Metabolic activity of size-fractionated microbial plankton in estuarine, near-shore, and continental shelf waters of Georgia

Abstract

Estimates of bacterial production and activity, and of planktonic community respiration, were made at a series of stations starting in an estuary, passing through the coastal front, and across the continental shelf to a distance of 120 km offshore in the Georgia Bight. Measurements were made on unfiltered and on 1.0 .mu.m (pore-size) filtered water samples to examine the small free-living bacterial size fraction relative to the larger plankton. In waters landward of the coastal front, ca 50 to 80% of bacterial and community activity was associated with the larger size fraction, indicating the importance of larger and particle-associated heterotrophs in the nearshore, highly turbid water. Seaward of the front, 80 to 99% of activity was in the < 1.0 .mu.m fraction. Thus, the < 1.0 .mu.m size fraction is responsible for most of the respiration in shelf-waters. Bacterial production and activity decreased markedly with distance from shore; community respiration also decreased across the shelf, but not as much as the bacterial production decreased. Bacterial growth efficiency was low in estuarine waters (11%), and decreased in nearshore (6%) and shelf (2%) waters.