Bacterial-algal interactions and stream biofilm composition: The role of resource availability and drainage network position

Epilithic biofilms composed of algae and bacteria play an important role in nutrient and carbon cycling in stream ecosystems and link to higher trophic levels through biofilm scrapers.  In aquatic ecosystems interactions among bacteria and algae change as a function of light, nutrient, and carbon availability. Nutrient-limited algae exude excess labile carbon. Bacteria are often carbon-limited and can exploit these algal exudates. Aquatic bacteria may also compete with algae for inorganic nutrients if there is an alternative carbon source. Light, nutrients, and terrestrial carbon inputs change in partially predictable patterns down drainage networks, so network position might be a useful predictor of interactions between algae and bacteria in streams.  We deployed solute-diffusing substrates (supplementing carbon [sucrose], nitrogen [nitrate] and phosphorous [phosphate]) down a drainage network (in streams with D.A. 0.6 to 140 km²) during the low-flow period in a forested watershed with a Mediterranean climate. Bacterial responses to nutrient and carbon supplementation suggest that bacteria were carbon limited and dependent on algae in many parts of the drainage network. Additionally, under high light conditions carbon supplementation facilitated resource competition between algae and bacteria resulting in lower chlorophyll A concentrations relative to control treatments. Microcosm experiments using selective biocide treatments crossed with light and nutrient manipulations confirm that under high light the presence of bacteria reduces algal growth. Ongoing work with selective biocide-diffusing-substrates will be used to reduce the algal or bacterial biofilm fraction in situ.  This will allow us to quantify interaction strengths and make predictions about the boundaries where these microbial interactions change in the drainage network.