Review of National Estuarine Research Reserve System Station Data During 2008 Gulf of Mexico Hypoxia Event

摘要

The National Oceanic and Atmospheric Administration's (NOAA) National Data Buoy Center (NDBC) creates, integrates, and maintains a broad network of buoys and Coastal Marine Automated Network (C-MAN) stations. Data from these stations are used by the National Weather Service (NWS) in creating and verifying forecasts, as well as by the general public for commercial and recreational purposes. In addition to serving quality-controlled data from NDBC, National Ocean Service (NOS), and Integrated Ocean Observing System (IOOS) (NOAA) partner stations, NDBC's Data Assembly Center (DAC) serves near-real-time quality-controlled data from a number of National Estuarine Research Reserve System (NERRS) stations. Hypoxic and anoxic events occur in the oceans when the level of oxygen consumed by pelagic animals and bottom feeders exceeds the level of oxygen replenished by phytoplankton and the atmosphere. This results in low oxygen levels and inability to sustain aquatic life. Waters are considered hypoxic when the amount of dissolved oxygen is measured at 2 milligrams (mg) O_2 L~(-1) and below. Anoxia occurs when there is no dissolved oxygen. A "dead zone" event, which is a popular name for an area of hypoxia, occurs when the waters become stratified in low-turbulent deep water a few kilometers offshore, preventing surface oxygen waters from resupplying bottom waters. The sinking of decomposing organic matter causes a hypoxic or, in extreme cases, an anoxic environment due to decomposers using up the limited oxygen in the near-bottom waters as they feast on the sinking organic matter. Along the Northern Gulf of Mexico coast, these events are becoming increasingly severe, strangling the fishing and shrimping industry, as well as endangering human health. One of the largest hypoxia zones is located along the western coast of Louisiana and is attributed to nutrients transported into the Gulf of Mexico by the Mississippi River and transported westward by the prevailing winds. The Mississippi River incorporates nutrients from the Ohio, Arkansas, Missouri, Illinois, and Tennessee rivers, making the Mississippi the largest watershed in North America. In 2008, the Mississippi River watershed experienced severe rains and flooding. More than the yearly average of nutrients from nitrogen-rich fertilizer and phosphorous-rich cattle ranches surged down the flooded Mississippi River. The Army Corps of Engineers saw it necessary to open the Bonnet Carre Spillway to alleviate the amount of water coming toward New Orleans via the Mississippi River, which diverted some of the water into Lake Pontchartrain. The effects of the spillway opening were observed within Lake Pontchartrain by several stations recording salinity, turbidity, and dissolved oxygen. There is evidence that the 2008 opening of the Bonnet Carre Spillway resulted in Mississippi River nutrients making their way 188.3 kilometers east from Lake Pontchartrain to the Bangs Lake, Grand Bay Reserve, Mississippi NERRS station GDQM6. A significant drop in dissolved oxygen at the station recorded the hypoxia/anoxia event and was supported by a significant increase in turbidity at the same station. The water gauges for the Grand Bay watershed reported no appreciable increase in fresh water discharge that would explain the June 2008 hypoxia event. Fish River, Weeks Bay Reserve, Alabama NERRS station WKQA1 detected a hypoxia event a month after the Grand Bay station. However, the gauges for this watershed do indicate an increase in discharge that could explain the hypoxia. The dissolved oxygen dropped below 2 mg O2 L~(-1) in a matter of hours. Six days prior to the event, WKQA1 reported average percentage oxygen saturated of 53.59 percent, and at the time of the event, the station reported below 10 percent oxygen saturation. NDBC data analysts only had station turbidity at both NERRS stations to verify the hypoxic event to be legitimate and not a sensor malfunction. Having near-real-time nutrient and chlo