Predicting effects of climate warming on nitrogen fixation and its ecological consequences in aquatic ecosystems

Organisms need nitrogen to build biomolecules that support growth and, therefore, nitrogen availability influences productivity, species interactions, and other element cycles, including carbon. Unlike other organisms, nitrogen-fixing species of algae access nitrogen gas from the air and can increase nitrogen input to aquatic ecosystems. Enzymatic processes, like nitrogen fixation, often respond positively to increasing temperature and, thus we hypothesize that nitrogen supply in aquatic ecosystems should increase as temperatures rise. The objective of this study was to determine the effect of temperature on nitrogen fixation rates and algal species composition in rivers in the Hengill watershed in Iceland, which is geothermally active and contains streams that vary in temperature by 25°C. Nitrogen fixation rates were measured in eight streams in summer 2012 by incubating representative samples of algae in gas-tight chambers filled with stream water saturated with nitrogen gas enriched with 15N, a stable isotope of nitrogen. Six replicate samples were collected for each algal type and subsamples of algae were collected before and after incubation with 15N gas in each stream. Algal samples were dried at 60°C and analyzed on an isotope ratio mass spectrometer. The degree of 15N enrichment (pre versus post incubation) was used to calculate nitrogen fixation rates. We found a strong positive relationship between temperature and nitrogen fixation rates, consistent with an increase in nitrogen supply with increased temperature. These results strongly suggest that as temperatures rise, aquatic ecosystems may become enriched in nitrogen, which may have profound effects on the services these ecosystems provide