Planktonic and bacterial respiration along an estuarine gradient: responses to carbon and nutrient enrichment

Previous observations have revealed that rates of plankton community respiration vary consistently along the main salinity gradient of Chesapeake Bay, USA, with maximum values occurring in the mesohaline region and significantly lower rates in both the oligohaline and polyhaline regions. This study tested the hypothesis that spatial variations in respiration are the result of differential resource limitation of the bacterial communities along the land-sea gradient. Effects of organic carbon and inorganic nutrients on microbial (<3 mu m) respiration rate were assessed in short-term (6 h) enrichment bioassays. Responses of bacterioplankton to enrichment were also investigated by means of re-growth culture experiments. Both sets of experiments were conducted during summer in 3 regions along the main salinity gradient of the estuary. Within the re-growth cultures, there was little variation in, or effect of enrichment on, specific growth rates, which were roughly 2 d(-1) in all cultures. In contrast, estimates of cell-specific respiration varied 2-fold along the salinity gradient (from 3.3 to 6.9 fg O-2 cell(-1) h(-1) in the Upper and Mid Bay regions, respectively) and showed significant responses to substrate enrichment. Resulting estimates of bacterial growth efficiencies exhibited similar variability, ranging from 20.4 to 41.3%, with spatial variations in efficiencies apparently related to nutrient concentrations. Results of substrate enrichment in both short-term bioassays and bacterioplankton re-growth cultures demonstrated a pattern of stimulation by organic carbon in the Upper Bay and by inorganic nutrients in the Lower Bay, with no significant response to substrate enrichment in the Mid Bay. Patterns of bacterioplankton response to enrichment in the nutrient-poor polyhaline region suggested a propensity for the bacterial community to maximize growth, rather than efficiency, via changes in specific respiration rates.