Abstract
The phylogenetic composition of bacterial communities varies along the salinity gradient of estuaries, one notable pattern being the prevalence of alpha‐ and beta‐proteobacteria in salt‐ and freshwater regions, respectively. We tested the hypothesis that bottom‐up forces (substrate supply) control these and other changes in bacterial community composition in the Delaware estuary. We measured the biomass and growth rate of four major phylogenetic groups of bacterioplankton (alpha‐, beta‐, and gamma‐proteobacteria and the Cytophaga‐Flavobacter cluster) in low‐ and high‐salinity regions of the estuary by the dilution culture approach combined with fluorescent in situ hybridization with rRNA‐targeted probes. Group‐specific growth rates were highly variable depending on location and season and could far exceed (up to nearly fourfold) the growth rates of total bacteria. The phylogenetic groups that exhibited the highest growth rates included alpha‐ and gamma‐proteobacteria at both low‐ and high‐salinity stations and betaproteobacteria at a low‐salinity station. These data help to explain the high abundance of beta‐proteobacteria in the freshwater region, but bottom up controls appear to account only partly for the variability in bacterial communities. The relationship between production and biomass suggested that bacterial communities at the low‐salinity site were controlled by substrate supply, whereas bacterial mortality appeared to be more important at the high‐salinity site. Our data demonstrate that group‐specific growth rates are useful parameters for examining the competitive advantages of individual groups and the mode of regulation of bacterial communities in estuarine environments.