A brief summary of hypoxia on the northern Gulf of Mexico continental shelf: 1985–1988

Abstract

Abstract Oxygen-deficient conditions occur from April to October on the inner to middle continental shelf of the northern Gulf of Mexico and may cover up to 9500 km 2 during mid-summer off the Louisiana coast. Hypoxic bottom waters are found in 5–60 m water depth, 5–60 km offshore and extend up to 20 m above the bottom. Salient forcing functions contributing to hypoxic water formation, maintenance and break-up are presented. Stratification is directly correlated with hypoxia in time and space, suggesting that reaeration of bottom waters is controlled by physical processes that are influenced by regional wind fields, river discharge and continental shelf scale currents. Phytoplankton biomass reaches the bottom waters in the hypoxic zones in large amounts (>5 µg/l), fueling water column and benthic respiration rates, but to a currently unknown degree. Increased nutrient loadings in the two major rivers and changes in the proportion of those nutrients essential to phytoplankton growth have probably changed both the phytoplankton community species composition and community production. It is not yet clear whether the extent and severity of hypoxia on the Louisiana continental shelf have changed as a result of these riverine water quality changes.