The relative importance of heterotrophic bacteria to pelagic ecosystem dynamics varies with reservoir trophic state

Abstract

We tested the hypothesis that nutrient sequestration and carbon flow through heterotrophic bacteria is relatively highest in oligotrophic systems and decreases with trophic state in 17 reservoirs in Texas and Ohio, U.S.A. The percentage of particulate carbon (C), nitrogen (N), and phosphorus (P) in bacteria (< 1 µm) was highest in oligotrophic reservoirs and decreased with chlorophyll a, our measure of trophic state. Patterns of nutrient sequestration in bacteria were very similar to Minnesota (U.S.A.) natural lakes. Bacterial production (BPr) and primary production (PPr) ranged from 13 µg C L−1 d−1 to 172 µg C L−1 d−1 and from 41 µg C L−1 d−1 to 1695 µg C L−1 d−1, respectively, and the BPr : PPr ratio decreased with trophic state. Inclusion of variables such as dissolved organic carbon (DOC), DOC: soluble reactive phosphorus, and DOC: dissolved inorganic nitrogen did not improve predictions of BPr : PPr over that based solely upon chlorophyll a. BPr : PPr was better predicted by DOC: particulate carbon (PC) and mixing depth (Zmix) than by chlorophyll a. The relationship between BPr : PPr and DOC: PC was primarily driven by the relationship between BPr : PPr and PC, which was a surrogate for trophic state. Unexpectedly, the strength of Zmix as a predictor BPr : PPr did not primarily reflect differences in the light environment. Our results support the hypothesis that the relative importance of heterotrophic bacteria is highest in oligotrophic systems and decreases with trophic state, but provide limited support for proposed mechanisms of this pattern. Patterns in microbial nutrient dynamics are similar in reservoirs and natural lakes.