Food Quality for Zooplankton in Humic Lakes

Food quality affects growth, reproduction and community structure of zooplankton, and has implications for nutrient cycling and the transfer of energy and matter in aquatic food chains. While the issue of food quality in clear water lakes has received great recent attention, studies on food quality for zooplankton in humic lakes are scarce. Humic lakes are largely influenced by carbon of terrestrial origin, and the effect of this large terrestrial carbon influence on the total quality of the resource available for zooplankton is poorly known. On the one hand, there is usually a large contribution of low-quality detritus and bacteria to the particulate carbon pool of humic lakes. On the other hand, cryptophytes and chrysophytes, algal groups generally considered of high quality for zooplankton, are rather common in humic lakes. In this thesis, I assess the quality of the food available for zooplankton in humic lakes, and compare it to lakes that are less influenced by terrestrial organic matter (clear water lakes). In a long-term laboratory experiment, in which zooplankton were kept in humic and in clear lake water in the presence or in absence of recent phytoplankton production, differences in zooplankton community composition between lake types as well as comparatively high zooplankton biomass were observed in the humic lake by the end of the study. It was unclear from that study whether the differences in zooplankton community composition and biomass could be due to differences in food quality between lake types. In a series of subsequent studies, I measured the content of fatty acids and elements in seston (suspended particulate matter) to assess the quality of the food available for zooplankton in humic and clear water lakes. I hypothesized that food quality is higher in humic than in clear water lakes of comparable trophy. In a field survey, I found higher food quality (in terms of fatty acids) in humic than in clear water lakes closer to an oligotrophic (low total phosphorus concentration) state. However, in meso- to eutrophic lakes, food quality (in terms of both elements and fatty acids) was lower in humic lakes than in clear water lakes, which resulted in comparatively low growth and reproduction of zooplankton in the humic lakes. My results show that there are differences in food quality between humic and clear water lakes that affect zooplankton growth and reproduction, and, consequently, energy transfer in the food chain. The large influence of terrestrial organic matter in humic lakes can have bearings on food quality for zooplankton, since it can affect, for instance, the relative contribution of algal (high food quality), non-algal and detrital (low food quality) carbon to the total organic carbon pool, as well as microplankton community structure and composition

Food quality for Daphnia in humic and clear water lakes

1. Growth and reproduction of Daphnia fed lake seston were measured in two categories of meso- to eutrophic lakes differing with respect to terrestrial organic matter influence (humic and clear water lakes). The content of highly unsaturated fatty acids (HUFA), P and N, as well as the taxonomical composition of seston were analysed.2. Seston HUFA and C : P ratios were similar between lake categories, whereas C : N ratios were lower in the clear water lakes in both spring and summer. Despite the similarity in HUFA and P content of seston, Daphnia growth rate, clutch size and the proportion of gravid females were, respectively, about 1.5, 3 and 6 times higher in the clear water lakes.3. Differences in growth and reproduction were related to a combination of higher N content and good fatty acid quality of the seston in the clear water lakes. Relatively high biomass of edible algae, such as Rhodomonas sp. and Cryptomonas sp., in the clear water lakes, and differences in water pH likely contributed to the observed differences in Daphnia growth and reproduction between lake categories. Additionally, it is possible that Daphnia was energy limited in the humic lakes despite high particulate organic carbon (POC) concentrations, as the contribution of non-algal and detrital C to the POC pool was high.4. Our results suggest that dietary HUFA content has the potential to improve herbivore growth and reproduction if N and P are not limiting. N merits more attention in studies of zooplankton nutrition

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) - 20:5 omega 3] 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 (mu g L-1) and relative content (mu g 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

Microbial food webs in the dark: independence of Lake Plankton from recent algal production

We investigated the development of a heterotrophic plankton food web with or without phytoplankton primary production in a long-term (>1 yr) laboratory experiment. Water from 3 Swedish lakes (humic, oligotrophic clearwater, eutrophic) was exposed to low light or kept in total darkness in triplicate 100 1 cylinders. Dissolved organic carbon (DOC) dynamics, bacterial growth and biomass of protozoans, rotifers and microcrustaceans were followed over 18 mo. In the dark treatments, no primary production was detected and DOC concentrations decreased by between 19 and 27% (1.3 to 3.2 mg C l(-1)). There was bacterial and protozoan growth in the dark during the whole experimental period. However, numbers and production of bacteria, as well as protozoan biomass, were significantly lower in darkness. Dissolved (DOM) and particulate organic matter (POM) initially present in the water (i.e. 18 mo old at the end of the experiment) helped to support substantial metazoan biomasses in dark treatments in the humic and eutrophic waters, but not in the oligotrophic clearwater lake. DOM in the humic water, thus largely of allochthonous origin, gave the highest and most prolonged support to zooplankton living in darkness. Our study indicates that a microbial food web, based on allochthonous organic matter and developing independently from phytoplankton, can act as a link to metazoan zooplankton, especially in oligotrophic humic lakes. These results confirm studies using stable C isotopes, showing a substantial incorporation of terrestrial carbon into zooplankton

Complementary UV protective compounds in zooplankton

Zooplankton accumulate several groups of photoprotective compounds to shield against damaging ultraviolet radiation (UV). One of these groups, the carotenoids, makes the animals more conspicuous to visually hunting predators, whereas others, such as the mycosporine-like amino acids (MAAs) may not. The blend of photoprotective compounds is therefore important for the UV defense but also for the ability to escape predation through crypsis. Here we assess laboratory and field data from different latitudes to examine how UV, predation threat, and pigment availability ( in food) affects the mixture of UV-protective compounds in copepods. Overall, the blend of MAAs and carotenoids was partly explained by the availability of MAAs in the food, the UV-threat, and the presence of predators. Copepods upregulated their MAA content when UV threat was increasing (i.e., if MAAs were abundant in food), and in field data this accumulation only occurred at high levels of predation threat. If MAAs were scarce, copepods instead compensated with higher carotenoid accumulation. However, when there was a high predation threat this carotenoid compensatory effect was disadvantageous, and low concentrations of both MAAs and carotenoids at high UV-threat resulted in lower reproduction. In all, these results showed that carotenoids and MAAs are complementary substances, i.e., one is high when the other is low, and copepods are, hence, able to adjust their blend of different UV-protective compounds to optimize their defenses to the threats of UV and predation. These defense systems may buffer against direct food-web interactions and help the zooplankton to survive in environments with high UV threat

Predicted warming and browning affect timing and magnitude of plankton phenological events in lakes: a mesocosm study

1. Aquatic ecosystems in Northern Europe are expected to face increases in temperature and water colour (TB) in future. While effects of these factors have been studied separately, it is unknown whether and how a combination of them might affect phenological events and trophic interactions. 2. In a mesocosm study, we combined both factors to create conditions expected to arise during the coming century. We focused on quantifying effects on timing and magnitude of plankton spring phenological events and identifying possible mismatches between resources (phytoplankton) and consumers (zooplankton). 3. We found that the increases in TB had important effects on timing and abundance of different plankton groups. While increased temperature led to an earlier peak in phytoplankton and zooplankton and a change in the relative timing of different zooplankton groups, increased water colour reduced chlorophyll-a concentrations. 4. Increased TB together benefitted cladocerans and calanoid copepods and led to stronger top-down control of algae by zooplankton. There was no sign of a mismatch between primary producers and grazers as reported from other studies. 5. Our results point towards an earlier onset of plankton spring growth in shallow lakes in future with a stronger top-down control of phytoplankton by zooplankton grazers