Solute evidence for hydrological connectivity of geographically isolated wetlands

Abstract

Hydrological connectivity describes the water-mediated transfer of mass, energy, and organisms between landscape elements and is the foundation for understanding how individual elements such as wetlands and streams integrate to support ecosystem services and nature-based solutions in the landscape. Hydrological connectivity of geographically isolated wetlands (GIWs)-that is, wetlands without persistent surface water connections-is particularly poorly understood. To better understand GIW hydrological connectivity, we use a novel chloride mass-balance approach to quantify the local runoff generation (defined as precipitation minus evapotranspiration, assuming negligible long-term water storage) for 260 GIW subcatchments across North America. To evaluate hydrological connectivity, we compare the estimated local runoff from GIW subcatchments with the catchment-average runoff. These comparisons provide three novel insights regarding the magnitude and variability of GIW hydrological connectivity. First, across 10 study regions, GIW subcatchments generate runoff at 120% of the mean catchment rate, implying they are well-connected elements of the larger hydrologic landscape. Second, there is substantial heterogeneity in runoff generation among GIW subcatchments, which may enable support for a wide array of ecosystem functions and services. Finally, observed heterogeneity in runoff generation was largely uncorrelated to simple linear geographic predictors, indicating that GIW landscape position cannot reliably predict hydrological connectivity. In stark contrast to a priori legal assumptions that GIWs exhibit low or no hydrological connectivity, our results suggest that GIW subcatchments are active landscape features in runoff generation. KEYWORDS ecosystem services, geographically isolated wetlands, hydrological connectivity, landscape management, nature-based solutions

Wetland Connectivity Thresholds and Flow Dynamics From Stage Measurements

“…Additional work is needed to link wetland and landscape‐scale dynamics in these settings and to draw contrast with settings where groundwater flow rates are sufficient to preclude surface spill dynamics, such as cypress domes in North Florida (McLaughlin & Cohen, ) and Nebraska sandhill lakes (Winter, ). We note that these landscapes, where connectivity is predominantly in the subsurface, do not necessarily generate less flow (Thorslund et al, ), which implies their support for a different portfolio of landscape functions. Many studies (e.g., McLaughlin et al, ; Neff & Rosenberry, ; Winter & LaBaugh, ) highlight the influences of subsurface connectivity on landscape‐scale hydrology and function; we assert that the CFS method is applicable to all depressional wetland systems and will yield insights into the timing and magnitude of both surface and subsurface connectivity.…”

“…Indeed, temporal inequality of watershed exports strongly suggests periodic but important “hot moments” of surface water and solute transport (Jawitz & Mitchell, ). For example, variability in periodic wetland surface connections is a primary control on chloride patterns both within wetlandscapes and in downstream receiving waters (Thorslund et al, ). However, the absence of prolonged surface connectivity between depressional wetlands and nearby streams has been interpreted as the absence of a meaningful role in regulating the physical, chemical, or biological integrity of downstream waters, which has led to limited U.S. Federal protections (Creed et al, ).…”

“…Quantifying water and material fluxes from individual wetlands in low‐gradient landscapes has been a challenge due to heterogeneity among wetlands and methodological constraints. Site attributes, such as topography, soil, and bedrock permeabilities (Winter, ), and temporal variation in climate control the mode (surface vs. subsurface) and magnitude of hydrologic exchange (Haque et al, ; Thorslund et al, ). As a result, accurate measurements of subsurface and surface fluxes often require dense distributed well networks, site‐level soil property characterization, and detailed topographic information, the combination of which is often cost or time prohibitive.…”

Modeling Connectivity of Non‐floodplain Wetlands: Insights, Approaches, and Recommendations

“…), a suite of environmental tracer data (e.g., Fossey and Rousseau ; Thorslund et al. ), and nutrient isotope data (Kendall et al. ), as well as qualitative data to conceptualize both expert knowledge and local residents’ experiences (e.g., Seibert and McDonnell ).…”

“…Recent improvements in high-resolution sensors (Epting et al 2018;Haque et al 2018), environmental tracer analyses ; Thorslund et al 2018), and remote sensing technology (DeVries et al 2017;Vanderhoof et al 2017a) have all significantly increased our ability to characterize hydrologic fluxes between NFWs and other watershed components. These empirical characterizations are important for developing a fundamental understanding of the system in question (see Burt and McDonnell 2015) but are often limited in spatial domain and measurement period.…”