Within fluvial networks, lakes can be sinks or sources of dissolved organic carbon (DOC) and nutrients, yet the controls over sink‐source behavior remain unclear. We investigated the influence that an in‐network lake exerted on DOC and nutrient export. Our investigation consisted of: (1) injecting a conservative tracer to determine lake travel times and flow paths; (2) sampling lake inflow, outflow, and surrounding groundwater to determine water and nutrient budgets; and, (3) sampling internal lake profiles to ascertain in‐lake physico‐chemical patterns through time. Conservative tracer data indicated considerable in‐lake retention and combined with inflow‐outflow discharge measurements revealed a decoupling of kinematic and solute pulses. Nitrate (NO3) was the dominant form of dissolved inorganic nitrogen (DIN) at lake inflow whereas ammonium (NH4) became the dominant component at lake outflow. The lake was a sink for NO3‐N and PO4, but a source for NH4‐N, DON, TDN, and DOC. We observed hydrologic controls on DOC concentrations and export patterns, but redox controls on DIN dynamics. Our results indicate that lakes within fluvial networks can be sources of dissolved organic material and reduced nitrogen (NH4) while simultaneously being sinks for NO3‐N and PO4‐P. Determining controls on sink‐source behavior and the cumulative effect of lakes on DOC and nutrient budgets is a necessary first step toward improved understanding of the role of lakes in network‐ to regional‐scale dynamics.
This content will become publicly available on October 30, 2024
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.
more » « less- Award ID(s):
- 1637630
- NSF-PAR ID:
- 10471718
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Biogeosciences
- Volume:
- 128
- Issue:
- 11
- ISSN:
- 2169-8953
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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