Long-term nitrogen concentration saturation in river corridors weakens removal capacity and increases regional export
Abstract
This summer was marked by record-sized hypoxic zones in estuaries and widespread beach closures following flood-driven toxic algal blooms, reminding us that excess nutrient export is resisting extensive management efforts and remains a serious water resource and economic concern. Nutrient storage legacies have emerged as a fundamental reason why conservation efforts such as riparian setbacks and restoration have shown marginal reduction in annual loading to downstream waters. Our study suggests that another underlying reason, as supported by a limited number of field studies, is increased concentration and reduced uptake efficiency that weaken the removal capacity of the river corridor that regulates nutrient delivery from headwaters to coasts. Here we estimate the nitrogen concentration saturation threshold and its effects on long-term regional and coastal delivery in the Northeastern United States, revealing a 44% concentration-induced reduction in river corridor removal capacity. The weakened capacity caused a 11% increase in the predicted long-term delivery of riverine nitrogen from urban and agricultural watersheds compared to estimates using first-order reaction kinetic assumptions. Our results suggest that nitrogen uptake falls below a first-order process at a surprisingly low chronic in-stream threshold concentration of 0.5 mg N L-1. This low concentration indicates that, near the threshold, the health of a river corridor is extremely sensitive to small increases in nitrogen loading. However, it also suggests that even modest mitigation of nitrogen sources to river corridors can cause a self-reinforcing boost to nitrogen removal efficiency and substantially reduce coastal delivery. Accounting for nitrogen concentration saturation will improve long-term forecasting of water quality changes and prioritization of specific management alternatives.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.H23A..03S
- Keywords:
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- 1803 Anthropogenic effects;
- HYDROLOGY;
- 1831 Groundwater quality;
- HYDROLOGY;
- 1834 Human impacts;
- HYDROLOGY;
- 1871 Surface water quality;
- HYDROLOGY