Effects of Extreme Climate on Nutrient and Sediment Retention in Natural Wetlands.

The effects of climatic extremes on the functioning of created and restored wetlands can be elucidated by examining the responses of natural wetland ecosystems.  Here I describe examples of extreme climate effects on nutrient and sediment retention in wetland ecosystems across the southeastern U.S. coastal plain that offer general insight into the processes controlling wetland function.  In Everglades wetlands, downstream transport of suspended sediment and nutrients occurs slowly due to the very slow water velocities, very low constituent concentrations, and interception by macrophytes and periphyton.  The passage of Hurricane Wilma over the Everglades in October 2005 caused substantial disturbance to vegetation, high water velocities, and long-lasting increases in sediment and nutrient concentrations, all of which greatly increased downstream loading rates of sediment and nutrients.  Rare flooding events in drier wetlands can also cause significant nutrient release.  In a short-hydroperiod floodplain wetland located on a low-order river in coastal Maryland, nitrate concentrations increased along a surface water flowpath activated during brief periods of overbank inundation on the floodplain.  Overbank flooding following freezing weather in the same floodplain resulted in substantial increases in the concentrations of all forms of dissolved inorganic nitrogen and phosphorus.  Conversely, drought can also reduce nutrient interception by wetlands.  Retention of annual river loads of nitrate by long-hydroperiod floodplain wetlands in Louisiana decreased by an order of magnitude during a year with severe drought due to limited channel-floodplain hydraulic connectivity.  Channelization of rivers can also reduce channel-floodplain interactions, similar to the effects of drought, resulting in lower accumulation rates of sediment and sediment-associated nutrients in floodplain wetlands.  In conclusion, wetland ecosystems are sensitive to hydrologic disturbance in the form of storms, floods, and droughts, which can lead to perturbations in nutrient biogeochemistry and increased loading of sediment and nutrients to downstream ecosystems.