Disconnectivity matters: the outsized role of small ephemeral wetlands in landscape-scale nutrient retention

Abstract

Wetlands protect downstream waters by filtering excess nitrogen (N) generated from agricultural and urban activities. Small ephemeral wetlands, also known as geographically isolated wetlands (GIWs), are hotspots of N retention but have received fewer legal protections due to their apparent isolation from jurisdictional waters. Here, we hypothesize that the isolation of the GIWs make them more efficient N filters, especially when considering transient hydrologic dynamics. We use a reduced complexity model with thirty years of remotely sensed monthly wetland inundation levels in 3,700 GIWs across eight wetlandscapes in the US to show how consideration of transient hydrologic dynamics can increase N retention estimates by up to 130%, with greater retention magnification for the smaller wetlands. This effect is more pronounced in semi-arid systems such as the prairies in North Dakota, where transient assumptions lead to 1.8 times more retention, compared to humid landscapes like the North Carolina Pocosins where transient assumptions only lead to 1.4 times more retention. Our results highlight how GIWs have an outsized role in retaining nutrients, and this service is enhanced due to their hydrologic disconnectivity which must be protected to maintain the integrity of downstream waters.

A Random Forest in the Great Lakes: Stream Nutrient Concentrations Across the Transboundary Great Lakes Basin

“…Interestingly, the regions with low wetland density are also regions with a high percent developed area, suggesting that the presence of wetlands in more agricultural and urban areas has the potential to provide the greatest ecosystem services with respect to water quality improvements. Wetlands remove nutrients and protect downstream waters (Cheng & Basu, 2017; Cheng et al., 2020, 2022; Creed et al., 2017; Golden et al., 2017; Hansen et al., 2018; Jordan et al., 2011; Marton et al., 2015; Thorslund et al., 2017; Van Meter & Basu, 2015); however empirical studies at the watershed scale often find little or no influence of wetland cover on riverine nitrate due to confounding effects of land use and other variables (Powers et al., 2013; Strayer et al., 2003). The PD plots control for the variability of the other variable and thus allow us to clarify the role of wetlands of dissolved N and P concentrations in streams.…”

Patterns of Wetland Hydrologic Connectivity Across Coastal‐Plain Wetlandscapes

“…As such, spatial variation in surface connectivity patterns is relevant to conservation planning. Likewise, slow subsurface hydrologic connectivity extends residence times that enabled further biogeochemical processing (Marton et al., 2015) including landscape nutrient retention (Cheng & Basu, 2017; Cheng et al., 2022). In short, the heterogeneity in connectivity within and between wetlandscapes documented here impacts a multitude of functions considered under the Clean Water Act (Creed et al., 2017; Lane et al., 2018; Sullivan et al., 2020; Wade et al., 2022; Ward et al., 2023).…”

“…Surface connectivity impacts the dynamic water balance and controls retention and export of solutes (Ameli & Creed, 2017; Jawitz & Mitchell, 2011; Smith et al., 2018), nutrients, contaminants, and dispersal of organisms, such as hydrochoric seeds and fish (Gurnell et al., 2008; Semlitsch & Bodie, 2003). For example, variability of wetland surface connectivity is a factor influencing chloride accumulation (Thorslund et al., 2018), and a strong regulator of landscape nutrient retention (Cheng & Basu, 2017; Cheng et al., 2022). However, because surface connectivity between depressional wetlands and adjacent water bodies is not persistent, these “geographically isolated wetlands” have been assumed to lack meaningful contributions to the hydrological and biogeochemical functions of downstream “waters of the United States”, with crucial implications for shifting U.S. Federal protections under the Clean Water Act (Creed et al., 2017; Sullivan et al., 2020; Wade et al., 2022; Ward et al., 2023).…”

Reviews and Syntheses: Variable Inundation Across Earth’s Terrestrial Ecosystems

“…Despite their relatively small size, ponds can have considerable variability in both community composition and in biogeochemical processes, in part due to differences in inundation regimes, where pond margins are more likely to be more frequently desiccated for longer periods than central regions (Reverey et al 2018). Models that explicitly incorporate remotely sensed variable inundation predict that ephemeral systems with shorter hydroperiods retain nitrogen at greater rates than larger systems with less variable inundation and longer hydroperiods, particularly in semi-arid regions like the Prairie Potholes of the North American northern Great Plains and playas in the south-central United States (Cheng et al 2023). In addition, research suggests reproduction is largely impacted by inundation.…”