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Barun Sen Gupta, Louisiana State University (United States)
Nancy Rabalais, Louisiana Universities Marine Consortium (United States)
R Eugene Turner, Louisiana State University (United States)
Jenn Lasseigne, Louisiana State University (United States)
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A large volume of coastal water in the northern Gulf of Mexico (Louisiana and eastern Texas shelf) becomes hypoxic (O2 <2 mg/L) in spring and summer. The affected bottom area may extend from nearshore (water depth 3-4 m) to over 100 km offshore (water depth ∼60 m), although the most severely hypoxic (including anoxic) waters are restricted to depths <40 m. Time-series monitoring shows that the extent of mid-summer hypoxia has averaged 13500 km2 in the past 23 years, and that the average for 1993-2007 was nearly twice the 1985-1992 average. Extensive areas of low oxygen were uncommon before the mid-1970s. This hypoxia is caused by the coupling of persistent water-column stratification and high algal production in surface waters, which, in turn, is fueled by river-derived nutrients. For the past several decades, the worsening of hypoxia has been closely related to anthropogenic increase in riverine nitrate concentration, and secondarily to a natural variation in the discharge of the Mississippi River system. In the sedimentary record of the past half century, proxies of bottom-water oxygen, including mineral, isotopic, phytoplankton pigment, and benthic foraminiferal indicators, support the scenario of worsening oxygen stress. This trend is best seen in areas of recent chronic hypoxia, and matches the pattern of increasing nitrogen load from the Mississippi River system beginning in the 1950s.
Within the group of paleohypoxia indicators, the benthic Foraminifera are particularly useful, because: (1) severe oxygen stress decreases their diversity and alters species proportions within the community, and (2) their shells are preserved well in sediments. While present-day foraminiferal diversity is, in general, low in the Louisiana Bight, comparisons among assemblages from areas of different oxygen depletion indicate how hypoxia has developed over time and extended into deeper water. In addition, several species/genus-based foraminiferal indices may be used as hypoxia proxies in the sedimentary column. Among these, an index based on relative abundances of Ammonia and Elphidium is a most promising paleohypoxia tracker also in other areas, because these taxa are widely distributed and well preserved in coastal sediments. Given an undisturbed, continuous record of rapid sedimentation, this index may track past coastal hypoxia on a decadal scale, and, in exceptional cases, on an annual scale.
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