Fatty acids distribution in marine, brackish and freshwater plankton during mesocosm experiments

Most studies on the fatty acid composition of phytoplankton are based on laboratory experiments, and numerous studies using fatty acids as food web traces were conducted with monoalgal cultures. However, fatty acid studies with natural phytoplankton communities (food quality) and the passage of these to the next food web levels (biomarkers) are not abundant. It is important to consider the differences between analyses of fatty acids from natural plankton and laboratory studies. Seston does not only contain phytoplankton, but also some portion of bacteria, protozoa and non-living particles (detritus), especially in regions with high nutrient contents. Because it is almost impossible to quantitatively separate algae from the other particles, the fatty acid composition of phytoplankton can be disguised in natural waters. The aim of this studies was to describe and compare the composition of fatty acids in natural phytoplankton and their transfer to the mesozooplankton community under different nutrient availability and grazing pressure. Furthermore the viability of using phytoplankton fatty acid biomarkers found in monoalgal culture studies was tested. The fatty acid composition of planktonic particulate matter (seston and zooplankton) was examined during summer and spring experiments carried out in Lake Schöhsee (Germany), the Hopavågen lagoon (Norway) and the Kiel Fjord (Germany). At all study sites, mesocosm experiments were carried out in polyethylene bags (volume ~1.5 or 3.4 m3) suspended in several floats. Each treatment consisted of a logarithmically scaled gradient of copedod or Daphnia densities. The copepods originated from natural assemblages, the Daphnia were laboratory-reared. In all experiments the nutrient availability influenced the fatty acid contents in seston. These results supported the idea that the ratio between SAFA, MUFA and PUFA change with variable nutrient limitation and may be used as an indicator of the physiological status of the algae. Increasing nutrient limitation lead to reduced food quality of phytoplankton because of the decrease in essential PUFA. Therefore nutrient limitation of phytoplankton can alter trophic interactions. The low food quality can inhibit zooplankton growth, because most zooplankton species are not able to synthesize de novo these fatty acids essential for growth and reproduction. However the results of the Kiel fjord experiment suggested that the lacking quality of seston can partially be compensated by seston quantity.