Planktonic nitrogen fixation from an isotopically-labelled long-term nutrient enrichment experiment

Nitrogen (N) fixation remains a critically understudied ecological process in freshwater lentic ecosystems. We conducted a three-year mesocosm experiment in oligotrophic freshwater ponds near Oxford, Mississippi from April 2019 to present. The mesocosms isolate the water column in an open-top and open-bottom design, allowing atmosphere-water and sediment-water interactions. Triplicate mesocosms were enriched with a single level of phosphorus (P) and four levels of isotopically-labelled reactive N (15N = 100) to create N:P ratios of 2.2, 16, 55, and 110 by atoms. We collected seston samples for carbon, N, and P, and 15N to derive a two source mixing model to estimate the fractional contribution of N fixation to the planktonic N pool. We also utilized whole-ecosystem metabolism data from each mesocosm to estimate instantaneous N fixation rates, N demand, and N fixation efficiency. Preliminary data indicate a predictable patterns of diazotrophy across treatments but unexpected patterns of N fixation efficiency relative to N demand by plankton.