Microzooplankton dominate carbon flow and nutrient cycling in a warm subtropical freshwater lake

In a series of grazer-gradient and dilution microcosm experiments, we compared grazing and nutrient mineralization by naturally co-existing crustacean and microzooplankton assemblages from mesoeutrophic Lake Kinneret. Across two distinct seasonal plankton assemblages, microzooplankton dominated both phytoplankton and bacteria grazing and nitrogen (N) and phosphorus (P) mineralization. Mass-specific ingestion rates by microzooplankton were ,20 times higher than ingestion rates of crustaceans. Although most carbon ingested by microzooplankton was in the form of bacteria, microzooplankton inflicted substantially higher mortalities (both in absolute terms and relative to maximum potential growth rates) on both phytoplankton and bacteria compared with crustaceans. Mass-specific P and N excretion rates were also higher (by 70 and 50 times, respectively) than crustacean excretion rates. These results suggest that microzooplankton grazing and nutrient mineralization are driving forces affecting bacteria and phytoplankton dynamics, playing important roles in carbon and nutrient transfer to upper trophic levels even in pelagic freshwater systems containing abundant crustaceans.