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 mesotroph ic Lake Erken, Sweden, were analysed for carbon (C), nitrogen (N) phos phorus (P), total lipids and fatty acid (FA) content to determine what differences and seasonal changes might exist in the quality of food a vailable to pelagic zooplankters and benthic invertebrates. 2. Matter collected in a plankton net was assumed to correspond to food availabl e to pelagic grazers, while matter collected in sedimentation traps pr ovided a measure of food available to benthic invertebrates. Furthermo re, food quality was assumed to be related to polyunsaturated fatty ac id (PUFA) content. 3. The results suggest that suspended particulate m atter, collected with a plankton net, is a much higher-quality food re source than sedimenting matter. Our data also show that only during th e spring and autumn do the benthic fauna have access to high-quality f ood, 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 th an trap samples (% of dry weight, DW, or mg g(-1) DW) for nearly all v ariables analysed: C, N, P, saturated FA (SAFA), monounsaturated FA (M UFA) PUFA, omega 3 FA and omega 6 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 mo st of the season, on average 10 and 7 (by weight), respectively. The l argest differences between the two food resources occurred in the PUFA content, including the important FA of the omega 3 type. The only FA with higher levels in the trap samples were some unidentified FA of re latively short chain lengths. Seasonal variation for most variables wa s also very large in both net and trap samples. 5. PUFA and omega 3 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.