Zooplankton nutrition: recent progress and a reality check

Evidence suggests that marine and freshwater zooplankton generally experience food levels above subsistence values in terms of carbon. However, the quality of this food may be poor due to an insufficiency of other essential nutrients. In this review, we examine recent progress in three main areas of food quality research: (1) elemental (especially P) limitation, (2) digestion resistance, and (3) biochemical (especially fatty acids) limitation. We evaluate laboratory and field evidence in each of these areas, look at new evidence about the life history implications of the elemental limitation hypothesis, and suggest future avenues for research. From a rather large number of seemingly heterogeneous studies, a single consistent picture of food quality emerges: both P and essential fatty acids are predicted to be important dietary factors, but at different places and times. Nevertheless, despite an abundance of valuable laboratory studies, our knowledge of food quality limitation in the field is still poor.     

The role of food quality for zooplankton: remarks on the state-of-the-art, perspectives and priorities

1. This paper summarizes the salient features of the contributions to the workshop on The Role of Food Quality for Zooplankton. In this paper we attempt critically to evaluate our present knowledge in the light of new studies. 2. For the growth and reproduction of zooplankton, the existing literature considers two main limiting factors in the diet, i.e. phosphorus (homeostasis theory) and fatty acids. Nevertheless, interpretations and opinions regarding the importance of these two factors are the subject of controversy in the literature. No attempts have been made to link these two potentially limiting factors, let alone give a coherent view based on the mechanisms behind limitation. Aquaculture studies provide some direct evidence of the importance of the long-chained poly unsaturated fatty acids (PUFA) for zooplankton. The presence of PUFA in phytoplankton is reported to affect the growth rates of zooplankton significantly. 3. Field data on carbon and phosphorus indicate a greater constancy of the C : P ratios of zooplankton than of their food. Empirical data and modelling studies suggest that zooplankton, especially Daphnia spp., may maintain nutrient homeostasis by incorporating a greater proportion of the limiting nutrients ingested and releasing more of nutrients in excess supply. The need for conserving nutrients in short supply increases with the increase in growth rates. 4. Phosphorus certainly influences zooplankton food directly. Direct supplementation of the P-insufficient algal diet with PO₄-P alone discernibly improves the growth in daphnids. It is highly plausible that P limitation and fatty acid limitation are not mutually exclusive alternatives. The two, separately or in conjunction, can control growth of at least some lake zooplankters, especially daphnids. 5. Besides a shortage of nutrient (P), other environmental factors (irradiance, UV-radiation, temperature) can also adversely affect the zooplankton diet, including its digestibility and assimilation efficiency. 6. It is not yet clear if PUFA deficiency in the diet is in some way related to or caused by P deficiency. It is, however, now known that the EPA (eicosapentaenoic acid, 20 : 5ω3) content of certain algae is markedly reduced under P-limitation and that it differs significantly among the different taxonomic groups of phytoplankton. Diatoms and flagellates are generally considered as good-quality foods because of their high EPA content. On the contrary, cyanobacteria are low-quality food, having both low EPA and P content. 7. Recent experiments reveal that the relative importance of fatty acids for daphnids increases with a decreasing C : P ratio in the food, i.e. if P is no longer limiting, and vice versa. For daphnids, there is possibly a switch between P-limitation and PUFA limitation at intermediate C : P ratios. At higher C : P ratios, P is more important but at lower ratios PUFA are more crucial for growth and reproduction. 8. Lastly, the accumulating evidence for P limitation is stronger than that for fatty acid limitation.     

Highly unsaturated fatty acids in nature: what we know and what we need to learn

The supply and demand of omega‐3 highly unsaturated fatty acids (ω‐3 HUFA) in natural ecosystems may lead to resource limitation in a diverse array of animal taxa. Here, we review why food quality in terms of ω‐3 HUFAs is important, particularly for neural tissue, across a diversity of animal taxa ranging from invertebrate zooplankton to vertebrates (including humans). Our review is focused on ω‐3 HUFAs rather than other unsaturated fatty acids because these compounds are especially important biochemically, but scarce in nature. We discuss the dichotomy between ω‐3 HUFA availability between aquatic primary producers, which are often rich in these compounds, and terrestrial primary producers, which are contain little to none of them. We describe the use of fatty acids as qualitative and quantitative tracers for reconstructing animal diets in natural ecosystems. Next, we discuss both direct and indirect ecological implications of ω‐3 HUFA limitation at the individual, population, food web, and ecosystem scales, which include: changes in behavior, species composition, secondary production rates, trophic transfer efficiency and cross‐ecosystem subsidies. We finish by highlighting future research priorities including a need for more research on ω‐3 HUFAs in terrestrial systems, more research their importance for higher order consumers, and more research on the food web and ecosystem‐scale effects of ω‐3 HUFA limitation. Synthesis Mismatches between the supply of and demand for omega‐3 highly unsaturated fatty acids (ω‐3 HUFA) in natural ecosystems have the potential to result in resource limitation across a diverse array of ecosystems. We combined perspectives from ecology and nutritional science to develop a unified synthesis of ω‐3 HUFA ecology. We reviewed the importance of ω‐3 HUFAs for animals, the striking differences in ω‐3 HUFA availability at the base of terrestrial versus aquatic food webs, and the implications of ω‐3 HUFA limitation for food webs. We finished by highlighting research priorities in the field including more research on ω‐3 HUFAs in terrestrial systems, on higher order consumers, and at the food web and ecosystem‐scales.     

Atmospheric nitrogen deposition is associated with elevated phosphorus limitation of lake zooplankton

Here, we present data that for the first time suggests that the effects of atmospheric nitrogen (N) deposition on nutrient limitation extend into the food web. We used a novel and sensitive assay for an enzyme that is over‐expressed in animals growing under dietary phosphorus (P) deficiency (alkaline phosphatase activity, APA) to assess the nutritional status of major crustacean zooplankton taxa in lakes across a gradient of atmospheric N deposition in Norway. Lakes receiving high N deposition had suspended organic matter (seston) with significantly elevated carbon:P and N:P ratios, indicative of amplified phytoplankton P limitation. This P limitation appeared to be transferred up the food chain, as the cosmopolitan seston‐feeding zooplankton taxa Daphnia and Holopedium had significantly increased APA. These results indicate that N deposition can impair the efficiency of trophic interactions by accentuating stoichiometric food quality constraints in lake food webs.     

Essential fatty acids and phosphorus in seston from lakes with contrasting terrestrial dissolved organic carbon content

1. It is often assumed that lakes highly influenced by terrestrial organic matter (TOM) have low zooplankton food quality because of elemental and/or biochemical deficiencies of the major particulate organic carbon pools. We used the biochemical [polyunsaturated fatty acids, especially eicosapentaenoic acid (EPA) – 20:5ω3] and elemental (C : P ratio) composition of particulate matter (PM) as qualitative measures of potential zooplankton food in two categories of lakes of similar primary productivity, but with contrasting TOM influence (clear water versus humic lakes). 2. C : P ratios (atomic ratio) in PM were similar between lake categories and were above 400. The concentration (μg L⁻¹) and relative content (μg mg C⁻¹) of EPA, as well as the particulate organic carbon concentration, were higher in the humic lakes than in the clear‐water lakes. 3. Our results show high fatty acid quality of PM in the humic lakes. The differences in the biochemical quality of the potential zooplankton food between lake categories can be attributed to the differences in their phytoplankton communities. 4. High biochemical quality of the food can result in high efficiency of energy transfer in the food chain and stimulate production at higher trophic levels, assuming that zooplankton are able to ingest and digest the resource available.     

Vallisneria natans detritus supports Daphnia magna somatic growth and reproduction under addition of periphyton

Submerged macrophytes are regarded as being hardly assimilated by zooplankton for their lack of essential nutrients such as polyunsaturated fatty acids (PUFAs) thus serve as poor quality food, contrary to field stable isotopic investigations with observed macrophyte carbon contributions to zooplankton. However, periphyton growing on them produces the PUFAs and is thus a nutrient supplement. We hypothesize that with this supplement, zooplankton can be supported by macrophyte carbon. To test this hypothesis, we fed zooplankton with (1) ¹³C enriched Vallisneria natans detritus, (2) periphyton and (3) a mix of the two. We compared growth and reproduction of zooplankton under these three food treatments and calculated zooplankton assimilation of macrophyte carbon when fed a mixed diet, using a stable isotope-mixing model. The fatty acid profile of the two carbon resources was also analyzed. Our results demonstrate that Daphnia magna can grow and reproduce well, and use V. natans carbon when a supplement of periphyton is available.

     

A comparison of the trophic transfer of fatty acids in freshwater plankton by cladocerans and calanoid copepods

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 (C₂₀, C₂₂; Daphnia, Ceriodaphnia, Boeckella) and less ω3 C₁₈ PUFAs (Daphnia, Ceriodaphnia, Boeckella) and ω6 C₁₈ 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.     

Survival,growth and reproduction of Daphnia galeata feeding on single and mixed Pseudomonas and Rhodomonas diets

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. 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.     

UV radiation and phosphorus interact to influence the biochemical composition of phytoplankton

1. Numerous laboratory studies have shown that food quality is suboptimal for zooplankton growth. However, little is known about how food quality is affected by the interaction of potential global change factors in natural conditions. Using field enclosures in a high altitude Spanish lake, seston was exposed to increasing phosphorus (P) concentrations in the absence and presence of UV radiation (UVR) to test the hypothesis that interactions between these factors affected the biochemical and stoichiometric composition of seston in ways not easily predicted from studies of single factors. 2. Phosphorus enrichment increased the content of total fatty acids (TFA), ω3‐polyunsaturated fatty acids (ω3‐PUFA) and chlorophyll‐a : carbon (Chl‐a : C) and C : N ratios in seston. The pronounced increase in ω3‐PUFA was largely explained by the enhancement of 18:3n‐3 (α‐linolenic acid). In contrast, P‐enrichment lowered the content of highly unsaturated fatty acids (HUFA), the HUFA : PUFA ratio and, at high P loads, seston C : P ratio. Although phytoplankton assemblages dominated by Chlorophytes were not rich in HUFA, seston in the control had substantially higher 20:4n‐6 (arachidonic acid, ARA) content (79% of HUFA) than did P‐enriched enclosures. 3. The UVR increased the content of ω3‐PUFA and TFA in seston at the two ends of the trophic gradient generated at ambient and high concentrations of P, but decreased seston C : P and HUFA at all points on this gradient. ARA was not detected in the presence of UVR. 4. The interaction between P and UVR was significant for seston HUFA and C : P ratios, indicating that the effect of UVR in reducing HUFA (decreased food quality) and C : P ratios (enhanced food quality) was most pronounced at the low nutrient concentrations characteristic of oligotrophic conditions and disappeared as P increased. Therefore, any future increase in UVR fluxes will probably affect most strongly the food quality of algae inhabiting oligotrophic pristine waters although, at least in the Mediterranean region, these effects could be offset by greater deposition of P from the atmosphere.     

Seston food quality and Daphnia production efficiencies in an oligo-mesotrophic Subalpine Lake

Because of major biochemical imbalances between plants and animals, ecological efficiency at this interface may have a major impact on overall energy flow in ecosystems. In order to study relationships between seston food quality and energy transfer between primary producers and herbivores, we conducted five microcosm experiments in Castle Lake, California, USA during the summer of 1996. We simultaneously performed life table experiments to determine the effects of highly unsaturated fatty acids (HUFA) on Daphnia rosea growth, reproduction and survival. The results of these experiments suggest strong energy limitation of D. rosea growth in Castle lake during the study. D. rosea production was coupled with primary production in Castle Lake and in the microcosm experiments. D. rosea production efficiencies, i.e., the ratios of D. rosea productivity to primary productivity, decreased towards the end of the summer. A food quality index based on phytoplankton species composition and seston carbon to phosphorus (C:P) ratio were good predictors of D. rosea production efficiencies. The predicted D. rosea production pattern based on phytoplankton composition and primary productivity matched the zooplankton biomass dynamics in Castle Lake during 1991. Life table experiments showed HUFA effects on D. rosea population growth rates, reproduction and survival in support of the HUFA limitation hypothesis.     

Simulated eutrophication and browning alters zooplankton nutritional quality and determines juvenile fish growth and survival

The first few months of life is the most vulnerable period for fish and their optimal hatching time with zooplankton prey is favored by natural selection. Traditionally, however, prey abundance (i.e., zooplankton density) has been considered important, whereas prey nutritional composition has been largely neglected in natural settings. High‐quality zooplankton, rich in both essential amino acids (EAAs) and fatty acids (FAs), are required as starting prey to initiate development and fast juvenile growth. Prey quality is dependent on environmental conditions, and, for example, eutrophication and browning are two major factors defining primary producer community structures that will directly determine the nutritional quality of the basal food sources (algae, bacteria, terrestrial matter) for zooplankton. We experimentally tested how eutrophication and browning affect the growth and survival of juvenile rainbow trout (Oncorhynchus mykiss) by changing the quality of basal resources. We fed the fish on herbivorous zooplankton (Daphnia) grown with foods of different nutritional quality (algae, bacteria, terrestrial matter), and used GC‐MS, stable isotope labeling as well as bulk and compound‐specific stable isotope analyses for detecting the effects of different diets on the nutritional status of fish. The content of EAAs and omega‐3 (ω‐3) polyunsaturated FAs (PUFAs) in basal foods and zooplankton decreased in both eutrophication and browning treatments. The decrease in ω‐3 PUFA and especially docosahexaenoic acid (DHA) was reflected to fish juveniles, but they were able to compensate for low availability of EAAs in their food. Therefore, the reduced growth and survival of the juvenile fish was linked to the low availability of DHA. Fish showed very low ability to convert alpha‐linolenic acid (ALA) to DHA. We conclude that eutrophication and browning decrease the availability of the originally phytoplankton‐derived DHA for zooplankton and juvenile fish, suggesting bottom‐up regulation of food web quality.     

Food sources and lipid retention of zooplankton in subarctic ponds

1. Subarctic ponds are seasonal aquatic habitats subject to short summers but often have surprisingly numerous planktonic consumers relative to phytoplankton productivity. Because subarctic ponds have low pelagic productivity but a high biomass of benthic algae, we hypothesised that benthic mats provide a complementary and important food source for the zooplankton. To test this, we used a combination of fatty acid and stable isotope analyses to evaluate the nutritional content of benthic and pelagic food and their contributions to the diets of crustacean zooplankton in 10 Finnish subarctic ponds. 2. Benthic mats and seston differed significantly in total lipids, with seston (62.5 μg mg^?1) having approximately eight times higher total lipid concentrations than benthic mats (7.0 μg mg^?1). Moreover, the two potential food sources differed in their lipid quality, with benthic organic matter completely lacking some nutritionally important polyunsaturated fatty acids (PUFA), most notably docosahexaenoic acid and arachidonic acid. 3. Zooplankton had higher PUFA concentrations (27–67 μg mg^?1) than either of the food sources (mean benthic mats: 1.2 μg mg^?1; mean seston: 9.9 μg mg^?1), indicating that zooplankton metabolically regulate their accumulation of PUFA. In addition, when each pond was evaluated independently, the zooplankton was consistently more ¹³C‐depleted (δ¹³C ?20 to ?33‰) than seston (?23 to ?29‰) or benthic (?15 to ?27‰) food sources. In three ponds, a subset of the zooplankton (Eudiaptomus graciloides, Bosmina sp., Daphnia sp. and Branchinecta paludosa) showed evidence of feeding on both benthic and planktonic resources, whereas in most (seven out of 10) ponds the zooplankton appeared to feed primarily on plankton. 4. Our results indicate that pelagic primary production was consistently the principal food resource of most metazoans. While benthic mats were highly productive, they did not appear to be a major food source for zooplankton. The pond zooplankton, faced by strong seasonal food limitation, acquires particular dietary elements selectively.     

Are zooplankton food resources poor in the vegetated littoral zone of shallow lakes?

1. The distribution of zooplankton in shallow lakes is negatively related to macrophyte density. However, the abundance of their food along density gradients of macrophytes is unknown. A common but untested assumption is that food quantity and quality for pelagic zooplankton is poor in the littoral zone owing to the deleterious influence of macrophytes on phytoplankton. 2. We tested this assumption with a combination of a field survey and laboratory experiments. We collected seston samples from the littoral and pelagic zones of four shallow temperate lakes and related food quantity (phytoplankton biovolume) and quality to macrophyte abundance (per cent volume infested). Seston food quality was assessed in three ways: N/C and P/C ratios, polyunsaturated fatty acid content and phytoplankton community composition. In the laboratory, we measured the growth and reproduction of Daphnia pulicaria on diets consisting of seston from the littoral and pelagic zones in one lake. 3. In our four study lakes, food quantity was not significantly influenced by macrophyte abundance, and food quality was generally high. Laboratory experiments showed increased juvenile growth, but no significant change in D. pulicaria reproduction, when feeding on littoral resources compared to pelagic resources. 4. Our results suggest that there is no nutritional cost to pelagic zooplankton inhabiting the littoral zone. Therefore, it is likely that other factors (e.g. predation, abiotic factors) are involved in determining zooplankton habitat use.     

Linking elements to biochemicals: effects of nutrient supply ratios and growth rates on fatty acid composition of phytoplankton species

Three species of marine phytoplankton, Rhodomonas sp., Isochrysis galbana Parke, and Phaeodactylum tricornutum Bohlin, were cultivated in semicontinuous cultures to test biochemical responses (fatty acids; FAs) to five nitrogen (N):phosphorus (P) supply ratios and four growth rates (dilution rates). The characteristic FA profile was observed for each algal species (representing particular algal class), which remained relatively stable across the entire ranges of N:P supply ratios and growth rates. For all species, significant direct effects of N:P supply ratios on FAs were found at lower growth rates. The highest saturated and monounsaturated fatty acid (SFA and MUFA) contents were observed under N deficiency at the lowest growth rate in all three species, while responses of polyunsaturated fatty acids (PUFAs) revealed no consistent pattern. Total FAs (and SFAs and MUFAs) in all species showed significant negative correlations with N cell quota (Q_N) under N deficiency, but PUFAs had species‐specific correlations with Q_N. The results show that characteristic FA profiles of algal genus or species (representing particular algal classes) underlie fluctuations according to culture conditions. The significant correlation between FAs and Q_N under N deficiency suggests that elemental and biochemical limitation of phytoplankton should be considered mutually as determinants of food quality for zooplankton in marine ecosystems.     

Lipid-ovary indices in food-limited Daphnia

The amount of lipid carried by a zooplankton individual is highlyvariable. Various lines of evidence have suggested that thenutritional condition of animals could be ascertained via thedetermination of lipid content. However, few studies have beenconducted where the lipid levels of animals of known conditionhave been measured. Here, we report on the lipid index and combinedlipid-ovary index values of Daphnia obrusa reared on foods madeup of varying quantity and quality of a unialgal (Scenedesmusacutus) diet. Lipid levels differed in different food types.However, in no cases were these indices positively correlatedto growth rate, such as would be necessary if one were to usethem to diagnose food limitation in the field. Thus, we urgecaution in the utilization of either the lipid or lipid-ovaryindex until more is known about the role of fatty acids in thediets of these animals.     

Polyunsaturated fatty acids in zooplankton: variation due to taxonomy and trophic position

1. Food quality has major effects on the transfer of energy and matter in food webs, and essential long‐chained polyunsaturated fatty acids (PUFAs) can affect the quality of phytoplankton as food. In a study of oligotrophic lakes in north‐western Sweden, we investigated the fatty acid composition of four planktonic cladocerans and two calanoid copepods, representing herbivorous and carnivorous species. We also collected seston samples. 2. The proportions of long‐chain PUFAs in the organisms increased with their increasing trophic position. Thus, both their quality as food for other organisms, as well as their requirement for fatty acids (FAs), differed among taxa and depended on their trophic position. 3. We found taxon‐specific differences in the FA composition of zooplankton that were not related to sestonic FA composition. This implies that the variation in zooplankton FA composition is constrained by phylogenetic origin, life history characteristics, or both. 4. The cladoceran taxa contained 12–23% eicosapentaenoic acid (EPA) but only 0.9–2.1% docosahexaenoic acid (DHA) of the total FA content. In contrast, the calanoid copepods contained 7–11% EPA and 14–21% DHA. Thus, our results show that differences in the PUFA content among zooplankton species could have repercussions for both food web structure and function.     

Associating particulate essential fatty acids of the ω3 family with phytoplankton species composition in a Siberian reservoir

1. We studied variation in the composition of fatty acids in the seston of a small freshwater reservoir with changes in phytoplankton composition during four growth seasons. We focused on the dynamics of the ω3 fatty acids because of their potential importance for zooplankton nutrition. 2. Total diatoms were related to the 20:5ω3 fatty acid (eicosapentaenoic, EPA) content in seston. Among two dominant diatom genera, Cyclotella was not associated with EPA content. In contrast, there was a significant correlation between Stephanodiscus and the percentage contribution and content of EPA throughout the study. Hence, freshwater diatoms can differ strongly in content of the essential EPA. 3. We considered abundant cyanobacteria as a potential source of 18:3ω3 fatty acid (linolenic, ALA) to aquatic food webs. Among four dominant cyanobacteria species, two (Anabaena flos‐aquae and Planktothrix agardhii) showed significant correlation with the ALA content of the seston, while the other two (Aphanizomenon flos‐aquae and Microcystis aeruginosa) did not. 4. Dinophyta had a relatively high level of 22:6ω3 (docosahexaenoic, DHA) for freshwater species and can be also a source of EPA to aquatic food webs. 5. Our results show that various species of diatoms as well as cyanobacteria can be of contrasting nutritional value for zooplankton because of their different content of the essential PUFAs. Diatoms, which are low in EPA, could not be considered as a valuable food, while some field populations of cyanobacteria might be valuable sources of essential ALA.     

两种水华藻——球形棕囊藻和铜绿微囊藻的脂肪酸组成特征与水华形成机制

浮游植物所含的不饱和脂肪酸是测定其作为食物质量的指标,并在浮游植物向浮游动物及其它动物能量转化过程中起着关键的作用,必需不饱和脂肪酸的缺乏有利于水华的形成。球形棕囊藻(Phaeocystis globosa)和铜绿微囊藻(Microcystis aeruginosa)分别是常见的海洋和淡水水华藻类,该文分析了它们在不同生长期的脂肪酸组成,探讨了这两种藻类的脂肪酸组成特征。球形棕囊藻和铜绿微囊藻的脂肪酸碳链长为14~20个碳原子,脂肪酸种类组成都比较简单,以饱和脂肪酸为主,未检测到二十碳五烯酸(Eicosapentaenoic acid,EPA)和二十二碳六烯酸 (Docosahexaenoic acid,DHA)等动物的必需脂肪酸。球形棕囊藻的总脂肪酸含量在247.294~735.44 μg·g^-1干重之间,在对数期和延滞期含量最高的脂肪酸分别是C14:0和C16:0;而两株铜绿微囊藻的总脂肪酸在1 405.095~6 087.617μg·g^-1干重之间,以C16:0含量最高。两株铜绿微囊藻的脂肪酸含量在对数期和延滞期差异明显(p<0.05),但球形棕囊藻的脂肪酸含量在不同生长期差别不大。由于缺乏必需脂肪酸EPA和DHA,球形棕囊藻和铜绿微囊藻不能为高营养级的生物提供必需的不饱和脂肪酸,不是浮游动物等生物的良好食物。因此球形棕囊藻和铜绿微囊藻作为浮游动物的食物质量较低,浮游动物对它们的捕食压力也较小,可能是这两种藻容易暴发水华的重要原因。     

Lipid‐rich zooplankton subsidise the winter diet of benthivorous Arctic charr (Salvelinus alpinus) in a subarctic lake

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Joint effect of phosphorus limitation and temperature on alkaline phosphatase activity and somatic growth in <Emphasis Type="Italic">Daphnia magna</Emphasis>

Alkaline phosphatase (AP) is a potential biomarker for phosphorus (P) limitation in zooplankton. However, knowledge about regulation of AP in this group is limited. In a laboratory acclimation experiment, we investigated changes in body AP concentration for Daphnia magna kept for 6 days at 10, 15, 20 and 25°C and fed algae with 10 different molar C:P ratios (95–660). In the same experiment, we also assessed somatic growth of the animals since phosphorus acquisition is linked to growth processes. Overall, non-linear but significant relationships of AP activity with C:P ratio were observed, but there was a stronger impact of temperature on AP activity than of P limitation. Animals from the lowest temperature treatment had higher normalized AP activity, which suggests the operation of biochemical temperature compensation mechanisms. Body AP activity increased by a factor of 1.67 for every 10°C decrease in temperature. These results demonstrate that temperature strongly influences AP expression. Therefore, using AP as a P limitation marker in zooplankton needs to consider possible confounding effects of temperature. Both temperature and diet affected somatic growth. The temperature effect on somatic growth, expressed as the Q ₁₀ value, responded non-linearly with C:P, with Q ₁₀ ranging between 1.9 for lowest food C:P ratio and 1.4 for the most P-deficient food. The significant interaction between those two variables highlights the importance of studying temperature-dependent changes of growth responses to food quality.