Impact of zooplankton grazing on phytoplankton in north temperate coastal lakes: changes along gradients in salinity and nutrients

Zooplankton grazing at similar nutrient levels is generally regarded as lower in brackish than in freshwater lakes, but experimental evidence of this is lacking. Accordingly, we conducted short-term zooplankton grazing experiments in bottles with water from 12 Danish brackish lakes covering a large gradient in salinity (0.3-17.4 parts per thousand) and nutrient concentrations as well as with water from 24 mesocosms established in the same area with various salinities (0.5-12 parts per thousand), two nutrient levels and low fish density. Grazing was low in 11 of the 12 lakes, even when they were dominated by edible phytoplankton and nutrient addition led to a major increase in phytoplankton biomass. By contrast, grazing was significant in most of the mesocosms, particularly at high nutrient levels and salinities of 8 parts per thousand or below where Daphnia dominated. Moreover, grazing decreased the biomass of most phytoplankton taxa, except for a few (e.g. Ankyra at 0.5-2 parts per thousand and Ochromonas and Chaetoceros at 8 parts per thousand). Our results provide experimental support for potentially significant grazing by zooplankton on phytoplankton in brackish lakes up to a salinity of 8 parts per thousand at low fish density; however, grazing in summer was generally low in the majority of the lakes, which we attribute to high predation on zooplankton.