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Sediment and Lower Water Column Oxygen Consumption in the Seasonally-hypoxic Region of the Louisiana Continental Shelf
Citation:
MURRELL, M. C. AND J. C. LEHRTER. Sediment and Lower Water Column Oxygen Consumption in the Seasonally-hypoxic Region of the Louisiana Continental Shelf. Estuaries and Coasts. Estuarine Research Federation, Port Republic, MD, 34(5):912-924, (2011).
Impact/Purpose:
The purpose of this study was to quantify sediment and water column respiration rates on the Louisiana Continental Shelf in the regions susceptible to summer hypoxia. Results from this and related studies support development of nutrient criteria under the Clean Water Act, Water Quality Multi-Year Plan, Long Term Goal 1. It also supports the goals of the Gulf of Mexico, The Mississippi River/Gulf of Mexico Watershed Nutrient Task Force which was established under the Harmful Algal Bloom and Hypoxia Control Act (HABHRCA).
Description:
We report sediment and bottom water respiration rates from 10 cruises from 2003-2007 on the Louisiana Continental Shelf (LSC) where summer hypoxia regularly occurs. Cruises were conducted during spring (5 cruises), summer (3 cruises) and fall (2 cruises). Cruise average sediment oxygen consumption (SOC) ranged from 3.9 to 31.2 mmol O2 m-2 d-1, averaging 18.3 ± 1.7 mmol O2 m-2 d-1. Bottom water plankton respiration (WR) ranged from 5.4 to 10.8 mmol O2 m-3 d-1, averaging 6.8 ± 0.6 mmol O2 m-3 d-1. Integrated below-pycnocline oxygen consumption (TR), calculated as the sum of SOC and integrated WR rates, ranged from 50.5 to 110.0 mmol O2 m-2 d-1, averaging 68.0 ± 5.5 mmol O2 m-2 d-1. While our sampling was not exhaustive, respiration rates appeared remarkably similar across the shelf, and similar among the different seasons sampled. In general, respiration rates agreed well with the literature, though we observe some stark differences with prior studies in the Gulf of Mexico. SOC appeared strongly limited by oxygen concentrations in the overlying water. In contrast to several prior studies conducted on the LCS, we found that the water column, rather than sediments, was the major sink for dissolved oxygen below the pycnocline, averaging 75% of the total below-pycnocline oxygen consumption. These findings add substantially to the data available for parameterizing and evaluating models of oxygen dynamics and hypoxia formation on the LCS.