Effects of light, dissolved organic carbon, and inorganic nutrients [2pt] on the relationship between algae and heterotrophic bacteria in stream periphyton

Depending on the chemical and physical environment, algae and heterotrophic bacteria in stream periphyton communities likely engage in both positive and negative interactions. We tested the hypothesis that bacteria are more closely associated with algae when allochthonous sources of labile DOC are low and algae are not light limited. Secondly, we tested the hypothesis that, under extremely oligotrophic conditions, bacteria will out-compete algae for inorganic nutrients if their carbon requirements are met by allochthonous sources. Experiments were carried out using in situ light manipulations and nutrient diffusing substrates (releasing inorganic nutrients and /or glucose) in Harts Run, an oligotrophic stream located in north central Kentucky. Although we found that both algal and bacterial biomass were higher under ambient light, bacteria did not respond to glucose in the dark. This may indicate that bacteria were associated with algae not as a carbon source, but as a substrate for colonization. In the nutrient × glucose manipulation, we found that bacteria were co-limited by inorganic nutrients. There was no evidence of algae being negatively affected by competition with bacteria for nitrogen and phosphorus. Although low temperatures might have played a role in preventing inorganic nutrient competition between these two groups of organisms, the results of both experiments may indicate that the quantitative link between periphytic bacteria and algae is stronger under oligotrophic conditions.