The Relative Importance of Heterotrophic Bacteria to Pelagic Ecosystem Dynamics Varies with Trophic State of Reservoirs




Caston, Charles Bradford

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Previous research in pelagic ecosystems suggests that nutrient sequestration and carbon flow through heterotrophic bacteria is relatively high in oligotrophic systems but decreases as trophic state increases. I assessed this hypothesis and mechanistic explanations in 17 reservoirs in Texas and Ohio, USA. I measured bacterial production (BPr), primary production (PPr), particulate nutrients, dissolved organic carbon (DOC), soluble reactive phosphorus (SRP), and dissolved inorganic nitrogen (DIN). The % of particulate carbon (C), nitrogen (N), and phosphorus (P) in the bacterial size fraction (< 1-pm) was highest in ohgotrophic reservoirs but decreased as trophic state increased. Comparison of particulate nutrient data to Minnesota natural lakes suggests a similarity in nutrient sequestration by bacteria in reservoirs and natural lakes. BPr and PPr ranged from 17 - 172 and from 41 - 1,695 pg C L'1 d'1, respectively. BPr:PPr decreased as our measure of trophic state, chlorophyll, increased (r2 = 0.42). DOC, DOC:SRP, and DOC:DIN did not improve predictions of BPr:PPr over that based solely upon chlorophyll. These variables may have limited use as predictors of BPr:PPr across a broad range of trophic state. BPr:PPr was better predicted by DOCrparticulate carbon (PC) (r2 = 0.47) and mixing depth (Zmix) (r2 = 0.61) than by chlorophyll. However, the relationship between BPr:PPr and DOC:PC was primarily driven by the relationship between BPr:PPr and PC (r2 = 0.41), which was a surrogate for trophic state. Unexpectedly, the strength of Zmix as a predictor BPriPPr did not primarily reflect differences in the light environment. My results support the hypothesis that the relative importance of heterotrophic bacteria is highest in oligotrophic systems and decreases as trophic state increases, but I found limited support for several of the proposed mechanisms of this pattern. My results suggest that patterns in microbial dynamics are similar in reservoirs and natural lakes.



heterotrophic bacteria, reservoirs, biotic communities, environmental aspects, Texas, Ohio


Caston, C. B. (2008). The relative importance of heterotrophic bacteria to pelagic ecosystem dynamics varies with trophic state of reservoirs (Unpublished thesis). Texas State University-San Marcos, San Marcos, Texas.


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