Balanced top-down and bottom-up control in coastal planktonic food webs

Aquatic food webs are controlled by physical and chemical factors (bottom-up) and by predation (top-down) processes. However, the relative importance of these controls is unclear, which impedes the understanding of ecosystem changes due to climatic or anthropogenic forcing. For example, while high nutrient levels are generally considered to shape coastal ecosystems, sustained or even increasing stocks of primary producers in coastal areas with decreasing nutrient input suggests a weak bottom-up control. To unravel the relevance of bottom-up and top-down regulation, we analyze 15 years of environmental conditions and seven trophic groups in the Wadden Sea, Southern North Sea. We use size- and trait- based feeding theory to aggregate species into trophic groups. Based on a novel explorative, theoretical framework linking causal and statistical relations, we correlate fluctuations of biomass on weekly-monthly and annual time scales. We found signs of bottom-up influences on the community structure and growth of phytoplankton during spring blooms. In late spring and summer, phytoplankton blooms seemed to be terminated by grazing with concurrent elevated concentration of zooplankton. In turn, zooplankton and dinoflagellates appeared to be controlled by their prey and by higher trophic levels. These findings provide an ensemble of possible trophic relationships, and suggest a similar importance of top-down processes compared to bottom-up, both at annual and sub-seasonal scales, which indicate an important link between the ecology of higher trophic levels and the plankton food-web and, ultimately, biogeochemical cycling.