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Abstract Terrestrial groundwater travels through subterranean estuaries before reaching the sea. Groundwater‐derived nutrients drive coastal water quality, primary production, and eutrophication. We determined how dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), and dissolved organic nitrogen (DON) are transformed within subterranean estuaries and estimated submarine groundwater discharge (SGD) nutrient loads compiling > 10,000 groundwater samples from 216 sites worldwide. Nutrients exhibited complex, nonconservative behavior in subterranean estuaries. Fresh groundwater DIN and DIP are usually produced, and DON is consumed during transport. Median total SGD (saline and fresh) fluxes globally were 5.4, 2.6, and 0.18 Tmol yr−1for DIN, DON, and DIP, respectively. Despite large natural variability, total SGD fluxes likely exceed global riverine nutrient export. Fresh SGD is a small source of new nutrients, but saline SGD is an important source of mostly recycled nutrients. Nutrients exported via SGD via subterranean estuaries are critical to coastal biogeochemistry and a significant nutrient source to the oceans.
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Small Reservoirs as Nitrogen Transformers: Accounting for Seasonal Variability in Inorganic and Organic Nitrogen Processing
Abstract Anthropogenic nitrogen (N) inputs to the landscape have serious consequences for inland and coastal waters. Reservoirs are effective at mitigating downstream N fluxes but measurements have generally focused on large reservoirs and have not considered seasonal variability or all N forms. In this study, we conducted an N mass balance in eight small reservoirs (surface area <0.55 km2) in coastal New England over annual time periods, including both inorganic and organic forms of N. We found that small reservoirs have high capacity for dissolved inorganic N (DIN) retention during low and moderate discharge, but are roughly in balance for DIN at higher discharge. Because proportional DIN retention occurred when N inputs were at their lowest, their effect on downstream N fluxes is small over annual time frames. Further, dissolved organic N (DON) was also evident during low flow late in the warm season. Accounting for DON production, the net effect of reservoirs on total dissolved N (TDN) fluxes was limited. These transformations between inorganic and organic N should be considered when evaluating the effect of small reservoirs on TDN fluxes over seasonal and annual timescales. With dam removal becoming a common solution to aging, unsafe dams, their ability to retain or produce N must be scrutinized at longer time scales while accounting for the complete N pool to better comprehend the effect their reservoirs have on downstream waters.
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- Award ID(s):
- 1637630
- PAR ID:
- 10485013
- Publisher / Repository:
- AGU
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Biogeosciences
- Edition / Version:
- 1.0
- Volume:
- 128
- Issue:
- 11
- ISSN:
- 2169-8953
- Page Range / eLocation ID:
- 1-18
- Subject(s) / Keyword(s):
- nitrogen, small streams
- Format(s):
- Medium: X Size: 2MB Other: pdf/A
- Size(s):
- 2MB
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
-
Accepted Manuscript
- Journal Article:
- https://doi.org/10.1029/2023JG007635
