Seasonal variations in food quality for pelagic and benthic invertebrates in Lake Erken – the role of fatty acids

1. Plankton net samples and sedimenting matter in traps from mesotrophic Lake Erken, Sweden, were analysed for carbon (C), nitrogen (N), phosphorus (P), total lipids and fatty acid (FA) content to determine what differences and seasonal changes might exist in the quality of food available to pelagic zooplankters and benthic invertebrates. 2. Matter collected in a plankton net was assumed to correspond to food available to pelagic grazers, while matter collected in sedimentation traps provided a measure of food available to benthic invertebrates. Furthermore, food quality was assumed to be related to polyunsaturated fatty acid (PUFA) content. 3. The results suggest that suspended particulate matter, collected with a plankton net, is a much higher‐quality food resource than sedimenting matter. Our data also show that only during the spring and autumn do the benthic fauna have access to high‐quality food, because of the dominance of diatoms during these periods. During summer, pelagic grazer production was probably limited by food quantity, whereas benthic invertebrate production was probably limited by food quality. 4. Plankton net samples showed consistently higher values than trap samples (% of dry weight, DW, or mg g –1 DW) for nearly all variables analysed: C, N, P, saturated FA (SAFA), monounsaturated FA (MUFA), PUFA, ω3 FA and ω6 FA. If N and P were expressed per C, the differences decreased and both data sets indicated moderate to no N or P limitation. N/P also showed well‐balanced ratios throughout most of the season, on average 10 and 7 (by weight), respectively. The largest differences between the two food resources occurred in the PUFA content, including the important FA of the ω3 type. The only FA with higher levels in the trap samples were some unidentified FA of relatively short chain lengths. Seasonal variation for most variables was also very large in both net and trap samples. 5. PUFA and ω3 FA showed good relationships with the P content of net samples, but not with that of trap samples. Eicosapentaenoic acid alone did not give a good relationship with P. Thus, P seems to be a good predictor of food quality in living phytoplankton but not in dead matter or detritus.