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Description:
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Six years of comprehensive data sets collected over the northern continental shelf
and upper slope of the Gulf of Mexico during the LATEX -A and NEGOM -COH
programs showed that low -oxygen waters ( <2 .4 mLà ·L -1 ) are found only in spring and
summer and only in water depths between 10 and 60 m . Four regions in the northern
Gulf show considerable differences in the occurrence of low -oxygen waters . Lowoxygen
waters are observed almost exclusively in regions subject to large riverine
influences : the Louisiana and Mississippi -Alabama shelves . Hypoxic waters (oxygen
concentrations <1 .4 mLà ·L -1 ) are found only over the Louisiana shelf . No low -oxygen
water is found over the Florida shelf which has minimum riverine influence . Lowoxygen
water is found at only one station on the Texas shelf ; this is during spring when
the volume of low -salinity water is at maximum . The distributions of low -salinity water
influenced the different distributions of low -oxygen and hypoxic waters in the four
regions . Low -oxygen occurrences are clearly related to vertical stratification . Lowoxygen
occurred only in stable water columns with maximum Brunt -Và ¤isà ¤là ¤ frequency
(Nmax ) greater than 40 cyclesà ·h -1 . When Nmax exceeded 100 cyclesà ·h -1 in summer over the
Louisiana shelf , oxygen concentrations dropped below 1 .4 mLà ·L -1 , and the bottom
waters became hypoxic . Salinity is more important than temperature in controlling
vertical stratification . Locations where temperature influence was larger were found in
summer in water depth greater than 20 m over the Louisiana shelf , along the near shore
areas of the Mississippi -Alabama shelf west of 87à ºW , and in the inner shelf waters of the
Texas shelf . The extent of oxygen removal at the bottom of these stable water columns is
reflected in the amount of remineralized silicate . Silicate concentrations are highest
closest to the Mississippi River Delta and decrease east and west of the Delta . EOF analyses show that more than 65 % of the oxygen variance is explained by the first mode .
The amplitude functions of the first EOF modes of bottom oxygen , water column Brunt -
Và ¤isà ¤là ¤ maxima , and bottom silicate are well correlated , indicating that much of the
variance in bottom oxygen is explained by water column stratification and bottom
remineralization . |