Microzooplankton: major herbivores in an estuarine planktonic food web

Abstract

Microzooplankton, on average, ingested 10 times more phytoplankton productivity than Acartia tonsa, which dominates the mesozooplankton community in Apalachicola Bay, Florida, USA. Microzooplankton ingested < 75 and > 75% of phytoplankton productivity during winter and summer, respectively. Ciliates, particularly ciliates < 20 m in size, were abundant in all samples. Phytoplankton was the main (> 50%) component of the microzooplankton diet during the summer high productivity period. Because microzooplankton are prey for organisms occupying higher trophic levels, such as copepods, fish larvae, and oysters, we propose that energy and mass flows mainly from phytoplankton through microzooplankton to higher trophic levels in Apalachicola Bay. A reduction in discharge from the Apalachicola River during our study period led to reductions in rates of grazing, ingestion, and productivity of microzooplankton at a particular salinity. Reduced river discharge also increased the areal extent of higher salinity water where ingestion and productivity of microzooplankton were relatively low. Because microzooplankton are key constituents of the estuarine food web in Apalachicola Bay, upstream water diversion that increases the areal extent of higher salinity water in the bay, can be expected to lower microzooplankton productivity and therefore reduce higher trophic level productivity in the bay.