Seasonal shifts in the diet of lake zooplankton revealed by phospholipid fatty acid analysis

We analysed the phospholipid fatty acid (PLFA) profiles of seston and of the dominant zooplankter, Daphnia longispina, through the open water period in a small, dystrophic lake to investigate seasonal variation in the diet of Daphnia. Phytoplankton, heterotrophic bacteria, green sulphur bacteria and methane-oxidizing bacteria (MOB) were all present in the water column of the lake, and previous studies have indicated that vertically migrating Daphnia can exploit all these potential food sources. For adult Daphnia, although there was some correspondence between the PLFA profile of Daphnia and the concurrent seston PLFA profile, strongest correlations were between the Daphnia PLFA profile and those of potential food sources determined 7 days earlier. This interval presumably reflects the time it takes for adult Daphnia to turn over their fatty acid pool. A correlation between the concentration of polyunsaturated fatty acids (PUFAs) in the epi- and metalimnion and measured primary production indicated that, within the total PLFA fraction, PUFAs can be useful biomarkers for phytoplankton in food-web studies. Algal PUFAs contributed appreciably to total PLFAs in adult Daphnia during spring and summer, but less so in autumn. Daphnia in the lake actually reached their highest biomass in autumn, when methanotrophic activity was also highest, and the highest magnitude of MOB-specific PLFAs was recorded in both adult and juvenile Daphnia. A strong relationship existed between delta C-13 values of Daphnia reported previously and the proportion of MOB-specific PLFAs in Daphnia. Autumnal mixing evidently stimulates bacterial oxidation of methane from the hypolimnion, and exploitation of the methanotrophic bacteria sustains a high Daphnia population late in the season. Our results show that the PLFA composition of freshwater zooplankton like Daphnia corresponds rather well to that of their in situ diet of phytoplankton and bacteria, with a lag period of around 1 week in the case of adult animals. The PLFA profile of seston revealed the dominant available food sources, and relating these to the Daphnia PLFA profile provided insights into seasonal changes in Daphnia diet.