Food Quality for Zooplankton in Humic Lakes

Sammanfattning: 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.