Sedimentary Oxygen Consumption and Nutrient Regeneration in the Northern Gulf of Mexico Hypoxic Zone

Seasonal summer stratification and enhanced nutrient loading of the Louisiana continental shelf (U.S.A.) west of the Mississippi River create hypoxic regions that affect large areas of the benthos. Total sediment oxygen uptake and nutrient recycling were measured during different, (e.g. pre, early, late and post) hypoxic regimes using shipboard Batch Micro-Incubation Chambers (BMICs) in 2004 to 2005 and again in 2007 to 2009. Sediment community oxygen consumption during oxic regimes (dissolved oxygen > 63 mu mol L-1) was -9.5 +/- 0.7 mmol O-2 m(-2) d(-1) (mean +/- SE), almost twice that measured (-5.8 +/- 0.6 mmol O-2 m(-2) d(-1)) during suboxic conditions. During the summer when hypoxia occurred, the benthos consumed nitrate and nitrite (-0.14 +/- 0.04 and -0.10 +/- 0.02 mmol N m(-2) d(-1) respectively) and produced ammonium (1.6 +/- 0.39 mmol N m(-2) d(-1)). Elevated sediment community oxygen consumption and nutrient remineralization occurred near terrestrial river inputs associated with the Mississippi and Atchafalaya Rivers. Net release of dissolved inorganic nitrogen, in the form of ammonium, peaked during late summer. Released ammonium may be a source of nutrients for primary production in bottom waters, and can also provide reduced nitrogen for nitrification and microbial respiration, both of which may reinforce the intensity and duration of hypoxia. Based on chamber results, sediments actively scavenged phosphate from the bottom waters (-98.4 +/- 21.3 mu mol P m(-2) d(-1)) and released silicate (2.62 +/- 0.31 mmol Si m(-2) d(-1)). The addition of reactive nitrogen and removal of phosphorous due to benthic community metabolism could potentially be accentuating phosphorous limitation on the continental shelf.