Nitrogen fixation associated with epiphytes on the seagrass Zostera marina in a temperate lagoon with moderate to high nitrogen loads

Abstract

As part of a long-term study on the effects of nitrogen (N) loading in a shallow temperate lagoon, we measured rates of N₂ fixation associated with seagrass (Zostera marina) epiphytes during the summer from 2005 to 2019, at two sites along a gradient from where high N groundwater enters the system (denoted SH) to a more well-flushed outer harbor (OH). The data presented here are the first such long-term N₂ fixation estimates for any seagrass system and one of the very few reported for the phyllosphere in a temperate system. Mean daily N₂ fixation was estimated from light and dark measurements using the acetylene reduction assay intercalibrated using both incorporation of ¹⁵N₂ into biomass and a novel application of the N₂:Ar method. Surprisingly, despite a large inorganic N input from a N-contaminated groundwater plume, epiphytic N₂ fixation rates were moderately to very high for a seagrass system (OH site 14-year mean of 0.94 mmol N m⁻² d⁻¹), with the highest rates (2.6 mmol N m⁻² d⁻¹) measured at the more N-loaded eutrophic site (SH) where dissolved inorganic N was higher and soluble reactive phosphorus was lower than in the better-flushed OH. Over 95% of the total N₂ fixation measured was in the light, suggesting the importance of cyanobacteria in the epiphyte assemblages. We observed large inter-annual variation both within and across the two study sites (range from 0.1 to 2.6 mmol N fixed m⁻² d⁻¹), which we suggest is in part related to climatic variation. We estimate that input from phyllosphere N₂ fixation over the study period contributes on average an additional 20% to the total daily N load per area within the seagrass meadow.