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PHYSICAL AND BIOLOGICAL CONTROLS ON DISSOLVED OXYGEN DYNAMICS IN A NORTHEASTERN GULF OF MEXICO ESTUARY
Citation:
Greene, R. PHYSICAL AND BIOLOGICAL CONTROLS ON DISSOLVED OXYGEN DYNAMICS IN A NORTHEASTERN GULF OF MEXICO ESTUARY. Presented at EPA Science Forum, Washington, DC, June 1-3, 2004.
Description:
Nutrient over-enrichment is one of the most often cited causes of 305b impairment in coastal waters. Excessive nutrients affect designated uses of the nation's aquatic resources, and pose risks to human health and the environment. The process of developing nutrient criteria for estuaries and coastal marine waters poses unique challenges to EPA, States and Tribes because each system can response differently to nutrient over-enrichment. Current scientific understanding of estuarine ecosystem function is inadequate to allow us to extrapolate from one system to another - thus, it is not possible to predict whether a given nutrient concentration or load will have measurable adverse impacts.
A long-term goal of ORD's Water Quality research is to provide the methods to develop and apply numeric nutrient criteria to support designated uses. ORD is developing load - response relationships targeting key response endpoints such as hypoxia, loss of submerged aquatic vegetation and food web changes. A key component of the research is to better define and quantify important ecological processes that contribute to adverse ecosystem effects. The present study examines the physical and biological processes that influence the depletion of dissolved oxygen in estuaries.
Nutrient enrichment may cause hypoxia (low dissolved oxygen) by increasing primary production and biological oxygen demand. Other factors, however, contribute to hypoxia and affect the susceptibility of coastal waters to hypoxia. Hypoxia formation requires net biological oxygen consumption and a physical environment that limits oxygen inputs. Oxygen dynamics were examined in Pensacola Bay, FL estuary using box models of water and oxygen transport during 2002-2003 and direct benthic and planktonic metabolic rate measurements in summer 2003. Unusually extensive hypoxia occurred in summer 2003 during a period of high freshwater inflow. Vertical diffusive exchange became negligible (exchange velocity <10 cm/d) in lower estuary areas that were highly stratified. Subtidal advective transport decreased from 3 to 5 cm/s to <1 cm/s. Measured bottom layer respiration (avg: 19 mmol O2/m2/d) was similar to estimates derived from the box model and lower than estimates reported for many estuaries. Water clarity affected oxygen dynamics by allowing oxygen production below the pycnocline, which offset oxygen demand on some dates. Thus, hypoxia occurred when strong stratification co-occurred with high turbidity. These results highlight the need to better understand and quantify processes influencing oxygen dynamics and the linkages between nutrient load and hypoxia.