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As part of a long-term study on the effects of nitrogen (N) loading in a shallow temperate lagoon, we measured rates of N₂fixation 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 N₂fixation estimates for any seagrass system and one of the very few reported for the phyllosphere in a temperate system. Mean daily N₂fixation was estimated from light and dark measurements using the acetylene reduction assay intercalibrated using both incorporation of¹⁵N₂into biomass and a novel application of the N₂:Ar method. Surprisingly, despite a large inorganic N input from a N-contaminated groundwater plume, epiphytic N₂fixation 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 N₂fixation 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⁻²d⁻¹), which we suggest is in part related to climatic variation. We estimate that input from phyllosphere N₂fixation over the study period contributes on average an additional 20% to the total daily N load per area within the seagrass meadow.

Reduced light is one of the primary threats to seagrass meadows in the coming decades, with reduced light reaching the benthos due to eutrophication. We assessed a multispectral photography technique using near‐infrared photography to estimate chlorophyll content in the seagrassZostera marina. Using near‐infrared and red wavelength cameras in the lab environment, we measured normalized difference vegetation index (NDVI) in photographs of sampled seagrass leaves. In samples taken from three different environments, we found a positive correlation between lab‐based NDVI and chlorophyll content, with variation attributable to leaf age. In samples grown under different light conditions, we found high levels of NDVI associated with lower light possibly due to seagrass photoacclimation. This method may be used in addition to existing seagrass monitoring methods to collect data on seagrass photic status and estimate chlorophyll content, and detect possible light limitation due to turbidity or high epibiota cover. The relatively low cost and time required for this method may make it useful where researchers are already collecting and imaging seagrass as part of routine monitoring.