The Impact of Agricultural Wetland Restoration on Adjacent Temporary and Perennial Streams

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Wetlands are known for the ecosystem services they provide, including hydrologic storage, sediment retention, nutrient processing, habitat provision, and carbon sequestration. Since European settlement, however, it is estimated that > 50% of wetlands within the conterminous United States have been lost, with a majority of loss attributed to drainage of freshwater wetlands for agriculture. In efforts to offset loss and restore ecosystem services, agricultural wetland restoration has become common. How wetland restoration impacts adjacent stream ecosystem structure and function, however, is poorly understood. Additionally, many freshwater wetlands have historically been considered geographically isolated and disconnected from adjacent surface waters. Recent U.S. Supreme Court rulings have called into question the jurisdictional status of so-called isolated wetlands and non-perennial streams, making investigation of wetland-stream connectivity particularly critical.

Comparing native forested, historical (i.e., prior-converted cropland), and hydrologically restored freshwater wetlands within the headwaters of the Choptank River watershed (Delmarva Peninsula, Maryland, USA), I examined the impact of agricultural wetland restoration on within-wetland structure and function and influences on adjacent temporary and perennial streams. In Chapter 1, I present evidence that recently restored wetland soils, although similar to historical wetland soils in physicochemical properties and denitrification potential, may be sediment and nutrient sinks. Chapter 2 shows that so-called isolated Delmarva bay wetlands may in fact be intimately linked to perennial stream networks via temporary stream flow and that land use influences connectivity. In Chapter 3, I investigate the role of temporary stream sediment drying and wetting on denitrification potential in restored and forested wetland-stream pairs and find that alterations in flow regime, a likely outcome of both land use change and climate change, may alter the capacity of temporary streams to denitrify. Chapter 4 considers the impact of cultivation on perennial stream dissolved organic matter (DOM) quantity and quality, and suggests agricultural wetland restoration may be a tool to recover more natural fluvial DOM.

Results from this research suggest geographically isolated wetlands may be both hydrologically and ecologically linked to adjacent temporary and perennial streams and that cultivation and subsequent restoration of historical wetlands exerts strong influence on these connections.