Seasonal phosphorus and nitrogen dynamics of open water during restoration of the Wood River Wetland, Klamath Falls, Oregon, USA

摘要

Draining wetlands for agricultural production is a global phe-nomenon that has often led to impaired ecosystem function, loss of connectivity with adjacent waters, and water quality impacts from agricultural runoff. Loss of wetlands shortens the time it takes rainfall to move through the catchment, contributes to soil erosion, and hastens nutrient and pollutant release into water bodies. In the United States, 22 states have lost 50% or more of their original wetlands in the last 2 centuries (DAHL 1990). Though efforts to conserve wetlands have increased, restoration falls short of loss rates. Over the past 50 years, approximately 30 000 ac of wetland adjacent to Upper Klamath Lake and connected Agency Lake (Fig. 1) have been diked and drained for livestock grazing and crop cultivation. Water quality in Upper Klamath Lake has simultaneously declined, and phosphorus (P) inputs from rivers that once flowed through wetland have resulted in large-scale algae blooms (predominantly Aphanizomenon flos-aquae), periodically depleting O_2 levels below those required by fish, including the endangered short-nosed (Chasmistes brevirosIris) and Lost River suckers (Deltistes luxatus; SNYDER & MORACE 1997). Wetland draining has also lowered the water table, which has increased air and oxygenated-water movement through subsurface soils facilitating aerobic peat decomposition and release of carbon (C), nitrogen (N), and P (SNYDER & MORACE 1997). Land surrounding the north end of the lakes has sub-sided 4-5 feet, requiring levees to prevent flooding of drained wetlands. A resource management plan to restore the Wood River Wetland was initiated in 1996 to provide long-term improvement in water quality and quantity entering Agency Lake and restore and enhance wetland habitat (U.S. BUREAU OF LAND MANAGEMENT 2005). In 2003 to 2005, we investigated the spatial and temporal variations in water chemistry during the initial phase of wetland recovery to assess its role as a nutrient source or sink. During the study, water flow through the wetland was nominal, and surface-water nutrients accumulated from sediment release and suggest ecosystem function had not fully recovered to process the large quantities of standing nutrients in the wetland.