Using stable isotope ratios to estimate atmospheric nitrogen fixed by cyanobacteria at the ecosystem scale

Diazotrophic cyanobacteria are capable of fixing atmospheric N-2 to satisfy their physiological nitrogen requirements. This process can result in the transfer of substantial amounts of new diazotrophic nitrogen (N-D) to aquatic ecosystems during blooms of these taxa. Using in situ measurements of plankton natural abundance stable isotope composition and a combination of underway and fixed site survey data, the total N-D flux into the Gippsland Lakes estuary (Australia) was estimated during a summer bloom of the diazotrophic cyanobacterium Nodularia spumigena. Over the course of the bloom, N-D increased in the upper water column of the estuary from 33% +/- 17% (mean +/- SD) to 73% +/- 13% of the standing pool of total particulate N. A conservative estimate of total N-D flux (146 Mg) equates to an estimated 177% of the summer total N load and 22% of the annual total N load to the estuary. Combining natural abundance stable isotope measurements with relatively simple fixed and underway survey designs can provide a cost-effective approach for monitoring the N-D flux into estuary or lacustrine environments. This approach relies on an isotopic differential between the diazotrophic and the non-diazotrophic components of the plankton community; it may not be appropriate in ecosystems that experience low-level blooms or blooms of intermittent N-fixing cyanobacteria. Large-scale blooms of diazotrophic cyanobacteria are considered uncommon in estuaries, yet it is clear that these blooms can represent major sources of new N to estuarine ecosystems when and where they occur.