Biological controls of aquatic nitrogen fixation rates in a northern california stream ecosystem

Background/Question/Methods

In a nitrogen (N) limited system, N-fixing species (N-fixers) should have a competitive advantage over non-fixing species. As such, N-fixers can potentially alleviate N-limitation; however, N-limitation persists, and the mechanisms that limit N-fixers are not well understood, especially in aquatic ecosystems. Potential biologic controls of aquatic stream N-fixers are: grazing, light availability, temperature, and phosphorus (P) availability. Our objective was to test the relative importance of these variables on N-fixation rates in periphyton communities across a watershed area gradient - from heterotrophic, closed-canopy streams to autotrophic, well-lit channels in the South Fork Eel River watershed in northern California. This stream network (main stem plus tributaries) spans natural gradients of temperature, light, and water column N and P concentrations. We excluded benthic invertebrates across the stream network to manipulate grazing effects. In small tributaries, we also found anomalous ‘light gaps’, microhabitats where photosynthetic active radiation (PAR) was significantly greater than the light availability typical of the surrounding reach. To investigate the effects of these variables (light, grazing, P availability, and temperature) on N-fixers, we measured rates of N-fixation across network gradients, grazer manipulations and ‘light gaps’ using both the acetylene reduction and the N₂-N¹⁵ enrichment method.

Results/Conclusions

N-fixation rates significantly (p<0.01) increased with drainage area and light availability, suggesting that N-fixers are primarily light limited. Grazer exclosures did not stimulate rates of N-fixation at any sites. However, grazing and stream size had a significant interactive effect on chlorophyll a concentration (p=0.013), with grazer exclusion leading to increased algal biomass in small streams. The lack of response of N-fixation rates to grazing suggests either avoidance of N-fixers by grazers, or increased N-fixation by remaining fixers in the algal assemblage. Linear mixed effects modeling, regressing N-fixation rate with all four variables (light, temperature, grazing and P availability), indicates that while light explains much of the variation in N-fixation rates (p=0.003), water column temperature and P availability are also significant (p=0.04 and 0.002, respectively). Overall, these data indicate that although grazing may affect community composition, N-fixation rates in this stream network are constrained by abiotic factors – primarily light, but also P availability and stream temperature.