Benthic Fluxes of Dissolved Inorganic Carbon (DIC) and Total Alkalinity (TA) in the Northern Gulf of Mexico Shelf Sediments

The seasonal development of hypoxic waters in the northern Gulf of Mexico (nGOM) along the Louisiana coastal shelf has large environmental and economic impact on the region. Coupled physical-biological models indicate that the hypoxia formation in the nGOM is sensitive to benthic dissolved O₂ consumption driven by the vertical flux of organic matter from surface eutrophied waters. The severity and duration of hypoxic conditions in the shelf bottom water raises the question of how shelf sediments impact the bottom water chemistry. The relative production or consumption of DIC and alkalinity from benthic sources as a result of aerobic and anerobic respiration in the sediments may be an essential component in controlling bottom water pH, but remains largely unknown. In order to assess the impact of sediment respiration on water column chemistry, both in situ and ex situ benthic flux measurements were carried out at five locations with contrasting bottom oxygen conditions during periods with high Mississippi River discharge in May 2017 and low river discharge in August 2016 and 2018. The benthic DIC and alkalinity fluxes in May were found to vary between 30.2 to 46.5 mmol/m²/d and -7.5 to -57.6 mmol/m²/d respectively. In August, the benthic DIC and alkalinity fluxes varied between 32.1 to 52.6 mmol/m²/d and -63.8 to 10.9 mmol/m²/d respectively, suggesting greater contribution of sediment anaerobic respiration to benthic water chemistry during hypoxia events.