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Abstract The tidal tributaries of the lower Chesapeake Bay experience seasonally recurring harmful algal blooms and the significance of submarine groundwater discharge (SGD) as a nutrient vector is largely unknown. Here, we determined seasonal SGD nutrient loads in two tributaries with contrasting hydrodynamic conditions, river‐fed (York River) vs. tidally dominated (Lafayette River). Radon surveys were performed in each river to quantify SGD at the embayment‐scale during spring and fall 2021. Total SGD was determined from a222Rn mass balance and Monte Carlo simulations. Submarine groundwater discharge rates differed by a factor of two during spring (Lafayette = 11 ± 17 cm d−1; York = 6 ± 10 cm d−1) and a factor of six during fall (Lafayette = 19 ± 27 cm d−1; York = 3 ± 7 cm d−1). Groundwater N concentrations and fluxes varied seasonally in the York (4–7 mmol N m−2d−1). In the Lafayette River, seasonal N fluxes (22–37 mmol N m−2d−1) were driven by seasonal water exchange rates, likely due to recurrent saltwater intrusion. Submarine groundwater discharge–derived nutrient fluxes were orders of magnitude greater than riverine inputs and runoff in each system. Additionally, sediment N removal by denitrification and anaerobic ammonium oxidation would only remove ~ 1–11% of dissolved inorganic nitrogen supplied through SGD. The continued recurrence of harmful algal blooms in the Bay's tidal tributaries may be indicative of an under‐accounting of submarine groundwater‐borne nutrient sources. This study highlights the importance of including SGD in water quality models used to advise restoration efforts in the Chesapeake Bay region and beyond.
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Nitrogen fixation associated with epiphytes on the seagrass Zostera marina in a temperate lagoon with moderate to high nitrogen loads
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.
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- PAR ID:
- 10493905
- Publisher / Repository:
- Springer
- Date Published:
- Journal Name:
- Biogeochemistry
- Volume:
- 166
- Issue:
- 3
- ISSN:
- 0168-2563
- Page Range / eLocation ID:
- 211 to 226
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1007/s10533-023-01083-2
