Three-dimensional Heterogeneity of Hypoxic Water Masses in the Mississippi Sound: The Geomorphology Connection

Author(s): Milroy, S.; Moshogianis, A.

Seasonal hypoxia is certainly common over the Louisiana-Texas (LATEX) shelf west of the Balize Delta, but over the last several years summer hypoxia has also been discovered east of the delta in the Mississippi Bight (Dillon et al. 2008, Brunner et al. 2009) and in the deeper reaches of the Mississippi Sound (Gundersen, pers. comm.). Hypoxia most commonly occurs during times of significant vertical stratification of the water column, caused by the complimentary effects of seasonal heating and freshwater discharge. These discharges, when laden with organic and inorganic nutrients, further exacerbate the geographic extent of these hypoxic water masses. While the causative agents of coastal hypoxia have been well-described, the synergies between coastal geomorphology and the net ecological burden (O2 production v. respiration) within the Mississippi Sound/Bight are less well-known. Over a series of cruises conducted from 01 APR–30 JUL 2010, vertical profiles from thirty repeat stations within a highly resolved (25 km2) grid were analyzed monthly for in situ CDOM/phycoerythrin/chl-a fluorescence, temperature, salinity, and dissolved oxygen. Results indicate that differences between surface and near-bottom chl-a, coupled with the unique geomorphology of the Mississippi Sound/Bight, can produce hypoxic water masses with significant heterogeneity over fine spatial scales.

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