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Climate change and eutrophication are among the main stressors of shallow freshwater ecosystems, and their effects on phytoplankton community structure and primary production have been studied extensively. However, their combined effects on the algal production of polyunsaturated fatty acids (PUFA), specifically, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) are currently unresolved. Moreover, the proximate reasons for changes in phytoplankton EPA and DHA concentrations are unclear, i.e., the relative importance of ecological (changes in the community composition) vs. ecophysiological (within taxa changes in EPA and DHA levels) factors. We investigated the responses of phytoplankton EPA and DHA concentrations to warming (IPCC climate scenario) and nutrient additions in mesocosms which had been run continuously at varying temperature and nutrient levels for 15 years prior to this study. Nutrient treatment had a significant effect on phytoplankton EPA and DHA concentrations and about 59 % of the variation in EPA and DHA concentrations could be explained by changes in the phytoplankton community structure. Increased biomass of diatoms corresponded with high EPA and DHA concentrations, while cyanobacteria/chlorophyte dominated mesocosm had low EPA and DHA concentrations. Warming had only a marginal effect on the EPA and DHA concentrations in these mesocosms. However, a significant interaction was observed with warming and N:P ratio. Our findings indicate that direct nutrient/temperature effects on algal physiology and PUFA metabolism were negligible and the changes in EPA and DHA concentrations were mostly related to the phytoplankton community structure and biomass. These results also imply that in shallow temperate lakes eutrophication, leading to increased dominance of cyanobacteria, will probably be a greater threat to phytoplankton EPA and DHA production than warming. EPA and DHA are nutritionally important for upper trophic level consumers and decreased production may impair secondary production., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 The Authors. Published by Elsevier B.V. All rights reserved.)

Lake size influences various hydrological parameters, such as water retention time, circulation patterns and thermal stratification that can consequently affect the plankton community composition, benthic-pelagic coupling and the function of aquatic food webs. Although the socio-economical (particularly commercial fisheries) and ecological importance of large lakes has been widely acknowledged, little is known about the availability and trophic transfer of polyunsaturated fatty (PUFA) in large lakes. The objective of this study was to investigate trophic trajectories of PUFA in the pelagic food web (seston, zooplankton, and planktivorous fish) of six large boreal lakes in the Finnish Lake District. Docosahexaenoic acid (DHA), eicosapentaenoic acid (EPA) and α-linolenic acid (ALA) were the most abundant PUFA in pelagic organisms, particularly in the zooplanktivorous fish. Our results show that PUFA from the n-3 family (PUFAn-3), often associated with marine food webs, are also abundant in large lakes. The proportion of DHA increased from ~4±3% in seston to ~32±6% in vendace (Coregonus albula) and smelt (Osmerus eperlanus), whereas ALA showed the opposite trophic transfer pattern with the highest values observed in seston (~11±2%) and the lowest in the opossum shrimp (Mysis relicta) and fish (~2±1%). The dominance of diatoms and cryptophytes at the base of the food web in the study lakes accounted for the high amount of PUFAn-3 in the planktonic consumers. Furthermore, the abundance of copepods in the large lakes explains the effective transfer of DHA to planktivorous fish. The plankton community composition in these lakes supports a fishery resource (vendace) that is very high nutritional quality (in terms of EPA and DHA contents) to humans., (Copyright © 2015 Elsevier B.V. All rights reserved.)

Stable isotope mixing models in aquatic ecology require δ13C values for food web end members such as phytoplankton and bacteria, however it is rarely possible to measure these directly. Hence there is a critical need for improved methods for estimating the δ13C ratios of phytoplankton, bacteria and terrestrial detritus from within mixed seston. We determined the δ13C values of lipids, phospholipids and biomarker fatty acids and used these to calculate isotopic differences compared to the whole-cell δ13C values for eight phytoplankton classes, five bacterial taxa, and three types of terrestrial organic matter (two trees and one grass). The lipid content was higher amongst the phytoplankton (9.5±4.0%) than bacteria (7.3±0.8%) or terrestrial matter (3.9±1.7%). Our measurements revealed that the δ13C values of lipids followed phylogenetic classification among phytoplankton (78.2% of variance was explained by class), bacteria and terrestrial matter, and there was a strong correlation between the δ13C values of total lipids, phospholipids and individual fatty acids. Amongst the phytoplankton, the isotopic difference between biomarker fatty acids and bulk biomass averaged -10.7±1.1‰ for Chlorophyceae and Cyanophyceae, and -6.1±1.7‰ for Cryptophyceae, Chrysophyceae and Diatomophyceae. For heterotrophic bacteria and for type I and type II methane-oxidizing bacteria our results showed a -1.3±1.3‰, -8.0±4.4‰, and -3.4±1.4‰ δ13C difference, respectively, between biomarker fatty acids and bulk biomass. For terrestrial matter the isotopic difference averaged -6.6±1.2‰. Based on these results, the δ13C values of total lipids and biomarker fatty acids can be used to determine the δ13C values of bulk phytoplankton, bacteria or terrestrial matter with ± 1.4‰ uncertainty (i.e., the pooled SD of the isotopic difference for all samples). We conclude that when compound-specific stable isotope analyses become more widely available, the determination of δ13C values for selected biomarker fatty acids coupled with established isotopic differences, offers a promising way to determine taxa-specific bulk δ13C values for the phytoplankton, bacteria, and terrestrial detritus embedded within mixed seston.

Terrestrial organic matter inputs have long been thought to play an important role in aquatic food web dynamics. Results from recent whole lake (13)C addition experiments suggest terrestrial particulate organic carbon (t-POC) inputs account for a disproportionate portion of zooplankton production. For example, several studies concluded that although t-POC only represented approximately 20% of the flux of particulate carbon available to herbivorous zooplankton, this food source accounted for approximately 50% of the C incorporated by zooplankton. We tested the direct dietary impact of t-POC (from the leaves of riparian vegetation) and various phytoplankton on Daphnia magna somatic growth, reproduction, growth efficiency, and lipid composition. By itself, t-POC was a very poor quality resource compared to cryptophytes, diatoms, and chlorophytes, but t-POC had similar food quality compared to cyanobacteria. Small additions of high quality Cryptomonas ozolinii to t-POC-dominated diets greatly increased Daphnia growth and reproduction. When offered alone, t-POC resulted in a Daphnia growth efficiency of 5 +/- 1%, whereas 100% Cryptomonas and Scenedesmus obliquus diets resulted in growth efficiencies of 46 +/- 8% (+/- SD) and 36 +/- 3%, respectively. When offered in a 50:50 mixed diet with Cryptomonas or Scenedesmus, the t-POC fraction resulted in a partial growth efficiency of 22 +/- 9% and 15 +/- 6%, respectively. Daphnia that obtained 80% of their available food from t-POC assimilated 84% of their fatty acids from the phytoplankton component of their diet. Overall, our results suggest Daphnia selectively allocate phytoplankton-derived POC and lipids to enhance somatic growth and reproduction, while t-POC makes a minor contribution to zooplankton production.

Temporal variability in lake phytoplankton is controlled largely by a complex interplay between hydrodynamic and chemical factors, and food web interactions. We explored mechanisms underlying phytoplankton interannual variability in Lake Washington (USA), using a 25-yr time series of water quality data (1975-1999). Time-series analysis and PCA were used to decompose chlorophyll data into modes of variability. We found that phytoplankton dynamics in Lake Washington were characterized by four seasonal modes, each of which was associated with different ecological processes. The first mode coincided with the period when the system was light limited (January-March) and phytoplankton patterns were driven by the amount of available solar radiation. The second mode (April-June) coincided with the peak of the spring bloom and the subsequent decline of phytoplankton biomass, and was largely controlled by total phosphorus levels and grazing pressure from cladoceran zooplankton. Evidence of co-dependence and tight relationship between phytoplankton and cladoceran dynamics were also found from July to October when a large portion of the phosphorus supply in the mixed layer was provided by zooplankton excretion. The fourth mode (November-December) was associated with the transition to thermal and chemical homogeneity and the winter phytoplankton minima (2-2.5 microg/l). Finally, we examined the effects of meteorological forcing and large-scale oceanic climate fluctuations (ENSO and PDO) on phytoplankton dynamics and assessed the significance of their role on the interannual variability in the lake.

Abstract: The deuterium ratio (2H/1H) in tissue is often used to estimate terrestrial subsidies to aquatic consumers because of strongly differentiated values between terrestrial and aquatic primary producers. However, quantitative deuterium‐based analyses of terrestrial resource assimilation are highly dependent on several poorly defined assumptions. We explored the sensitivity of these estimates to assumptions regarding environmental water contributions to consumer deuterium content (ω) and algal photosynthetic hydrogen discrimination (εH). We also tested whether 13C/12C and 2H/1H‐based estimates of terrestrial resource assimilation give similar outcomes. The average of the 12 experiments that have directly estimated proportional contributions of environmental water to consumer tissue 2H/1H was 0.27 ± 0.11 (mean ± SD), with similar values for invertebrates and fish. Conversely, of the 28 field studies that have used 2H/1H to characterize aquatic food webs, all but one assume a value that is less than our current best estimate, usually substantially less. A reanalysis of the raw data from four recent case studies indicates the calculated terrestrial contribution to aquatic consumers is extremely sensitive to this assumption. When the authors’ original assumptions were used (i.e., ω = 0.16 ± 0.05), the estimated proportional contribution of terrestrial resources to aquatic consumers (θT) averaged 29 ± 17%, and when ω = 0.27 was used the average estimated assimilation of allochthonous resources was ≈0.00. A compilation of published photosynthetic hydrogen discrimination values for microalgae averaged εH = −150 ± 27‰ (SD, n = 99), and a sensitivity analysis showed the outcomes of these calculations were also strongly influenced by uncertainty in εH. There was no statistical association between 13C/12C and 2H/1H‐based estimates of terrestrial subsidies (r = −0.12, n = 274). This analysis indicates that the assumptions in deuterium‐based estimates of terrestrial resource assimilation are highly influential but poorly constrained; therefore, the impact of these assumptions on calculated outputs must be carefully assessed and thoroughly reported. Due to the highly uncertain assumptions inherent in deuterium‐based analyses, we urge much more caution when using this approach to estimate terrestrial subsidies to consumers in aquatic ecosystems. [ABSTRACT FROM AUTHOR]

Understanding which factors affect the algal bioavailability of dissolved organic nitrogen (DON) compounds in natural surface waters is important for our understanding of nutrient biogeochemistry and water quality management. We used nitrogen uptake kinetics and algal cell yield to characterize the algal bioavailability of 22 dissolved DON compounds that are commonly found in natural surface waters and wastewater treatment plant effluents, including urea, amino acids, amino sugars, nucleotides, pyrimidines, oraganonitriles, polyacrylamide, EDTA, caffeine, phenolic compounds and humic acids. Twelve of these compounds were highly bioavailable, including urea, dissolved free amino acids, bovine serum albumin, DNA, RNA, ATP, AMP, acetonitrile and caffeine. Four compounds had intermediate bioavailability including two humic acids (Elliott Soil and Pahokee Peat), glycylglycine, RNA and uracil. The remaining six compounds were classified as recalcitrant, i.e., EDTA, 2,3-Dinitrophenol, aminobenzoic acid, polyacrylamide and Aldrich humic acid. For many of the compounds tested, the algal cell yield was only 60–80% of expected relative to DON uptake. These results help explain why some DON compounds are more likely to persist in natural systems, and why the DON pool is often recalcitrant in surface waters. [ABSTRACT FROM AUTHOR]

The zooplankton is a key link in the transfer of energy from primary producers up through aquatic food webs. Previous efforts to quantify the importance of basal resources to aquatic consumers have used stable isotopes ( SI) and simple ternary models, including only 'bulk' phytoplankton, bacteria or terrestrial particulate organic matter (t- POM)., We used a novel Bayesian mixing model based on fatty acids ( FA) to quantify the dietary assimilation of seven basal resources, including five phytoplankton groups, pelagic bacteria and t- POM, to Cladocera in large boreal lakes in Finland. To account for trophic enrichment of FA from the diet to consumers, we parameterised the model with a resource library, from many feeding trials, consisting of Daphnia magna fed 22 diverse basal taxa., The results of the feeding trials show that the distinctive FA profiles of algal groups are transferred to consumers. Moreover, the large number of FA variables ( n = 22) used in the model avoids the limitations of underdetermined mixing problems, common to SI modelling, in cases when the number of resources outnumbers the tracer variables., We show that cladocerans were generally supported by phytoplankton (86-94%), with little use of t- POM (1-9%) and bacteria (1-3%). Cladocerans used primarily high-quality phytoplankton (cryptophytes, diatoms and dinoflagellates) in both summer (51 ± 22%) and autumn (79 ± 12%), and the relative importance of medium-quality resources (cyanobacteria, chlorophytes and chrysophytes) declined from 37 ± 23% in the summer to 8 ± 2% in the autumn., High-quality resources, rich in essential biochemical compounds, are critical in fuelling food webs in large lakes, even those with high concentrations of allochthonous organic matter. [ABSTRACT FROM AUTHOR]

1. Analyses of zooplankton fatty acid (FA) composition in laboratory experiments and samples collected from lakes in New Zealand spanning a wide gradient of productivity were used to assess the extent to which FAs might infer their diet. We used the cladocerans, Daphnia and Ceriodaphnia, and the calanoid copepod, Boeckella, as test organisms, and monocultures of cryptophytes, chlorophytes and cyanobacteria as food. Based on reproductive success, cryptophytes were the highest food quality, chlorophytes were intermediate and cyanobacteria the poorest. 2. Several FA groups were highly correlated between zooplankton and their diets. They were monounsaturated fatty acids (MUFAs), and ω3 and ω6 polyunsaturated fatty acids (PUFAs) for cladocerans, and saturated fatty acids (SAFAs) and ω3 PUFAs for copepods. Several FAs varied significantly less in the zooplankton than in their monoculture diets, e.g. MUFAs in Daphnia, and ω3 and ω6 PUFAs in Ceriodaphnia, despite clear dietary dependency for these FAs. 3. Zooplankton collected from lakes in New Zealand had more eicosapentaenoic acid (EPA) ( Daphnia), more highly unsaturated ω3 and ω6 FAs (C, C; Daphnia, Ceriodaphnia, Boeckella) and less ω3 C PUFAs ( Daphnia, Ceriodaphnia, Boeckella) and ω6 C PUFAs ( Daphnia, Ceriodaphnia) than measured in the same species reared on phytoplankton in the laboratory. 4. Analyses of FA composition of seston and freshwater zooplankton globally showed that, in general, zooplankton had a significantly higher proportion of arachidonic acid and EPA than seston, and copepods also had a higher percentage of docosahexaenoic acid than seston. 5. These results suggest that zooplankton selectively incorporate the most physiologically important FAs. This could be a consequence of preferential assimilation, selective feeding on more nutritious cells or locating and feeding within higher food quality food patches. [ABSTRACT FROM AUTHOR]

We developed a mechanistic model of nutrient, phytoplankton, zooplankton and fish interactions to test the effects of phytoplankton food quality for herbivorous zooplankton on planktonic food web processes. When phytoplankton food quality is high strong trophic cascades suppress phytoplankton biomass, the zooplankton can withstand intense zooplanktivory, and energy is efficiently transferred through the food web sustaining higher trophic level production. Low food quality results in trophic decoupling at the plant-animal interface, with phytoplankton biomass determined primarily by nutrient availability, zooplankton easily eliminated by fish predation, and poor energy transfer through the food web. At a given nutrient availability, food quality and zooplanktivory interact to determine zooplankton biomass which in turn determines algal biomass. High food quality resulted in intense zooplankton grazing which favored fast-growing phytoplankton taxa, whereas fish predation favored slow-growing phytoplankton. These results suggest algal food quality for herbivorous zooplankton can strongly influence the nature of aquatic food web dynamics, and can have profound effects on water quality and fisheries production. [ABSTRACT FROM AUTHOR]

Abstract: Complex environmental models are often criticized as being difficult to analyze and poorly identifiable due to their nonlinearities and/or their large number of parameters relative to data availability. Others consider overparameterized models to be useful, especially for predicting system dynamics beyond the conditions for which the model was calibrated. In this paper, we present a complex eutrophication model that has been developed to simulate plankton dynamics in Lake Washington, USA. Because this model is to be used for testing alternative managerial schemes, the inclusion of multiple elemental cycles (org. C, N, P, Si, O) and multiple functional phytoplankton (diatoms, green algae and cyanobacteria) and zooplankton (copepods and cladocerans) groups was deemed necessary. The model also takes into account recent advances in stoichiometric nutrient recycling theory, and the zooplankton grazing term was reformulated to include algal food quality effects on zooplankton assimilation efficiency. The physical structure of the model is simple and consists of two spatial compartments representing the lake epilimnion and hypolimnion. Global sensitivity analysis showed background light attenuation, the maximum phytoplankton growth rate, the phytoplankton basal metabolic rate, the zooplankton maximum grazing rate and the grazing half saturation constant have the greatest impact on model behavior. Phytoplankton phosphorus stoichiometry (maximum and minimum internal concentrations, maximum uptake rate) interacts with these parameters and determines the plankton dynamics (epilimnetic and hypolimnetic phytoplankton biomass, proportion of cyanobacteria and total zooplankton biomass). Sensitivity analysis of the model forcing functions indicated the importance of both external and internal loading for simulating epilimnetic and hypolimnetic plankton dynamics. These results will be used to calibrate the model, to reproduce present chemical and biological properties of Lake Washington and to test this lake''s potential response to different external nutrient loading scenarios. [Copyright &y& Elsevier]

Abstract: We developed a complex eutrophication model to simulate the current chemical and biological properties of Lake Washington (USA). The model reproduces the key epilimnetic and hypolimnetic temporal patterns of the system and results in a good fit between simulated and observed monthly values. The relative error of model estimates was below 20% for most of the water quality parameters (phytoplankton, phosphate, total phosphorus, total nitrogen, dissolved oxygen). Discrepancies between simulated and observed ammonium levels were mainly due to the explicitly modeled egestion of excess nitrogen during zooplankton feeding. This indicates that the relation between secondary production and nutrient recycling has significant effects on the fractionation of the major elements (C, N and P) and regulates their distribution between the particulate/dissolved and inorganic/organic pools. The model was forced by 1962 nutrient loadings, when the lake received large quantities of treated wastewater treatment effluent, and accurately predicted the phytoplankton community responses (phytoplankton biomass and cyanobacteria dominance) and the nitrogen and phosphorus annual cycles for these conditions. We used Monte Carlo simulations to reproduce the meteorological forcing (air temperature, solar radiation, precipitation and subsequent river inflows) that in large part regulates phytoplankton interannual variability for the last 25 years in the lake. We found three seasonal components (modes of variability). The first component (January, May, November, December) is associated with the conditions that determine the abundance of the herbivorous cladocerans; the second component (June–September) coincides with the summer-stratified period, and the third component (February–April) is associated with the initiation and peak of the spring bloom. Finally, an illustrative application of two scenarios of nutrient loading increase at 25% of the 1962 levels indicated that both phytoplankton and cyanobacteria growth are likely to be stimulated. The three seasonal components still characterize phytoplankton dynamics, but changes in the relationships between summer phytoplankton/cyanobacteria biomass and total phosphorus/phosphate concentrations indicate the likelihood of structural shifts towards relaxation of the present phosphorus-limiting conditions and promotion of cyanobacteria dominance. Integration of the present eutrophication model with a hydrodynamic model with enhanced vertical resolution will allow more realistic predictions. [Copyright &y& Elsevier]

Determining the factors that control food web interactions is a key issue in ecology. The empirical relationship between nutrient loading (total phosphorus) and phytoplankton standing stock (chlorophyll a) in lakes was described about 30 years ago and is central for managing surface water quality. The efficiency with which biomass and energy are transferred through the food web and sustain the production of higher trophic levels (such as fish) declines with nutrient loading and system productivity, but the underlying mechanisms are poorly understood. Here we show that in seston (fine particles in water) during summer, specific ?3-polyunsaturated fatty acids (?3-PUFAs), which are important for zooplankton, are significantly correlated to the trophic status of the lake. The ?3-PUFAs octadecatetraenoic acid, eicosapentaenoic acid (EPA) and docosahexaenoic acid, but not a-linolenic acid, decrease on a double-logarithmic scale with increasing total phosphorus. By combining the empirical relationship between EPA-to-carbon content and total phosphorus with functional models relating EPA-to-carbon content to the growth and egg production of daphnids, we predict secondary production for this key consumer. Thus, the decreasing efficiency in energy transfer with increasing lake productivity can be explained by differences in ?3-PUFA-associated food quality at the plant-animal interface. [ABSTRACT FROM AUTHOR]

The impact of nutrient additions, zooplankton grazing and light intensity on phytoplankton net growth with depth and season was studied with five microcosm experiments in meso-oligotrophic, subalpine Castle Lake, California, during the period of summer stratification in June-September 1994. The incubations (4 day) were performed at 5 m intervals from the surface to the bottom using natural phytoplankton and zooplankton assemblages, with enrichments of phosphorus and nitrogen. The phytoplankton community was only limited by nutrients in the upper 5 m (epilimnion), as indicated by change in chlorophyll concentration. Nutrient enrichments had the greatest effect on the phytoplankton net growth in June and July. High light inhibited the phytoplankton net growth at the surface. Low light intensities limited phytoplankton at 20 m and below, and at the end of the growing season already around 10-15 m. A deep chlorophyll maximum in the hypolimnion in June-August was not limited by either light or nutrients. The results showed variation in grazers' impact on phytoplankton. These results suggest the importance of nutrient limitation only in the epilimnion with light inhibition at the surface, light limitation in the hypolimnion, and varying impact of zooplankton grazing in influencing the development of the phytoplankton in Castle Lake. [ABSTRACT FROM PUBLISHER]

1. Polyunsaturated fatty acids (PUFA) are almost exclusively synthesized by plants. Animals can convert from one form of PUFA to another through elongation and desaturation, but very few can synthesize PUFA de novo. PUFA play an important role in regulating cell membrane properties, serve as precursors for important animal hormones and are essential for animals. 2. In aquaculture studies, highly unsaturated fatty acids (HUFA), a subset of PUFA, have been found to be critical for maintaining high growth, survival and reproductive rates and high food conversion efficiencies for a wide variety of marine and freshwater organisms. 3. The plankton literature suggests high food-quality algae species are rich in HUFA and low food-quality algae are poor in HUFA. Adding semi-pure emulsions of HUFA to algae monocultures can markedly increase the growth rates of zooplankton feeding on these mixtures. 4. A study measuring zooplankton biomass accrual when feeding on natural phytoplankton found a strong correlation between phytoplankton HUFA (specifically eicosapentaenoic acid) content and herbivorous zooplankton production. 5. The aquatic ecology literature suggests that planktonic foodwebs with high HUFA content phytoplankton have high zooplankton to phytoplankton biomass ratios, while systems with low HUFA phytoplankton have low zooplankton biomass. Also, the seasonal succession of plankton in many temperate lakes follows patterns tied to phytoplankton HUFA content, with intense zooplankton grazing and 'clear-waterphases' characteristic of periods when the phytoplankton is dominated by HUFA-rich species. 6. Herbivorous zooplankton production is constrained by the zooplankton's ability to ingest and digest phytoplankton. It is becoming increasingly clear, however, that much of the phytoplankton which is assimilated may be nutritionally inadequate. HUFA may be key nutritional constituents of zooplankton diets, and may determine energetic efficiency across the plant-animal interface, secondary production and the strength of trophic coupling in aquatic pelagic foodwebs. [ABSTRACT FROM AUTHOR]