Summertime hydrography of the nearshore Louisiana Continental Shelf: Effects of riverine outflow, shelf morphology, and the presence of sand shoals on water quality

摘要

River-influenced ocean margins are dynamic and productive coastal environments. Nearshore Louisiana exemplifies the strong influence of the Mississippi and Atchafalaya rivers. Coastal salinities, temperatures, turbidities, and nutrients are directly affected, and the zone annually experiences the second largest eutrophication-induced marine hypoxic event. During the summers of 2013-14 we assessed the extent of river influence across the nearshore region of the Louisiana Continental Shelf by sampling water quality in water depths of 3.7-18 m. Single-variable spatial analyses and multivariate factor analysis suggested that the nearshore zone comprises three functionally-distinct hydrographic regions. Differences in bathymetry and freshwater input associated with delta morphology explained significant differences in mixing and stratification patterns among East, Central, and West regions of nearshore Louisiana. Additionally, intraregional differences occurred between waters overlying sandy shoals and those overlying the surrounding muddy sediments and with wind. Mean interregional surface-to-bottom density differences were 0.9, 3.2, and 9.2 kg m(-3), (West, Central, and East, respectively) and intraregional density differences were 2.1 over sand and 4.3 kg m(-3) over mud. The mean vertical extent of hypoxia (DO < 50% saturation) over sand was 0.6 m lower than over mud. Mean DO saturation was 33.1% and 21.4% greater over sand than over mud in the Central and West regions, respectively, but did not differ in the East. Mean DO saturation was up to 57.6% higher with southeasterly rather than northwesterly winds. These physicochemical patterns demonstrate the importance of physical factors influencing hypoxia and drive substantially different habitat quality within this dynamic, estuarine environment.