Comparison of continuous records of near-bottom dissolved oxygen from the hypoxia zone along the Louisiana coast.

Rabalais, N.N. W.J. Wiseman,Jr., and R.E. Turner


Oxygen depletion is a seasonally dominant feature of the lower water column on the highly-stratified riverine-influenced continental shelf of Louisiana. the aerial extent of hypoxia (bottom waters ( mg l-1 dissolved oxygen) in mid-summer may encompass up to 9,500 km2, from the Mississippi River delta to the upper Texas coast, with the spatial configuration of the zone varying interannually. We placed two continuously recording oxygen meters (Endeco 1184) within 1 m of the seabed in 20-m wat er depth at two locations 77 km apart where we previously documented mid-summer bottom water hypoxia. The oxygen meters recorded considerably different oxygen conditions for a 4-mo. deployment from mid-June through mid-October. At the station off Terreb onne Bay (C6A), bottom waters were severely depleted in dissolved oxygen and often anoxic for most of the record from mid-June through mid-August, and there were no strong diurnal or diel patterns. At the station 77 km to the east and closer to the Missi ssippi River delta (WD32E), hypoxia occurred for only 50% of the record, and there was a strong diurnal pattern in the oxygen time-series data. There was no statistically significant coherence between the oxygen time-series at the two stations. Coherenc e of the oxygen records with wind records was weak. The dominant coherence identified was between the diurnal peaks in the WD32E oxygen record and the bottom pressure record from a gauge located at the mouth of Terrebonne Bay, suggesting that the dissolv ed oxygen signal at WD32E was due principally to advection by tidal currents. Although the oxygen time-series were considerably different, they were consistent with the physical and biological processes that affect hypoxia on the Louisiana Shelf. Differ ences in time-series were most intimately tied to the topographic cross-shelf gradients in the two locations, that is, station C6A of Terrebonne Bay was in the middle of a broad, gradually sloping shelf and station WD32E in the Mississippi River Delta Big ht was in an area with a steeper cross-shelf depth gradient and likely situated near the edge of a hypoxic water mass that was tidally advected across the study site.
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