Factors controlling nitrogen fixation in temperate seagrass beds

Nitrogen fixation is an ecologically significant process in marine systems because nitrogen is typically the key limiting nutrient controlling productivity. Seagrass beds are often hot spots for nitrogen fixation owing to the mutualistic relationship between seagrass and nitrogen-fixing sulphate-reducing bacteria. The objectives of this study were to: (1) investigate the factors that controlled nitrogen fixation within seagrass beds (Zostera muelleri and Z. nigricaulis) on a system scale in a temperate Australian embayment (Port Phillip Bay), and (2) investigate differences in nitrogen isotope ratios (N-15/N-14, delta N-15) in seagrass and Ulva spp. tissue as a proxy for nitrogen fixation. Nitrogen fixation rates ranged between 3 and 90 mu mol m(-2) h(-1) and were related to plant biomass during both summer and spring, except at a highly nitrogen-enriched site adjacent to a sewage treatment plant outlet. During spring, biomass-specific nitrogen fixation rates were strongly positively related to leaf C: N ratio, suggesting that nitrogen fixation rates increased with nitrogen limitation during the growing season. Leaves of Zostera spp. had delta N-15 values that were consistently depleted by 2.4 +/- 1.5% relative to Ulva spp. collected from the same site. We postulate that this isotopic difference arises as a consequence of Zostera spp. mainly assimilating N from newly mineralized nitrogen within the sediment (vs. a negligible fraction from N fixation), which is isotopically depleted owing to fractionation during nitrogen mineralization.