Seasonal variation in bacterial and flagellate communities of deep-sea sediments in a monsoonal upwelling system

We investigated bacterial and nanoflagellate densities in the bottom sediments and overstanding water, during both upwelling and non-upwelling seasons, along transects (depths 704350 m) in the Arabian Sea. Flagellate bacterivory was studied in bottom-water samples. There was a significant decrease with depth in benthic bacteria and benthic nanoflagellates, but except for the shallowest station, not in water column bacteria or flagellates. There were two significant differences between the seasons: bacterial density, biomass and cell volume were significantly larger in the bottom water during the upwelling season, whereas benthic flagellates were significantly more numerous in the sediments in the non upwelling season. Grazing rates were low, but bacterivory had significantly greater impact on bacterial standing in the bottom water during the non-upwelling season.