Regulation of bacterial growth efficiency in a large turbid estuary

Bacterial secondary production transforms organic C from the environment in to new bacterial biomass. Bacterial respiration generates energy and conver ts assimilated organic C into CO2. Two decades of research have given us a good understanding of the magnitude and regulation of bacterial production in pelagic ecosystems, but much less is known about bacterial respiration. Bacterial growth efficiency [BGE = BP/(BP + BR)] relates measurements of ba cterial production and respiration. Recent reviews demonstrate a large rang e in BGE among and within systems; the regulation of this variance is not w ell. understood. We made direct measurements of both BP and BR over a full seasonal cycle in the Hudson River, New York, and in a series of manipulati ve experiments. BGE was well correlated with BP and ranged from 0.04 to 0.6 6, with a majority (69%) between 0.2 and 0.5. BR and BP were correlated (r = 0.65; p < 0.0001) but BR was less variable than BP. Thus. much of the var iation in BGE could be explained by the variation in BP. The relationship ( based on 24 h bioassays) between BP and BGE fit a rectilinear hyperbola [BG E = 0.10 + 0.68BP(5.21 + BP)] and explained 70% of the variation in BGE (p < 0.001). During the relatively long incubation (24 h) required to measure- BR, conditions diverge from ambient. BP, BR and BGE all increase during thi s incubation period. We used the relationships between BGE and BP and BR (a bove) to calculate realistic ambient estimates of BGE from short-term measu rements (<1 h) of BP. Based on this approach, modeled BGE for the Hudson av eraged 0.16 +/- 0.05 (range = 0.07 to 0.23), about 50% lower than the value s based on 24 h bioassays. Using this relationship we estimate pelagic BR i n the tidal, freshwater Hudson River to be between 176 and 229 g C m(-2) yr (-1).