Nitrogen fixation in restored eelgrass meadows

Abstract

Biological nitrogen (N2) fixation is the primary input of new nitrogen (N) to marine systems, and is important in meeting the N demands of primary producers. In this study, we deter- mined whether restoration of the eelgrass Zostera marina L. in a shallow coastal bay facilitated increasing rates of N2 fixation as the meadows aged. Rates of N2 fixation were measured in a system that had been devoid of eelgrass following local extinction in the 1930s until restoration by seeding began in 2001. Restored meadows of different ages were compared to nearby bare sediment sites during summer peak metabolism over 2 yr. Nutrient addition by N2 fixation was enhanced as the meadows aged. Rates of N2 fixation in the older (7 to 8 yr old) meadows were 2.7 times more than the younger (2 to 3 yr old) meadows (average 390 and 146 µmol N m �2 d �1 , respectively), and 28 times more than bare sediments (average 14 µmol N m �2 d �1 ). Heterotrophic epiphyte bacteria fixed approximately 90% of the total N2 in Z. marina meadows of both age classes. Both sediment and epiphyte N2 fixation were strongly related to Z. marina density and sediment organic content, suggesting that shoot density increases the positive feedback of plant presence on N2 fixation through the release of organic carbon exudates into the rhizosphere and phyllosphere, and the build up of sediment organic matter also increases. The N provided through fixation represented a large fraction (20.5 to 30%) of the total N demand to support eelgrass aboveground growth during this period of peak summertime production.