A review of water column processes influencing hypoxia in the northern Gulf of Mexico |
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Authors: | Michael J Dagg James W Ammerman Rainer M W Amon Wayne S Gardner Rebecca E Green Steven E Lohrenz |
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Institution: | 1.Louisiana Universities Marine Consortium,Chauvin;2.Institute of Marine and Coastal Sciences,Rutgers University,New Brunswick;3.Department of Marine Sciences,Texas A & M University at Galveston,Galveston;4.Marine Science Institute,University of Texas at Austin,Port Aransas;5.Ocean Sciences Division,Naval Research Laboratory,Stennis Space Center;6.Department of Marine Science,University of Southern Mississippi,Stennis Space Center |
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Abstract: | In this review, we use data from field measurements of biogeochemical processes and cycles in the Mississippi River plume
and in other shelf regions of the northern Gulf of Mexico to determine plume contributions to coastal hypoxia. We briefly
review pertinent findings from these process studies, review recent mechanistic models that synthesize these processes to
address hypoxia-related issues, and reinterpret current understanding in the context of these mechanistic models. Some of
our conclusions are that both nitrogen and phosphorus are sometimes limiting to phytoplankton growth; respiration is the main
fate of fixed carbon in the plume, implying that recycling is the main fate of nitrogen; decreasing the river nitrate loading
results in less than a 1:1 decrease in organic matter sinking from the plume; and sedimenting organic matter from the Mississippi
River plume can only fuel about 23% of observed coastal hypoxia, suggesting significant contributions from the Atchafalaya
River and, possibly, coastal wetlands. We also identify gaps in our knowledge about controls on hypoxia, and indicate that
some reinterpretation of our basic assumptions about this system is required. There are clear needs for improved information
on the sources, rates, and locations of organic matter sedimentation; for further investigation of internal biogeochemical
processes and cycling; for improved understanding of the rates of oxygen diffusion across the pycnocline; for identification
and quantification of other sources of organic matter fueling hypoxia or other mechanisms by which Mississippi River derived
organic matter fuels hypoxia; and for the development of a fully coupled physical-biogeochemical model. |
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