Daniel L. McLaughlin

Do geographically isolated wetlands influence landscape functions

Geographically isolated wetlands (GIWs), those surrounded by uplands, exchange materials, energy, and organisms with other elements in hydrological and habitat networks, contributing to landscape functions, such as flow generation, nutrient and sediment retention, and biodiversity support. GIWs constitute most of the wetlands in many North American landscapes, provide a disproportionately large fraction of wetland edges where many functions are enhanced, and form complexes with other water bodies to create spatial and temporal heterogeneity in the timing, flow paths, and magnitude of network connectivity. These attributes signal a critical role for GIWs in sustaining a portfolio of landscape functions, but legal protections remain weak despite preferential loss from many landscapes. GIWs lack persistent surface water connections, but this condition does not imply the absence of hydrological, biogeochemical, and biological exchanges with nearby and downstream waters. Although hydrological and biogeochemical connectivity is often episodic or slow (e.g., via groundwater), hydrologic continuity and limited evaporative solute enrichment suggest both flow generation and solute and sediment retention. Similarly, whereas biological connectivity usually requires overland dispersal, numerous organisms, including many rare or threatened species, use both GIWs and downstream waters at different times or life stages, suggesting that GIWs are critical elements of landscape habitat mosaics. Indeed, weaker hydrologic connectivity with downstream waters and constrained biological connectivity with other landscape elements are precisely what enhances some GIW functions and enables others. Based on analysis of wetland geography and synthesis of wetland functions, we argue that sustaining landscape functions requires conserving the entire continuum of wetland connectivity, including GIWs.

A significant nexus: Geographically isolated wetlands influence landscape hydrology

Recent U.S. Supreme Court rulings have limited federal protections for geographically isolated wetlands (GIWs) except where a “significant nexus” to a navigable water body is demonstrated. Geographic isolation does not imply GIWs are hydrologically disconnected; indeed, wetland-groundwater interactions may yield important controls on regional hydrology. Differences in specific yield (Sy) between uplands and inundated GIWs drive differences in water level responses to precipitation and evapotranspiration, leading to frequent reversals in hydraulic gradients that cause GIWs to act as both groundwater sinks and sources. These reversals are predicted to buffer surficial aquifer dynamics and thus base flow delivery, a process we refer to as landscape hydrologic capacitance. To test this hypothesis, we connected models of soil moisture, upland water table, and wetland stage to simulate hydrology of a low-relief landscape with GIWs, and explored the influences of total wetland area, individual wetland size, climate, and soil texture on water table and base flow variation. Increasing total wetland area and decreasing individual wetland size substantially decreased water table and base flow variation (e.g., reducing base flow standard deviation by as much as 50%). GIWs also decreased the frequency of extremely high and low water tables and base flow deliveries. For the same total wetland area, landscapes with fewer (i.e., larger) wetlands exhibited markedly lower hydrologic capacitance than those with more (i.e., smaller) wetlands, highlighting the importance of small GIWs to regional hydrology. Our results suggest that GIWs buffer dynamics of the surficial aquifer and stream base flow, providing an indirect but significant nexus to the regional hydrologic system.

Integrating geographically isolated wetlands into land management decisions

Wetlands across the globe provide extensive ecosystem services. However, many wetlands - especially those surrounded by uplands, often referred to as geographically isolated wetlands (GIWs) - remain poorly protected. Protection and restoration of wetlands frequently requires information on their hydrologic connectivity to other surface waters, and their cumulative watershed-scale effects. The integration of measurements and models can supply this information. However, the types of measurements and models that should be integrated are dependent on management questions and information compatibility. We summarize the importance of GIWs in watersheds and discuss what wetland connectivity means in both science and management contexts. We then describe the latest tools available to quantify GIW connectivity and explore crucial next steps to enhancing and integrating such tools. These advancements will ensure that appropriate tools are used in GIW decision making and maintaining the important ecosystem services that these wetlands support.

Drying Rates of Ephemeral Wetlands: Implications for Breeding Amphibians

Ephemeral wetlands provide breeding habitat for many amphibian species, and wetland hydrology plays a crucial role in determining amphibian breeding success. We discuss the potential influence of recession rates (i.e., rate of water level decline) and empirically evaluate them in wetlands inhabited by the endangered reticulated flatwoods salamander (Ambystoma bishopi). Rapid water level declines are potentially problematic for reticulated flatwoods salamanders because this species has a long development period, with metamorphosis generally occurring from March to May when groundwater losses are combined with high evapotranspiration rates. To evaluate magnitude, variability, and drivers of recession rates, we monitored water levels in 33 wetlands in the Florida panhandle and examined recession rates during the flatwoods salamander reproductive period. After controlling for the effects of specific yield, standardized recession rates were, on average, 3.9 times daily potential evapotranspiration rates, suggesting that groundwater fluxes are an important driver of water level declines in these wetlands. Standardized recession rates were variable across the landscape and increased with decreasing wetland size, indicating that larger wetlands are often hydrologically more suitable for flatwoods salamanders. This work points to these and other controls on wetland recession rates and their role in regulating amphibian reproductive success.

Estimating restorable wetland water storage at landscape scales

Globally, hydrologic modifications such as ditching and subsurface drainage have significantly reduced wetland water storage capacity (i.e., volume of surface water a wetland can retain) and consequent wetland functions. While wetland area has been well documented across many landscapes and used to guide restoration efforts, few studies have directly quantified the associated wetland storage capacity. Here, we present a novel raster-based approach to quantify both contemporary and potential (i.e., restorable) storage capacities of individual depressional basins across landscapes. We demonstrate the utility of this method by applying it to the Delmarva Peninsula, a region punctuated by both depressional wetlands and drainage ditches. Across the entire peninsula, we estimated that restoration (i.e., plugging ditches) could increase storage capacity by 80%. Focusing on an individual watershed, we found that over 59% of restorable storage capacity occurs within 20 m of the drainage network, and that 93% occurs within 1 m elevation of the drainage network. Our demonstration highlights widespread ditching in this landscape, spatial patterns of both contemporary and potential storage capacities, and clear opportunities for hydrologic restoration. In Delmarva and more broadly, our novel approach can inform targeted landscape-scale conservation and restoration efforts to optimize hydrologically mediated wetland functions.

Stream phosphorus dynamics of minimally impacted coastal plain watersheds

1636 Copyright

Hydrologic implications of smoldering fires in wetland landscapes

Smoldering fires in organic soils have negative effects on air quality and motorist safety as well as global implications from their release of large quantities of refractory C. However, the ecological implications of their occurrence are relatively unexplored despite their potential importance to the management of wetland ecosystems. We developed a conceptual model of the ecohydrologic implications of peat-consuming fires that explores the interactive effects of fire, hydrology, and C dynamics on hydrology. We modify an existing wetland hydrology model parameterized with climate, soil, and spatial data from a low-relief region in southern Florida (USA) to explore hypothesized pyrogeomorphic changes to upland water table elevation, wetland inundation (depth and hydroperiod), and groundwater exchange as a function of fire severity (area and depth of burn). Smoldering fires increase hydroperiod and storage in organic soils in burned wetlands by changing soil elevation. After fire, negative feedbacks to fire occurrence are likely because of increased hydroperiods in burned areas. However, adjacent, unburned wetland areas and uplands may experience drier conditions that increase fire frequency in distal locations. Simulation results indicate that increasing the area of soil combustion or depth of burn increases wetland hydroperiod, flooding depths, and groundwater exchange between wetlands and surrounding uplands. Additional field data characterizing fire effects on organic soil elevations and wetland bathymetry are needed, but the model supports our hypothesis about the effects of soil-consuming fires on hydrology and habitat, and these results will inform future work on the ecological role of peat-consuming fires.

Forested versus herbaceous wetlands: Can management mitigate ecohydrologic regime shifts from invasive emerald ash borer?

Wetlands self-organize through reciprocal controls between vegetation and hydrology, but external disturbance may disrupt these feedbacks with consequent changes to ecosystem state. Imminent and widespread emerald ash borer (EAB) infestation throughout North American forested wetlands has raised concern over possible ecosystem state shifts (i.e., wetter, more herbaceous systems) and loss of forest function, calling for informed landscape-scale management strategies. In response, we employed a large-scale manipulative study to assess the ecohydrologic response of black ash wetlands to three alternative EAB management strategies: 1) a do-nothing approach (i.e., simulated EAB infestation via tree girdling), 2) a preemptive, complete harvesting approach (i.e., clearcut), and 3) an overstory replacement approach via group selection. We analyzed six years of daily water table and evapotranspiration (ET) dynamics in six blocks comprising black ash wetlands (controls) and management strategy treatments to quantify potential for hydrologic change and subsequent recovery. In both the do-nothing approach and complete harvesting approach, we found persistent changes in hydrologic regime defined by shallower water tables and lower ET rates coupled with increased herbaceous vegetation growth, indicating ecosystem state shifts driven by vegetation-water table interactions. The do-nothing approach showed the least hydrologic recovery after five years, which we attribute to reduction in overstory transpiration as well as greater shade (via standing dead trees) that reduces open water evaporation and herbaceous layer transpiration compared to complete harvesting. We found no evidence of ecohydrologic disturbance in the overstory replacement approach, highlighting its potential as a management strategy to preserve forested wetland habitat if periodically executed over time before EAB infestation. Although the scale of potential disturbance is daunting, our findings provide a baseline assessment for forest managers to develop preemptive mitigation strategies to address the threat of EAB to ecological functions in black ash wetlands.