in individuals of the marine copepod Calanus sinicus were measured, In general, the metal AEs were comparable in copepods feeding on T. weissflogii P. minimum, They decreased by 1.4 to 2.0x, 1.2 to 1.5x and 2.2 to 2.7x for Cd, Se, and Zn, respectively, after a 20x increase in food concentration (0.054 to 1.076 mg C 1(-1) for T. weissflogii, 0.174 to 3.470 mg C 1(-1) for P. minimum). The physiological turnover-rate constant was, however, independent of food density. There was no evidence of any effect of starvation on metal AEs, The AE of Cd in the copepods was significantly correlated with its distribution in the algal cytoplasm, whereas no such relationship was found for Se and Zn, This study also indicated that the metal AEs were positively related to the gut transit-time of food particles and the gut passage-time of metals in the copepods, implying a more efficient digestion and absorption when the food particles and metals were retained longer in the digestive tract, The gut physiology of copepods may thus considerably affect metal assimilation in diverse food environments. Most unassimilated Cd and Zn was lost by copepods through feces egestion, whereas excretion contributed substantially to the loss of unassimilated Se from the copepods, Excretion was a major route by which assimilated metals were depurated and turned over from the copepods. The relative contribution of fecal egestion to the total metal loss from copepods increased with increasing food concentration. Physiological responses of metal uptake by marine copepods in response to phytoplankton blooms may thus influence the fate of metals in aquatic systems.

Radiotracer experiments were conducted to examine the influence of the concentration of Cd, Se, and Zn in ingested phytoplankton (dinoflagellate Prorocentrum minimum and diatom Thalassiosira weissflogii) and in ambient seawater on metal assimilation and elimination efficiencies of three marine copepods, Acartia spinicauda, Paracalanus aculeatus, and Calanus sinicus. The assimilation efficiencies (AEs) decreased by 1.7 to 2.0 times, 1.4 to 4.1 times, and 1.3 to 2.2 times in the copepods with an increase in metal concentration in ingested algae by 16 to 84 times, 14 times, and 45 to 153 times, for Cd, Se, and Zn, respectively. However, the physiologic turnover rate constant was relatively independent of the metal concentration in copepods. No evidence was found of any interaction between Cd and Zn in their assimilation by copepods. Assimilation efficiencies of Cd were higher in copepods feeding on the dinoflagellate P. minimum, whereas the AEs of Zn were higher in copepods feeding on the diatom T. weissflogii. Differences in metal distribution in al-al cytoplasm at different ambient metal concentrations may be partially responsible for the observed influence of metal concentration in algal cells on metal assimilation in copepods. However, metal desorption within the gut of the copepod may have little influence on metal assimilation, as a result of the short gut residence time of food particles and the neutral gut pH. Our study also indicated that the ingestion rate of copepods was reduced by a higher concentration of Cd and Se, but was not affected by Zn concentration in the food particles. Consequently, partial regulation of metal trophic transfer in response to increasing metal contamination may be achieved by a change in metal assimilation efficiency and the ingestion activity of the copepod, but not by changes in metal turnover rates from the animals.

The assimilation efficiencies (AEs) of 3 trace elements (Cd, Se, and Zn) from ingested algal food (the diatom Thalassiosira weissflogii and the dinoflagellate Prorocentrum minimum) in individuals of the marine copepod Calanus sinicus were measured, In general, the metal AEs were comparable in copepods feeding on T. weissflogii P. minimum, They decreased by 1.4 to 2.0x, 1.2 to 1.5x and 2.2 to 2.7x for Cd, Se, and Zn, respectively, after a 20x increase in food concentration (0.054 to 1.076 mg C 1(-1) for T. weissflogii, 0.174 to 3.470 mg C 1(-1) for P. minimum). The physiological turnover-rate constant was, however, independent of food density. There was no evidence of any effect of starvation on metal AEs, The AE of Cd in the copepods was significantly correlated with its distribution in the algal cytoplasm, whereas no such relationship was found for Se and Zn, This study also indicated that the metal AEs were positively related to the gut transit-time of food particles and the gut passage-time of metals in the copepods, implying a more efficient digestion and absorption when the food particles and metals were retained longer in the digestive tract, The gut physiology of copepods may thus considerably affect metal assimilation in diverse food environments. Most unassimilated Cd and Zn was lost by copepods through feces egestion, whereas excretion contributed substantially to the loss of unassimilated Se from the copepods, Excretion was a major route by which assimilated metals were depurated and turned over from the copepods. The relative contribution of fecal egestion to the total metal loss from copepods increased with increasing food concentration. Physiological responses of metal uptake by marine copepods in response to phytoplankton blooms may thus influence the fate of metals in aquatic systems.

We examined the biological uptake of iron by a coastal diatom (Thalassiosira pseudonana, clone 3H) and natural phytoplankton assembledge and its subsequent trophic transfer to marine copepods under various macronutrient regimes. Diatom cells maintained in semi-continuous cultures generally exhibited a higher intracellular Fe concentration with increasing nitrate concentration, whereas the ammonium concentration did not significantly affect the intracellular Fe uptake in the cells. At the similar concentrations, the calculated Fe uptake rate by the diatoms was 1.5-2.4 X higher in cells maintained in nitrate substrate than in cells maintained in ammonium substrate. Intracellular Fe uptake was also much higher in diatom cells maintained under N-enriched conditions than in cells under N starvation. In contrast, the intracellular Fe uptake in diatoms was not dependent on the ambient P and Si conditions. The significant influence of macronutrients was also observed in the natural phytoplankton assembledge collected from a harbor. There was marked increase in Fe uptake with the addition of all major nutrients or P alone, whereas N addition alone did not greatly affect Fe uptake because of the high background N concentration. More Fe was also partitioned into the intracellular compartment with the addition of macronutrients. The Fe assimilation efficiency (AE) in two marine copepods (Calanus sinicus and Acartia spinicauda) increased with increasing nitrate concentration in diatoms, primarily due to a greater partitioning of Fe in the intracellular and cytoplasmic pools of the cells. Under N-limited conditions, a greater fraction of phytoplankton Fe may be packed into fecal pellets and transported to deeper waters. Conversely, the higher biological uptake of Fe by phytoplankton and the higher assimilation by marine copepods under nitrate-enriched conditions may increase the Fe residence time in the surface waters. (C) 2001 Elsevier Science B.V. All rights reserved.

Radiotracer experiments were conducted to examine the influence of the concentration of Cd, Se, and Zn in ingested phytoplankton (dinoflagellate Prorocentrum minimum and diatom Thalassiosira weissflogii) and in ambient seawater on metal assimilation and elimination efficiencies of three marine copepods, Acartia spinicauda, Paracalanus aculeatus, and Calanus sinicus. The assimilation efficiencies (AEs) decreased by 1.7 to 2.0 times, 1.4 to 4.1 times, and 1.3 to 2.2 times in the copepods with an increase in metal concentration in ingested algae by 16 to 84 times, 14 times, and 45 to 153 times, for Cd, Se, and Zn, respectively. However, the physiologic turnover rate constant was relatively independent of the metal concentration in copepods. No evidence was found of any interaction between Cd and Zn in their assimilation by copepods. Assimilation efficiencies of Cd were higher in copepods feeding on the dinoflagellate P. minimum, whereas the AEs of Zn were higher in copepods feeding on the diatom T. weissflogii. Differences in metal distribution in al-al cytoplasm at different ambient metal concentrations may be partially responsible for the observed influence of metal concentration in algal cells on metal assimilation in copepods. However, metal desorption within the gut of the copepod may have little influence on metal assimilation, as a result of the short gut residence time of food particles and the neutral gut pH. Our study also indicated that the ingestion rate of copepods was reduced by a higher concentration of Cd and Se, but was not affected by Zn concentration in the food particles. Consequently, partial regulation of metal trophic transfer in response to increasing metal contamination may be achieved by a change in metal assimilation efficiency and the ingestion activity of the copepod, but not by changes in metal turnover rates from the animals.

The assimilation efficiencies (AEs) of 3 trace elements (Cd, Se, and Zn) from ingested algal food (the diatom Thalassiosira weissflogii and the dinoflagellate Prorocentrum minimum) in individuals of the marine copepod Calanus sinicus were measured, In general, the metal AEs were comparable in copepods feeding on T. weissflogii P. minimum, They decreased by 1.4 to 2.0x, 1.2 to 1.5x and 2.2 to 2.7x for Cd, Se, and Zn, respectively, after a 20x increase in food concentration (0.054 to 1.076 mg C 1(-1) for T. weissflogii, 0.174 to 3.470 mg C 1(-1) for P. minimum). The physiological turnover-rate constant was, however, independent of food density. There was no evidence of any effect of starvation on metal AEs, The AE of Cd in the copepods was significantly correlated with its distribution in the algal cytoplasm, whereas no such relationship was found for Se and Zn, This study also indicated that the metal AEs were positively related to the gut transit-time of food particles and the gut passage-time of metals in the copepods, implying a more efficient digestion and absorption when the food particles and metals were retained longer in the digestive tract, The gut physiology of copepods may thus considerably affect metal assimilation in diverse food environments. Most unassimilated Cd and Zn was lost by copepods through feces egestion, whereas excretion contributed substantially to the loss of unassimilated Se from the copepods, Excretion was a major route by which assimilated metals were depurated and turned over from the copepods. The relative contribution of fecal egestion to the total metal loss from copepods increased with increasing food concentration. Physiological responses of metal uptake by marine copepods in response to phytoplankton blooms may thus influence the fate of metals in aquatic systems.

We examined the biological uptake of iron by a coastal diatom (Thalassiosira pseudonana, clone 3H) and natural phytoplankton assembledge and its subsequent trophic transfer to marine copepods under various macronutrient regimes. Diatom cells maintained in semi-continuous cultures generally exhibited a higher intracellular Fe concentration with increasing nitrate concentration, whereas the ammonium concentration did not significantly affect the intracellular Fe uptake in the cells. At the similar concentrations, the calculated Fe uptake rate by the diatoms was 1.5-2.4 X higher in cells maintained in nitrate substrate than in cells maintained in ammonium substrate. Intracellular Fe uptake was also much higher in diatom cells maintained under N-enriched conditions than in cells under N starvation. In contrast, the intracellular Fe uptake in diatoms was not dependent on the ambient P and Si conditions. The significant influence of macronutrients was also observed in the natural phytoplankton assembledge collected from a harbor. There was marked increase in Fe uptake with the addition of all major nutrients or P alone, whereas N addition alone did not greatly affect Fe uptake because of the high background N concentration. More Fe was also partitioned into the intracellular compartment with the addition of macronutrients. The Fe assimilation efficiency (AE) in two marine copepods (Calanus sinicus and Acartia spinicauda) increased with increasing nitrate concentration in diatoms, primarily due to a greater partitioning of Fe in the intracellular and cytoplasmic pools of the cells. Under N-limited conditions, a greater fraction of phytoplankton Fe may be packed into fecal pellets and transported to deeper waters. Conversely, the higher biological uptake of Fe by phytoplankton and the higher assimilation by marine copepods under nitrate-enriched conditions may increase the Fe residence time in the surface waters. (C) 2001 Elsevier Science B.V. All rights reserved.

Radiotracer experiments were conducted to examine the influence of the concentration of Cd, Se, and Zn in ingested phytoplankton (dinoflagellate Prorocentrum minimum and diatom Thalassiosira weissflogii) and in ambient seawater on metal assimilation and elimination efficiencies of three marine copepods, Acartia spinicauda, Paracalanus aculeatus, and Calanus sinicus. The assimilation efficiencies (AEs) decreased by 1.7 to 2.0 times, 1.4 to 4.1 times, and 1.3 to 2.2 times in the copepods with an increase in metal concentration in ingested algae by 16 to 84 times, 14 times, and 45 to 153 times, for Cd, Se, and Zn, respectively. However, the physiologic turnover rate constant was relatively independent of the metal concentration in copepods. No evidence was found of any interaction between Cd and Zn in their assimilation by copepods. Assimilation efficiencies of Cd were higher in copepods feeding on the dinoflagellate P. minimum, whereas the AEs of Zn were higher in copepods feeding on the diatom T. weissflogii. Differences in metal distribution in al-al cytoplasm at different ambient metal concentrations may be partially responsible for the observed influence of metal concentration in algal cells on metal assimilation in copepods. However, metal desorption within the gut of the copepod may have little influence on metal assimilation, as a result of the short gut residence time of food particles and the neutral gut pH. Our study also indicated that the ingestion rate of copepods was reduced by a higher concentration of Cd and Se, but was not affected by Zn concentration in the food particles. Consequently, partial regulation of metal trophic transfer in response to increasing metal contamination may be achieved by a change in metal assimilation efficiency and the ingestion activity of the copepod, but not by changes in metal turnover rates from the animals.

The green mussel Perna viridis and the clam Ruditapes philippinarum have been frequently used as biomonitors of coastal contamination in subtropical and tropical waters, yet the physiological processes controlling metal uptake in these bivalves are unknown. Assimilation efficiency (AE) is an important physiological parameter quantifying metal bioavailability from ingested food. We determined the AEs of Cd, Cr, and Zn in these bivalves feeding on five species of phytoplankton and one natural seston. The influences of the cytoplasmic distribution of metals in the algal cells and the digestive physiology of bivalves on metal AEs were also examined. Among the three metals, Zn was generally assimilated at the highest efficiency, i.e., 21 to 36\% in the mussels and 29 to 59\% in the clams. Cr was the least assimilated metal, with AEs being 10 to 16\% in the mussels and 11 to 24\% in the clams. The AEs of Cd and Zn in the clams were 1.8 to 4.7 and 1.1 to 1.9 times higher, respectively. than the AEs in the mussels. Assimilation efficiencies of Cr were, however, comparable between the mussels and the clams. A positive significant relationship between the metal AE and the percent of metals in the algal cytoplasm was found only for Cd in the clams, suggesting that Cd fractionation in the algal cells influenced its assimilation. No significant relationship, however, was found for other metals in both bivalves. A significant relationship between Cr assimilation efficiency and gut passage time (GPT) was documented in the mussels, indicating a higher assimilation when Cr was retained longer in the gut. There was also significant correlation of metal AEs among the three metals, which were probably subjected to the same digestive pathway in the bivalves. Our study demonstrated that both the green mussels and the clams were able to accumulate metals from ingested food source, and food quality appeared to have different effects on metal assimilation in different bivalve species.