A recent, exciting phase of limnology, which may be called biochemical limnology, is evolving rapidly. One of its many challenges is the impetus to couple community dynamics with physiological processes driven at the biochemical level. For example, aquatic organisms at the plant-animal interface convey nutrients such as polyunsaturated fatty acids (PUFA) to animals at higher trophic levels. PUFA are important components of membrane lipids, precursors of signalling molecules and play a vital role in somatic growth, development, and eventually neurological functioning. These beneficial physiological effects are often counterbalanced by the concurrent dietary uptake, transfer, and accumulation of contaminants. One of the most toxic contaminants in the aquatic food web is the neurotoxin methyl mercury (MeHg), which is highly bioavailable for aquatic organisms and can cause decreased somatic growth in fish and severe health problems in humans. Therefore, it is important to understand how such essential nutrients and contaminants are transferred along the planktonic food chain and how their trophic transfer differs with time and among aquatic ecosystems.

I will present studies that demonstrate how MeHg concentrations of planktonic organisms increase significantly with plankton size, independent of their taxonomic composition, and throughout the season. In contrast to increasing MeHg concentrations with increasing zooplankton size, essential dietary nutrients do not necessarily increase in concentration with increasing zooplankton size. It will be shown that such inconsistent retention of essential nutrients is a function of taxonomic composition of organisms at the base of the aquatic food chain. A series of studies provides evidence that different essential nutrients (PUFA) are preferentially and consistently retained in zooplankton, whereas the toxic MeHg biomagnifies along the planktonic food chain, regardless of its taxonomic composition. Because of the preferential retention of some growth-enhancing essential nutrients, it was recently suggested that Hg concentrations may decrease in aquatic consumers, per unit biomass, when their diet provides a favorable PUFA profile conducive to enhanced somatic growth (‘principle of natural growth dilution’). Finally, it is a clear challenge to aquatic scientists to better understand and eventually manage pathways of both, potentially toxic and essential dietary nutrients that exert different control over bioaccumulation of contaminants and retention of essential nutrients in aquatic food webs. This seminar will discuss these points.