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To reach the goal of large-scale seaweed cultivation in Norway, new knowledge concerning commercially important species like the kelp Saccharina latissima is essential. This includes fundamental understanding of physiological mechanisms like nutrient uptake kinetics to better understand its ecological niche and nutritional requirements. The initial short-term nitrate (NO3−) uptake kinetics in cultivated S. latissima juvenile sporophytes were evaluated under nitrogen-saturated and nitrogen-limited conditions. The uptake was measured for concentrations in a gradient from 2 to 18 μM NO3− which is representative of Norwegian coastal waters in the main growth season of winter and early spring. Preconditioning treatments led to internal nitrogen pools (total tissue nitrogen and intracellular nitrate) that were significantly lower for the N-limited than for the N-saturated sporophytes prior to the experiment. Nitrate uptake rates, related to biomass (V) and intracellular nitrogen content (U), were linearly related to the substrate concentrations for both N-limited and N-saturated sporophytes, indicating that S. latissima requires high ambient nitrate concentrations to maintain rapid growth. The sporophytes with deficient internal nitrogen pools exhibited higher uptake rates of NO3− than sporophytes with higher internal pools of nitrogen. Mathematical modelling was used to investigate the temporal development of total tissue nitrogen (QN) based on the nitrogen-specific uptake rates (U) and revealed a near linear response of U to changes in QN. The model also found that a maximum estimated value for QN was only approached after more than 20 days at external NO3- concentrations of 8 μM. These results expand the physiological understanding of cultivated S. latissima and are important for a sustainable upscaling of seaweed farm production. © 2020 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

With the increasing interest and activities regarding seaweed cultivation in Europe, it is becoming crucial to utilize the opportunities that lies within environmental resources, such as light and nutrients, to produce biomass of a high yield and quality. The chemical composition of seaweed varies between seasons and depths as an effect of resource supply and environmental conditions. These factors are particularly important to assess for economically feasible species like the kelp Saccharina latissima. This paper examines how differences in light conditions and nutrient availability affect the growth and intracellular nitrogen in S. latissima. This was done through cultivating sporophytes in land-based tanks with four different combinations of high/low light and high/low nutrient supply over an experimental period of 20 days, with measurements of growth rate and subsequent analysis of tissue nitrogen metabolites. The results revealed that the mean growth rate and the intracellular nitrogen components of the sporophytes were positively related to the external nitrate concentration during the experimental period, indicating that S. latissima requires high nutrient concentration to maintain a rapid growth. Copyright © 2020 Jevne, Forbord and Olsen. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

Juvenile scallops of Pecten maxiumus were studied to see the capability to clear out and incorporate salmon feed and feces (30 µg L-1). Algae were also given, in a low and high concentration in addition to feed and faeces, to mimic a winter and summer situation in Norwegian waters. Rhodomonas baltica and Chaetoseris muelleri were provided in a concentration of 50 µg L-1 and 300 µg L-1. The feeding trial lasted for 27 days. Clearance rate was measured to study filtration characteristics, while fatty acid profiling and stable isotopes of nitrogen and carbon were used to trace the uptake of salmon feed and feces in the digestive gland and muscle of juvenile scallops (30–35 mm shell height). The results show that the scallops could clear out and retain both salmon feed and feces particles, although at a statistically lower clearance rate than the algae. Fatty acid profiling revealed that the scallops assimilated and incorporated the consumed salmon feed and feces, given with either high or low algae concentrations, in their tissues, where the fatty acid C18:1n9 was used as a tracer fatty acid. The digestive gland of the scallops that were fed salmon feed and feces contained a higher share of C18:1n9 than those that were only fed algae. The digestive gland reflected the fatty acid composition of the diet, while the fatty acid composition of the muscle, which also changed, reflected a more complex relation between diet and metabolic processes in the tissue. The use of stable isotopes of carbon and nitrogen to trace food sources was inconclusive in this study due to low differences between samples fed different feeds. Fatty acid profiling was a more sensitive method for tracing low concentrations of salmon feed and feces in the algae diet of scallops. Our results suggest that P. maximus could be a candidate for integrated multi trophic aquaculture (IMTA) and that scallops have the potential to utilize small particles of wasted salmon feed and feces during a winter situation with low phytoplankton concentration and during an algal bloom in Norwegian waters. Copyright © 2019 Bergvik, Stensås, Handå, Reitan, Strand and Olsen. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY)

Rotifer cultures of Brachionus Nevada and Cayman were grown semi-continuously, fed live Rhodmonas baltica algae, and cultures were diluted daily (20% day−1). The enrichment diets Marol E (triacylglycerol-rich) and PL (cod roe emulsion, phospholipid-rich) were added to both cultures (Nevada 40 ng ind−1 day−1 and Cayman 25 ng ind−1 day−1, respectively). After 4 days of enrichment (dilution rate: 20% day−1), Cayman rotifers–fed Marol E had comparable (P > 0.05) quantitative amounts of DHA (docosahexaenoic acid, 22:6n-3) to natural harvested copepods, which were analysed for comparison, and the Cayman strain had generally more efficient incorporation of HUFAs (highly unsaturated fatty acids) than the Nevada strain. However, the percentage DHA of the total fatty acids in copepods were much higher than in enriched rotifers (P

The interest to develop an industrialized cultivation of several macroalgae in Europe is growing rapidly. Here, we demonstrate cultivation of the sugar kelp Saccharina latissima in integration with salmon (Salmo salar) aquaculture in a Norwegian coastal area. Sporophytes of S. latissima were deployed at 2, 5 and 8 m depths at a salmon farm and at a reference station 4 km away in August, November, February and June (2010–2011). The growth was good in late autumn and in spring, with peak lengths of the sporophytes in June, while being poor in winter and summer. As a result of a faster initial growth at the fish farm than at the reference station from August to November, the August-sporophytes reached a significantly longer length than those at the reference station in 5 out of 10 sampling months at 2 m depth, and in 9 out of 10 months at 5 m depth (p

The growth of Calanus finmarchicus nauplii was studied through three spawning seasons (2007, 2009 and 2011) using flow-through tubes. Natural seston was collected every second day and added as food source using a peristaltic pump. A mixture of cultured microalgae supplied in surplus concentration was used as a control treatment. No significant correlation between growth and food concentration measured as Chlorophyll a (Chla) or particulate organic carbon (POC) was detected, but the growth rate was significantly related to the content of EPA (20:5n-3) and DHA (22:6n-3) in the seston. The growth rate was overall higher for nauplii fed cultured microalgae compared to the nauplii fed natural seston. Although the nauplii fed algae cultures were fed surplus food, the growth did vary between the growth periods. Furthermore, the growth rate for nauplii fed natural seston and for nauplii fed cultured algae were positively related, suggesting that the maternal condition and the food quality experienced by the mothers could explain some of the variation in naupliar growth rate. We present lipid class data on Calanus finmarchicus eggs from field samples that, contrary to previous studies, showed a high content of wax esters. Fatty acid analyzes of eggs, nauplii stages and copepodites from field samples showed that eggs and nauplii have a similar fatty acid composition and that the main increase in the content and share of DHA and EPA was from nauplii to copepodite. The secondary production measured as naupliar growth was compared to the secondary production measured as carbon specific female egg production rate. The secondary production measured as egg production was generally higher than the secondary production measured as naupliar growth early in the spring, whereas the opposite situation was observed during post-bloom situations in late spring/early summer.

Ceramic clay has been increasingly used to improve contrast and prey detection in tanks for rearing of fish larvae. In contrast to live microalgae or algae pastes, clay increases turbidity without contributing to the organic matter load. In addition, clay may aggregate and sediment organic matter and bacteria, facilitating its removal. Marine larvae are sensitive to infections by opportunistic bacteria. Fish, algae, and live feed increase the microbial carrying capacity of the rearing water which allow exponential growth of bacteria and favor fast-growing opportunists. Reducing substrate levels by replacing microalgae with clay may reduce bacteria proliferation and benefit larvae. We compared the effects of three rearing regimes including live Isochrysis galbana, Nannochloropsis oculata paste, and ceramic clay on the bacterial community, concentration of organic matter, and growth and survival of Atlantic cod larvae (Gadus morhua L.). The application of clay resulted in reduced substrate levels for bacteria in the rearing water compared to the addition of live algae or algae paste. To some extent, clay aggregated and transported organic matter to the bottom of the larval fish tanks, where it could be effectively removed. Fish tanks receiving clay showed a lower abundance of bacteria in the water than tanks added algae paste or live algae. Fish tanks with algae paste showed a higher abundance of bacteria and a higher share of cultivable bacteria and TCBS counts than the other two treatments. Tanks with live algae showed low relative abundances of opportunistic bacteria and TCBS counts in both water and rotifers. Cod larvae in tanks with clay or live algae initiated exponential growth earlier than larvae in tanks with algae paste. Larvae in tanks receiving clay had significantly higher dry weight than larvae in tanks receiving algae paste at day 5 and 20 post hatching. The survival of larvae in the tanks added clay was variable. Two of the three tanks with clay had significantly higher larval survival than the tanks with live algae or algae paste. However, one tank with clay underwent 100% mortality. It is not possible to conclude whether this was related to the use of clay or an incidental development of a harmful microbial community in this tank. The effects of clay addition on larval performance should be studied further. Clay addition appears to be an easy way to reduce bacterial load during early first feeding of marine larvae without compromising the beneficial effects of turbidity.

A recent meta-analysis indicates that trophic cascades (indirect effects of predators on plants via herbivores) are weak in marine plankton in striking contrast to freshwater plankton (Shurin et al. 2002, Ecol. Lett., 5, 785–791). Here we show that in a marine plankton community consisting of jellyfish, calanoid copepods and algae, jellyfish predation consistently reduced copepods but produced two distinct, opposite responses of algal biomass. Calanoid copepods act as a switch between alternative trophic cascades along food chains of different length and with counteracting effects on algal biomass. Copepods reduced large algae but simultaneously promoted small algae by feeding on ciliates. The net effect of jellyfish on total algal biomass was positive when large algae were initially abundant in the phytoplankton, negative when small algae were dominant, but zero when experiments were analysed in combination. In contrast to marine systems, major pathways of energy flow in Daphnia-dominated freshwater systems are of similar chain length. Thus, differences in the length of alternative, parallel food chains may explain the apparent discrepancy in trophic cascade strength between freshwater and marine planktonic systems.

Copepods and ciliates are 2 functionally different types of herbivores that dominate marine ecosystems. Ciliates have a higher reproductive capacity than copepods, but copepods can graze on ciliates. Hence we have an omnivorous system in which the inferior grazer can graze on its competitor. We conducted a 3 2 factorial mesocosm experiment with natural plankton communities to explore how the copepod -ciliate interaction affects algal development on the timescale relevant for algal blooms. We used nutrient load and copepod biomass as the experimental variables. Ciliate bio- mass was positively correlated to nutrient addition and negatively correlated to copepod biomass, with copepod biomass as the predominant factor at the highest densities tested. Both increasing copepod biomass and nutrient addition resulted in increased algal biomass, with the strongest effect on nanoalgae. The mechanism involved was carnivory by copepods and a resulting trophic cascade through ciliates to algae. We conclude that even a moderately high biomass of copepods may pro- mote algal blooms when the initial size distribution of the algal community is appropriate (i.e. domi- nance of algae eaten primarily by ciliates).

Artemia franciscana was grown on Isochrysis galbana Green (clone T. Iso) at saturated food concentrations (13 to 20 mg C l−1) for 11 d at 26 to 28 °C, and 34 ppt salinity. Three groups of brine shrimp were used in the feeding experiments: metanauplius III and IV (Group 1), post-metanauplius II and III (Group 2) and post-metanauplius VIII (Group 3), corresponding to 4-, 7- and 11-d-old animals, respectively. The ingestion rate, clearance rate and carbon balance were estimated for these stages at different concentrations of 14C-labeled I. galbana ranging from 0.05 to 30 mg C l−1. The handling time of algae was determined for all three groups. The ingestion rate (I, ng C ind−1 h−1) increased as a function of animal size and food concentration. In all three groups, the ingestion rate increased to a maximum level (I max) and remained constant at food concentrations ≥10 mg C l−1 (saturated food concentrations). The clearance rate (CR, μl ind−1 h−1) increased with increasing food concentration up to a maximum rate (CR max), after which it decreased for even higher food concentrations. The functional response of A. franciscana was most consistent with Holling's Type 3 functional response curve (sigmoidal model), which for the two oldest groups (Group 2 and 3) differed significantly from a Type 2 response (p

Individual dry weight and carbon content, ingestion rate, growth rate and production rate were determined for different developmental stages of Artemia franciscana feeding on six different concentrations of Isochrysis galbana (Tahiti). The cultures were grown at nearly constant food concentrations, ranging from 0.2 to 20 mg C l−1 for 12 days at a temperature of 26–28°C and 34 ppt salinity. The individual body dry weight increased exponentially with age to a point of inflection before the growth rate gradually decreased when the animals reached the pre-adult stage. The growth of A. franciscana was strongly influenced by the food concentration. Minimum food concentration required for maximum growth was 10 mg C l−1. At this food concentration the individual dry weigh increased from 2.3 μg ind−1 (newly hatched nauplii) to 195±7.03 μg ind−1 (mean±SE) when the animals had reached pre-adult stage (11 days growth). In the same period A. franciscana grown at lower food concentrations, 7, 5 and 3 mg C l−1, reached 134±3.41, 88±3.53 and 29±3.09 μg (mean±SE), respectively. At the lowest food concentration tested (0.2 mg C l−1) the individual dry weight was reduced by 14–18% after 5 days of cultivation, indicating that the animals were starving. The carbon and nitrogen content (% of dry weight) of newly hatched nauplii was 45.5 and 10.1%, respectively. During the growth period the carbon content varied between 40.1 and 45.5%, and the nitrogen content between 9.1 and 10.5%. The specific growth rate (μ, day−1) and the ingestion rate (ng C ind−1 day−1) were strongly affected by the size of the animals and the food concentration. Both rates increased with increasing food concentration and reached a maximum level at an algal concentration of 10 mg C l−1. The specific growth rate and the ingestion rate were not influenced by an increase of the food concentration above 10 mg C l−1. At these food concentrations the specific growth rate showed a maximum value when the animals were in the range 6–51 μg ind−1 (μmax=0.9–1.15 day−1). The weight specific ingestion rate (IW, h−1) also increased with increasing food concentration. A. franciscana in the range of 7–20 μg ind−1, grown at food concentrations ≥10 mg C l−1, ingested on carbon basis approximately 20–28% of the individual carbon content per hour (IW=0.20–0.28 h−1). At the lower food concentrations (3 and 5 mg C l−1) the ingestion rate was reduced and the weight specific ingestion rate varied between 0.05 and 0.1 h−1. The production rate (Pr, μg C ind−1 day−1) showed a distinct maximum for animals grown at the two highest food concentrations. When individual dry weight exceeded 6 μg ind−1 (post-metanauplius II) animal size affected both the production rate and the relative production rate (%, day−1). Thereafter the food concentration had a more pronounced effect on the production rate of the animals. The conversion of ingested food into biomass (yield, day−1) was dependent on food concentration and animal size. Animals fed at the lowest food concentrations (3, 5 and 7 mg C l−1) attained a yield of 0.50–0.65 at post-metanauplius stage III, and remained at this level until post-metanauplius stage VII (dry weight; 10–50 μg ind−1). Animals at the same size but fed at the highest food concentrations (10 and 20 mg C l−1), showed a yield close to 0.15–0.20 at post-metanauplius stage III increasing to 0.50 at post-metanauplius stage VI–VII. In the older stages the yield decreased from a mean of 0.46 to 0.05–0.10 when the animals attained pre-adult stage.

In the first-feeding of larval, turbot (Scophthalmus maximus) and halibut (Hippoglossus hippoglossus), microalgae are used in the production of rotifers (Brachionus plicatilis) in order to transfer essential nutrients from the algae to the live food. In addition, the algae may be given directly to the larvae along with the live food. In this circumstance they act both as food for the fish larvae and for the live food. Microalgal addition to the first-feeding tanks along with the rotifers improved growth and survival of larvae, whereas short-term enrichment of rotifers with algae did not improve growth and survival of larvae in tanks without algae added. The algae in larval tanks tended to modify and stabilize the nutritional quality of the rotifers in the period before they were consumed by the larvae. The lipid content and fatty acid composition of the rotifers reflected the composition of the algal diets, and the algal species used may be an effective tool to control the fatty acid content (especially DHA, 22:6n−3, docosahexaenoic acid, and EPA, 20:5n−3, eicosapentaenoic acid). The content of n−3 polyunsaturated fatty acids varied between algal species, and to some extent, with the growth limitation of the algal cells. Young stages of marine fish larvae ingested microalgae, but the algal cells were assimilated to a different extent in halibut and turbot. The ingested microalgae may have triggered the digestion process or contributed to the establishment of an early gut flora. The algae in larval tanks most probably modified the bacterial flora of the water and the rotifers. In addition, the algae in larval tanks may have modified the light milieu for the larvae.

The effect of three different rotifer enrichments was examined on growth, survival, pigmentation and viability of first feeding turbot larvae. The diets differed in rotifer content of protein, lipid and ratio of protein/lipid. The diets were fed to turbot with or without algae ( Isochrysis galbana ) added to the larval tanks. The turbot larvae were fed rotifers for 10 days and thereafter the same Artemia diet was fed to all treatments for the rest of the experimental period. Growth and survival of fish larvae were higher in tanks containing algae than in tanks where no algae were added. Independent of algal addition, the highest growth rate and survival was obtained by feeding rotifers containing the highest protein content. Larvae reared in greenwater consumed higher numbers of rotifers during the stagnant period than larvae kept in clearwater conditions, while analysis of the larval gut contents showed lower rotifer numbers in the gut of larvae reared in greenwater conditions. This must imply longer residence time of the food in the larval gut, and presumably also higher digestion and assimilation efficiencies of larvae maintained without algae than in larvae maintained with algae. Calculation of protein and carbon conversion efficiency showed higher utilization in larvae maintained without algae (18–28% for protein, 12–19% for carbon) than in larvae maintained with algae (6–9% for protein, 4–7% for carbon). No significant differences in pigmentation rate and stress sensitivity were observed among the larvae of the various treatments.

Large-scale artificial upwelling was tested as a method to enhance the environmental conditions for the growth of non-toxic algae in a Norwegian fjord (61°0′N, 6°22′E). The experiment was designed to evaluate if nutrient-rich seawater, brought up from below the mixed zone of a stratified fjord to the euphotic zone by air bubbling, would stimulate the growth of non-toxic relative to toxic algae. Pumping 44 m3 min−1 of air at 1 atm through a pipe diffuser submerged at 40 m depth formed a buoyancy flux that lifted 60 m3 s−1 of deep water to the upper 17 m over a period of 21 days. The supply of silicate, inorganic nitrogen and phosphate to the upper 10 m in the fjord increased, and a significant increase in the biomass of non-toxic algae was observed. The upwelling gave an increased growth of the non-toxic dinoflagellates Ceratium furca and C. tripos. After termination of the experiment, the phytoplankton biomass decreased significantly, whereas a distinct increase occurred in the relative biomass of the potentially toxic Dinophysis spp. The result is considered promising when it comes to creating controlled geographical areas with non-toxic food for mussel production.

Phosphorus release rates from Daphnia galeata, the dominant metazoan, were estimated in eutrophic Lake Nesjovatn during summer by applying our previously published mass balance model. For both juveniles (2.0 pg C ind.-‘) and adults (6.6 pg C ind.-‘), three different estimates of P release were made by two different parametrizations and by considering the importance of ingestion of detrital particles. As a basis for estimates of P release, clearance rates of algae and bacteria were determined as well as a thorough characterization of the food compartments. The specific release rate of P varied between 0.16 and 1.18 ,ug P mg C--l h-’ for juveniles and between 0.06 and 0.96 for adults, independent of estimate. These release rates are considerably lower than those obtained by previously published models. Most of the observed variation was due to variation in the P content of the food, thus emphasizing the importance of including this variable in studies of P release from zooplankton. Calculations of rates of population P release suggest that D. galeata contributed < *I3 of the P requirements of algae and bacteria.

Phaeodactylum tricornutum and Chaetoceros sp. (Badllariophyceae), Isochrysis galbana (clone T-Iso) and Pavlova lutheri (Prymnesiophyceae), Nannochloris atomus (Chlorophyceae), Tetraselmis sp. (Prasinophyceae), and Gymnodinum sp. (Dinophyceae) were cultured at different extents of nutrient-limited growth: 50 and 5% of μmax. The lipid content of the algae was in the range 8.3–29.5% of dry matter and was generally higher in the Prymnesiophyceae than in the Prasinophyceae and the Chlorophyceae. Increasing extent of phosphorus limitation resulted in increased lipid content in the Bacillariophyceae and Prymnesiophyceae and decreased lipid content in the green flagellates N. atomus and Tetraselmis sp. The fatty acid composition of the algae showed taxonomic conformity, especially for the Bacillariophyceae, where the major fatty adds were 14:0, 16:0, 16:1, and 20:5n-3. These fatty acids were dominant also in the Prymnesiophyceae together with 22:6n-3. An exception was I. galbana, in which 18:1 was the major monounsaturated fatty add and 20:5n-3 was absent. The fatty acids of N. atomus and Tetraselmis sp. varied somewhat, but 16:0, 16:1, 18:1, 18:3n-3, and 20:5n-3 were most abundant. Gymnodinum sp. contained mainly 16:0, 18:4n-3, 20: 5n-3, and 22:6n-3. An increased level of nutrient limitation (probably phosphorus) resulted in a higher relative content of 16:0 and 18:1 and a lower relative content of 18:4n-3, 20:5n-3, and 22:6n-3. The nutrient limitation probably reduced the synthesis of n-3 polyunsaturated fatty acids.

The effect of different lipid compositions of live feed on the survival, growth rate and pigmentation success of turbot larvae, Scophthalmus maximus (L.), was investigated. Rotifers, Brachionus plicatilis, together with the algae Tetraselmis sp., were administered until day 12, and Artemia was fed until day 27. The experimentally treated live feeds were enriched with four formulated emulsions, resulting in a gradient in the relative contents of Ω3 HUFA (highly unsaturated fatty acids) and in DHA (docosahexaenoic acid, 22:6 Ω3)/EPA (eicosapentaenoic acid, 20:5 Ω3) ratios in both the rotifers and Artemia.

The drinking rate of water, and the ingestion- and assimilation rates of algae in yolk-sac larvae of Atlantic halibut were examined by use of3H-dextran and14C-labelled algae. The drinking rate throughout the yolk-sac stage was in the range of 7–160 nl per larva h−1, with a slight increase towards the end of the period. The ingestion rate of algae (Tetraselmis sp.) was very low before day 30 and showed a peak between day 43 and 48 at 5° C. The assimilation efficiency of the algae was in the range of 1-5%. The clearance rate of algae was 100–1000 times higher than the drinking rate, and was correlated to the distance between the gill arches. Bigger cells were more effectively retained than smaller ones. The results suggested that the larvae acted as filter feeders from day 30 up to the point when they are expected to start feeding on bigger prey.

The effect which microalgal addition and rotifer enrichment with algae had on survival, growth rate and fry viability during first-feeding were examined for turbot (Scophthalmus maximus L.). Addition of the microalgae Isochrysis galbana or Tetraselmis sp. together with rotifers Brachionus plicatilis, grown on yeast and oil emulsion, and Artemia greatly improved rearing success, whereas short-term enrichment of the rotifers with Tetraselmis sp. gave only improved viability. The algae modified the relative fatty acid composition of rotifers. Rotifers with I. galbana exhibited and increased level of 22:6n-3 and a lower level of 20:5n-3, whereas the opposite change occurred when Tetraselmis sp. was used. Addition of either of the two algal species at 1 mg cl−1 to the larval tanks resulted in a constant lipid level and high egg ratio of the rotifers, and thereby also high individual biomass content. Without algal addition, the lipid content of the rotifers decreased by 20% day−1 in the early phase, and their egg ratio became close to zero. In addition to improved nutritional conditions of the larvae, some other factor, e.g. some trigger mechanism or changed microbial or light conditions, also can operate to explain the increased early appetite of larvae with microalgae added and must be taken into consideration.

The larval stage is regarded to be the main bottleneck of halibut production. Halibut eggs were obtained from captive broodstock both by stripping and by natural spawning. Artificial photoperiods were used to increase the total spawning season. Yolk sac larvae are presently produced either in small stagnant units or in large flow through systems. A major consideration is to avoid stress of the larvae, caused by mechanical disturbances of the larvae and by high bacterial load or high ammonia levels in the water. The experiments showed that halibut larvae began to ingest algae earlier than rotifers Brachionus sp. Supplementation of algae to first feeding tanks resulted in enhanced survival and growth rate of the larvae. Both rotifers and Artemia can be enriched with very high levels of highly unsaturated n-3 fatty acids (n-3 HUFA). Enriched live feed, containing high levels of n-3 HUFA and total lipids, enhanced both survival and growth of the halibut larvae. The highest growth rates were obtained with wild zooplankton and addition of algae, but enriched cultivated feed combined with algae resulted in growth of the same magnitude as with wild zooplankton.

The effects of fish elimination in the eutrophic Lake Haugatjern, Sør-Trøndelag County, Norway, were studied during 1979–84. Total elimination of the fish populations by rotenone in late 1980 resulted in a fourfold decrease in the algal biomass. The species composition changed from a dominance by the large-sized Anabaena flos-aquae and Staurastrum luetkemuelleri to smaller, fast-growing species and gelatinous green algae. The total zooplankton biomass remained at the same level in all years, but while the rotifers almost disappeared after the rotenone treatment, the daphnids increased their share of the biomass from 49–63% during 1979–80 to 74–90% during 1982–84. The mean individual size of the adult daphnids increased in the same period from 1.3 to 1.8 mm. A 30% drop in the total phosphorus concentration in the lake after the biomanipulation was explained by increased sedimentation of zooplankton and reduced phosphorus release from the epilimnetic sediments because of the lowered pH. The fish elimination also resulted in a lower yield of biomass per unit of phosphorus in the lake.

In Artemia franciscana, enriched for 24 h with emulsified lipids, the fatty acid DHA (22:6w3) is unstable. In rotifers the level of DHA can be stabilised by adding algae rich in DHA to the fish tanks. To test this method for A. franciscana,enriched A. franciscana was incubated with the alga Isochrysis galbana, which is rich in lipids and DHA, at six concentrations ranging from 1 to 20 mg C 1−1 at 13–14 °C. After lipid enrichment, A. franciscana contained 15% lipids of which 79% (116 mg g DW−1) were fatty acids. Of the fatty acids, 17% was DHA (19.8 mg g DW−1). After 72 hours incubation with I. galbana, the level of DHA decreased to 1.6–3.2 mg g DW−1, which was a reduction by 84–92%. This was independent of the algal concentration. The amount of total fatty acids decreased to 53–73 mg g DW−1, a reduction by 37–54%, independent of the algal concentration. The rates of loss of the fatty acids, and especially DHA, was greater during the first 24 h at the highest algal concentrations (8, 10 and 20 mg C 1−1). Enriched A. franciscana should therefore not be incubated with high algal concentrations (>6 mg C 1−1) during first feeding of marine fish larvae.

Size selective feeding by Brachionus plicatilis was investigated with algae and bacteria (0.3–3.5 μm) and mono-disperse latex beads (0.3–3.0 μm) in short term feeding experiments.

Interspecific competition for phosphorus between the blue-green alga Anabaena flos-aquae and the green alga Staurastrum luetkemuelleri was studied in enclosure experiments. Both algal populations increased in number upon introduction of fish, but addition of herbivorous zooplankton led to an increase solely in the Staurastrum population. This could not be attributed to grazing by zooplankton, chemical or physical conditions (light, pH, inorganic carbon), or the total supply or rate of supply of phosphorus. The observed differences were apparently due to the patchy release of phosphorus by the fish, and the more homogenous release of this nutrient by the zooplankton. This would allow the Anabaena species, which was capable of rapid uptake of temporary pulses of phosphate, to coexist with Staurastrum in the enclosures with fish.

The flux of phosphorus in a zooplankton-phytoplankton system is described by a simple com- partment model, with differential equations for grazing, growth and reproduction of animals, and release and recycling of P. The mean release rates from zooplankton for a given period are derived for cases restricted to animals feeding on algae at densities abcve the incipient limiting food concentration. The model is used to analyze short term incubat .on experiments with Daphnia pulex feeding on Scenedesmus sp. and Daphnia longispina fcediiig on Rhodomonas lacustris in dim light. When food organisms were saturated with P, total releitse rates were in the range 1. l- 1.5 pg P (mg dry wt-' h-l in accordance with a maximal excretio 1 rate of 0.8 pg P (mg dry wt)-l h-l predicted by the model of Peters. Phosphorus-starved phytoplankton, containing less P, gave much lower release rates, about 0.05 fig P (mg dry wt)-* h-l, only lo'% of the excretion rate predicted - .- - by the model of Peters.

The phosphorus status [total phosphorus (P), surplus P, and alkaline phosphatase activity (APA)] of phytoplankton and bacteria were followed in a eutrophic Norwegian lake. On average 36% (range 20-60) of the particulate P and 57% (range 10-l 00) of the surplus P were bound by bacteria. The P : C ratio of phytoplankton and bacteria varied between 2.5 and 55 (median 5.1) and 34 and 360 (median 89) pg P (mg C))‘, indicating that bacteria have a substantially higher P requirement than do phytoplankton. The inverse relation found between surplus P : C and APA : C ratios during a Microcystis aeruginosa bloom and the positive relation between P: C ratio and growth rate of the cryptophytedominated community in summer indicate that the phytoplankton were P limited during most of the ice-free period. This is supported by the fact that the data from the cryptophyte-dominated community could be described by the Droop model for nutrient-limited growth. For the bacteria a positive relation was found between growth rate and cellular P (i.e. the P : C and the surplus P : C ratios), as predicted by the Droop model. Although the bacteria obviously were P subsaturated, it cannot be concluded that they were P limited since C may have been supplied at an even lower rate. Because of their high P requirements the bacteria acted as consumers of inorganic P and their net consumption of P was four times higher than that of the phytoplankton.

The filamentous blue-green alga Nostoc calcicola Geitler, strain 79WA01, showed a peak production of ∼70% of its biomass as a mixture of exocellular proteoglycan complexes, containing 3–30% of a polypeptide moiety and >70% of a complex glycuronoglycan. The former contained high proportions of Asp, Glu, Arg, and amido-NH 3 , in addition to ∼35% of “hydrophobic” amino-acids. The latter varied in composition in different fractions: GalA (2.5–10.3%), GlcA (4.7–11.5%), Glc (11.7–39.0%), Xyl (5.7–17.9%), Man (2.7–9.5%), Gal (5.7–9.5%), Fuc (1.5–11.1%), Ara (1.9–4.3%), and Rha (1.4–4.4%). None of the fractions showed a stoichiometric ratio of sugar residues. Palmelloid cells of three unicellular green soil-algae of the genus Chlamydomonas yielded ∼70% of their dry weight as capsular mucilage. About 50% of the sodium salt of this material was soluble in water, and contained 3–12% of polypeptide and 88–97% of glycuronoglycan (GlcA:Glc:Xyl = 1:1:3 for C. humicola Lucksch, and GlcA:Gal = 1:2 for C. peterfii Gerloff and C. sajao Lewin). These categorical differences in sugar composition, together with narrow composition distributions, suggested regular structures for the glycuronoglycans. The remainder of the mucilages contained essentially the same glycuronoglycan chains, but a higher proportion of polypeptide. These materials did not form true solutions in water, but dispersed as microgel particles.

In two enclosure experiments, Daphnia pulex ingested cryptophytes, bacteria, and probably detritus particles. The specific clearance rate of the zooplankton increased when the concentration of food decreased. The P : C ratio of the food also increased. More than 92% of the particulate phosphorus was located in the living cells (algae and bacteria); the detritus was practically phosphorus-free. The specific release rate of phosphorus estimated for the daphnids by use of the recycling model increased as the P : C ratio of the food increased and became zero at a critical low P : C ratio, Q. of 6-8 pg P mg-’ C. At this concentration, all the ingested phosphorus is needed for growth and reproduction, and no release of the element can be expected. This indicates that Daphnia may experience P limitation in nature, since the P : C ratio of P-starved algae and detritus may be considerably ~6-8 pg P mg-l C. The use of mass balance also appears to be suitable for modeling phosphorus and carbon fluxes through zooplankton in pelagic ecosystems.

The efficiency of carbon assimilation in Daphnia magna was estimated as a function of the 14C-labeling time of the food alga Scenedesmus acutus. The assimilation efficiency of 14C was up to 24% higher at low labeling times (≤ 1.4 generations) (P < 0.05) than at high labeling times (≥ 4.2 generations). In uniformly labeled cells the assimilation efficiency in D. magna was 71 ± 2% SE. Variation in the assimilation efficiency with labeling time is associated with change in the relative distribution of 14C among the different cell compartments in the food alga. Uniform labeling of algae is achieved effectively via exponential growth over several generations in batch cultures from sparse inocula.

A detailed study of the life history of the rotifer Brachionus plicatilis was done at 20 °C, 20 ppt salinity, and 90 mg C 1−1 food concentration. Rotifers were grown individually in culture plate wells (150 µl culture volume) and fed Isochrysis galbana Tahiti, Tetraselmis sp., Nannochloris atomus, or a l : 1 mixture (weight) of two of the algae. Observations were made every 2–8 hr and rotifers were sized and transferred to new food daily. A total of 19 different parameters were compared. Rotifers fed Isochrysis averaged 21 offspring per female, a 6.7 day reproductive period, a lifespan of 10.5 days and a mean length of 234 µm. After Isochrysis, the foods giving the highest growth, survival, and reproduction in decreasing order were Isochrysis + Nannochloris, Nannochloris, Isochrysis + Tetraselmis, Tetraselmis + Nannochloris, and Tetraselmis. Although the small volume culture system used in this study seems appropriate for studying life history of B. plicatilis, the results cannot always be directly applied to larger cultures.