Simultaneous measurement of bacterio-plankton production and protozoan bacterivory in estuarine water

Abstract

Simultaneous measurements were made of bacterioplankton productivity ([3H]thymidine assay) and of bacterial mortality due to protozoan grazing (measured via uptake of fluorescently labeled bacterioplankton, FLB). Water samples were taken from a salt marsh tidal creek and from an estuarine sound near Sapelo Island, Georgia, USA at low tide over a 2 wk period in late summer. In control experiments performed to test the extent of selectivity of estuarine bacterivorous protozoa for or against FLB compared to natural bacterioplankton, we found no evidence for consistent discrimination. Rates of bacterial production and of protozoan bacterivory were greater in the tidal creek than in the open sound. Ciliates were responsible for the largest fraction of total protozoan consumption of bacteria in tidal creek water, and colorless flagellates in open estuary water. Bacterial production and protozoan bacterivory were not always in balance in individual samples, with the largest discrepancies in the open estuary. Estimated bacterivory was, on average, 80 % of bacterial production in the tidal creek and 50 >/o of production in the open estuary. Explanations for the measured shortfall in bacterial mortality include methodological problems with the assays used or alternate fates of bacterial production besides protozoan grazing