Zooplankton-associated and free-living bacteria in the York River, Chesapeake Bay: comparison of seasonal variations and controlling factors

Zooplankton provide microhabitats for bacteria, but factors which influence zooplankton-associated bacterial abundance are not well known. Through a year-long field study, we measured the concentration of free-living bacteria and bacteria associated with the dominant mesozooplankters Acartia tonsa and Balanus sp. Free-living bacteria peaked in the summer, while zooplankton-associated bacteria peaked in summer and winter. No relationships were found between bacterial abundance per individual and zooplankter width, length, surface area or body volume. Multiple regression analyses indicated that free-living and Acartia-associated bacterial concentrations were explained by temperature, salinity, ammonium, chl a, and all term interactions. Balanus-associated bacterial concentration was explained by ammonium and phosphate. Ammonium significantly influenced all sampled bacterial communities. In laboratory experiments, copepods raised under high ammonium concentration had higher bacterial concentrations than those raised under low ammonium condition. Transplant experiments showed that high ammonium favored loosely attached bacteria, whereas low ammonium selected for firmly attached bacteria, suggesting greater exchange between free-living and zooplankton-associated bacterial communities in nutrient-rich systems. Additional sampling of other zooplankton taxa all showed high bacterial concentrations, supporting the notion that zooplankton function as microbial hotspots and may play an important, yet overlooked, role in marine biogeochemical cycles.