Factors contributing to hypoxia in rivers, lakes, and streams

We investigated physical, chemical, and biological variables contributing to biochemical oxygen demand (BOD) in 17 North Carolina lotic and lentic water bodies affected by mild to severe hypoxia. Phytoplankton production created the dominant reservoir of labile carbon driving BOD, and subsequent hypoxia, in a Piedmont river subject to algal blooms, three urban streams, a set of anthropogenically affected tidal creeks, and two urban lakes. Autotrophic phytoplankton production contributed to the BOD load in some rural streams. Autochthonous heterotrophic processes, stimulated primarily by phosphorus and secondarily by nitrogen loading, were the major influences on BOD in two large black water rivers and some rural black water streams. Inputs of biochemical oxygen-demanding materials from storm water runoff contribute to BOD in some urban and rural streams and black water rivers. We suggest that reductions of hypoxia can be better achieved by a system-specific approach based on an array of factors that potentially influence BOD, including both autochthonous and allochthonous variables. In some circumstances targeting the nutrient(s) stimulating phytoplankton blooms will suffice to reduce hypoxia, but in other situations targeting nutrient(s) limiting bacterial production will be necessary. Reduction of non–point source inputs of biochemical oxygen-demanding materials derived from urbanization or other land disrupting activities will be critical in some cases. Hypoxia is a commonly recognized symptom of eutrophic waters (Bricker et al. 1999; Burkholder 2001). While acute hypoxia or anoxia can be caused by organic waste loading from allochthonous sources (Van Dolah and Anderson 1991; Mallin et al. 2002), chronic hypoxia is often caused by autochthonous processes within a water body (NRC 2000; Burkholder 2001; Wetzel 2001). Chronic hypoxia has been well documented in large ecosystems such as Chesapeake Bay (Officer et al. 1984; Boesch et al. 2001) and the Gulf of Mexico (Rabalais et al. 2001). Consequences of anoxia or hypoxia include fish and invertebrate kills, loss of habitat for resident organisms, enhanced susceptibility to disease, and changes in predator–prey interactions among affected species (Diaz and Rosenberg 1995; Breitburg et al. 1997; Lenihan and Peterson 1998). Biochemical oxygen demand (BOD) is a measure of the dissolved oxygen (DO) required by the microbial community in decomposing the organic matter present in a water sample by aerobic biochemical action (Clark et al. 1977; Boyd 2000). This measurement is commonly used by environmental engineers to determine the carrying capacity of 1 Corresponding author ([email protected]). 2 Present address: Institute of Marine Sciences, University of North Carolina at Chapel Hill, Morehead City, North Carolina 28557. 3 Present address: Division of Water Quality, Groundwater Section, North Carolina Department of Environment and Natural Re sources, Wilmington, North Carolina 28405.