Nitrogen fixation in restored eelgrass meadows

Abstract

Biological nitrogen (N 2 ) 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 determined whether restoration of the eelgrass Zostera marina L. in a shallow coastal bay facilitated increasing rates of N 2 fixation as the meadows aged. Rates of N 2 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 N 2 fixation was enhanced as the meadows aged. Rates of N 2 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 ). Heterotrophic epiphyte bacteria fixed approximately 90% of the total N 2 in Z. marina meadows of both age classes. Both sediment and epiphyte N 2 fixation were strongly related to Z. marina density and sediment organic content, suggesting that shoot density increases the positive feedback of plant presence on N 2 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.

The balance between nitrogen fixation and denitrification on vegetated and non‐vegetated intertidal sediments

“…The percentage contribution of nitrogen fixation to the overall nitrogen demand of the Zostera muelleri meadows (up to 32%; Fig. a) is in agreement with the range reported by Cole and McGlathery () of up to 29.8%. This further supports the hypothesis that nitrogen fixation was a key process in meeting the nitrogen demand of the Western Port environment.…”

Non‐seagrass carbon contributions to seagrass sediment blue carbon

“…). Past studies have observed OM and SOC concentration peaks between 3 cm and 6 cm below the sediment–water interface in this meadow, the zone of maximum Z. marina root and rhizome biomass (Cole and McGlathery ; Greiner et al ; Oreska et al ). The sediment samples collected in this study exactly captured this 3–6 cm bed depth interval.…”

“…The macroalgae δ 15 N range is similar to that for S. alterniflora , which was not a major contributor at these sites. These four sites hint at another SOC source, possibly N‐fixers in the microphytobenthos or epiphyte communities (Cole and McGlathery ), which would explain why the site δ 15 N values were biased towards atmospheric δ 15 N. The δ 15 N ratios at these sites could also be attributable to buried phytoplankton; however, the phytoplankton collected from this system by Hondula and Pace () yielded lower δ 13 C ratios (< −25‰).…”

Dissolved inorganic nitrogen uptake kinetics and δ15N of Zostera marina L. (eelgrass) in a coastal lagoon with oyster aquaculture and upwelling influence

“…Additional currently unpublished data shows that the relatively low δ 15 N signal in Z. marina plants at i-BF is a persistent pattern, indicating that these plants incorporate a higher proportion of isotopically depleted N-compounds into biomass, probably supplied by nitrogen fixation . In the sediments, N-fixation which can occur at high rates in seagrass meadows (Welsh, 2000;Romero et al, 2006;Cole and McGlathery, 2012), could reduce the isotopic signal of plant tissues. Alternatively, plants at i-BF may also be supplied with isotopically light DIN (and DON) exported during ebbing tides from the adjacent salt marsh where N-fixation may be particularly intense (Moseman, 2007).…”