Although heterotrophic bacteria are important components of stream ecosystems, little is known about the factors that contribute to their abundance. We used data from 69 streams to examine both the direct and indirect effects of algal biomass (chlorophyll a), dissolved organic C (DOC), NO3−, soluble reactive P (SRP), and NH4+ on bacterial density. All potential relationships were simultaneously evaluated using path analysis. Streams in 2 ecoregions (unglaciated and glaciated) were used to determine if important factors varied between ecoregions. The results indicated that algal biomass was the best predictor of bacterial density in both ecoregions. Furthermore, inorganic nutrients appeared to mainly affect bacteria indirectly through their influence on algal biomass. However, a quantitative relationship between algae and bacteria was not observed when algal biomass was <5 μg chlorophyll a/cm2. The lack of relationship between algae and bacteria at low algal biomass may help to explain why some studies have observed strong quantitative relationships between bacteria and algae in streams and others have not. Although the current study design was not capable of assessing the bioavailability of DOC to bacteria, the proposed conceptual framework used in the path analysis coupled with better assessments of the relationship between DOC and bacteria should prove useful for evaluating the importance of DOC, inorganic nutrients, and algae on epilithic bacteria in a number of different stream ecosystems.
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