Freshwater salinization from anthropogenic activities threatens water quality and habitat suitability for many lakes and rivers in North America. Recognizing that salinization is a stress on freshwater environments globally, research on watershed salt transport is necessary for informed management strategies. Prior to this research, there were few studies that examined salt export regimes along a river–lake continuum to investigate the drivers, temporal dynamics, and modulators of freshwater salinization. Here, we use high-frequency in situ monitoring to assess specific conductance–discharge (cQ) relationships, chloride concentrations and fluxes, and the role of lakes in downstream salt transport. The Upper Yahara River Watershed in southern Wisconsin, USA, is a mixed urban and agricultural watershed where the lakes' chloride concentrations have risen from < 5 mg L−1 in the 1940s to > 50–80 mg L−1 in 2021. Our results suggest cQ behavior depends on land use, with urban areas exhibiting more frequent mobilization events during stormflow and agricultural areas exhibiting predominantly dilution dynamics. In addition, chloride loading is driven by hydrology and watershed size whereas concentrations and yields are a function of anthropogenic drivers like urbanization. We demonstrate how an in-network lake attenuates downstream salinity, dampening the hydrologic, anthropogenic, and seasonal patterns observed in rivers upstream of the lake. Importantly, biogeochemical processes in lakes overlay a seasonal signal on salinity that must be considered when investigating temporal dynamics of anthropogenic salinization. This research contributes to understanding of temporal dynamics of salt export through watersheds and can be used to inform management strategies for habitat protection.
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Impact of salinization on lake stratification and spring mixing
Anthropogenic freshwater salinization affects thousands of lakes worldwide, and yet little is known about how salt loading may shift timing of lake stratification and spring mixing in dimictic lakes. Here, we investigate the impact of salinization on mixing in Lakes Mendota and Monona, Wisconsin, by deploying under-ice buoys to record salinity gradients, using an analytical approach to quantify salinity thresholds that prevent spring mixing, and running an ensemble of vertical one-dimensional hydrodynamic lake models (GLM, GOTM, and Simstrat) to investigate the long-term impact of winter salt loading on mixing and stratification. We found that spring salinity gradients between surface and bottom waters persist up to a month after ice-off, and that theory predicts a salinity gradient of 1.3–1.4 g kg-1 would prevent spring mixing. Numerical models project that salt loading delays spring mixing and increases water column stability, with ramifications for oxygenation of bottom waters, biogeochemistry, and lake habitability.
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- PAR ID:
- 10316439
- Date Published:
- Journal Name:
- Limnology and Oceanography Letters
- ISSN:
- 2378-2242
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Scripts, model configurations and outputs to process the data and recreate the figures from Ladwig, R., Rock, L.A, Dugan, H.A. (-): Impact of salinization on lake stratification and spring mixing. This repository includes the setup and output from the lake model ensemble (GLM, GOTM, Simstrat) ran on the lakes Mendota and Monona. Scripts to run the models are located under /numerical and the scripts to process the results for the discussion of the paper are in the top main repository. The scripts to derive the theoretical solution are located under /analytical. Buoy monitoring data are located under /fieldmonitoring. The final figures are located under /figs_HD.
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Abstract Reductions in ice cover duration and earlier ice breakup are two of the most prevalent responses to climate warming in lakes in recent decades. In dimictic lakes, the subsequent periods of spring mixing and summer stratification are both likely to change in response to these phenological changes in ice cover. Here, we used a modeling approach to simulate the effect of changes in latitude on long‐term trends in duration of ice cover, spring mixing, and summer stratification by “moving” a well‐studied lake across a range of latitudes in North America (35.2°N to 65.7°N). We found a changepoint relationship between the timing of ice breakup vs. spring mixing duration on 09 May. When ice breakup occurred before 09 May, which routinely occurred at latitudes < 47°N, spring mixing was longer and more variable; when ice breakup occurred after 09 May at latitudes > 47°N, spring mixing averaged 1 day with low variability. In contrast, the duration of summer stratification showed a relatively slower rate of increase when ice breakup occurred before 09 May (< 47°N) compared to a 109% faster rate of increase when ice breakup was after 09 May (> 47°N). Projected earlier ice breakup can result in important nonlinear changes in the relative duration of spring mixing and summer stratification, which can lead to mixing regime shifts that influence the severity of oxygen depletion differentially across latitudes.
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Abstract Human‐induced salinization increasingly threatens inland waters; yet we know little about the multifaceted response of lake communities to salt contamination. By conducting a coordinated mesocosm experiment of lake salinization across 16 sites in North America and Europe, we quantified the response of zooplankton abundance and (taxonomic and functional) community structure to a broad gradient of environmentally relevant chloride concentrations, ranging from 4 to ca. 1400 mg Cl−L−1. We found that crustaceans were distinctly more sensitive to elevated chloride than rotifers; yet, rotifers did not show compensatory abundance increases in response to crustacean declines. For crustaceans, our among‐site comparisons indicate: (1) highly consistent decreases in abundance and taxon richness with salinity; (2) widespread chloride sensitivity across major taxonomic groups (Cladocera, Cyclopoida, and Calanoida); and (3) weaker loss of functional than taxonomic diversity. Overall, our study demonstrates that aggregate properties of zooplankton communities can be adversely affected at chloride concentrations relevant to anthropogenic salinization in lakes.
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Abstract The salinity structure in an estuary is controlled by time‐dependent mixing processes. However, the locations and temporal variability of where significant mixing occurs is not well‐understood. Here we utilize a tracer variance approach to demonstrate the spatial and temporal structure of salinity mixing in the Hudson River Estuary. We run a 4‐month hydrodynamic simulation of the tides, currents, and salinity that captures the spring‐neap tidal variability as well as wind‐driven and freshwater flow events. On a spring‐neap time scale, salinity variance dissipation (mixing) occurs predominantly during the transition from neap to spring tides. On a tidal time scale, 60% of the salinity variance dissipation occurs during ebb tides and 40% during flood tides. Spatially, mixing during ebbs occurs primarily where lateral bottom salinity fronts intersect the bed at the transition from the main channel to adjacent shoals. During ebbs, these lateral fronts form seaward of constrictions located at multiple locations along the estuary. During floods, mixing is generated by a shear layer elevated in the water column at the top of the mixed bottom boundary layer, where variations in the along channel density gradients locally enhance the baroclinic pressure gradient leading to stronger vertical shear and more mixing. For both ebb and flood, the mixing occurs at the location of overlap of strong vertical stratification and eddy diffusivity, not at the maximum of either of those quantities. This understanding lends a new insight to the spatial and time dependence of the estuarine salinity structure.
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1002/lol2.10215
