PARTICULATE MATTER AND HYPOXIA ON TEXAS/LOUISIANA SHELF DURING SUMMER, 2012

Particulate matter in hypoxic areas of the TX/LA shelf have three major sources – river plumes, primary production, and resuspended sediments. The sources and processes controlling distribution and transport of these particles were investigated through in-situ sampling and optical profiling. Discrete samples were filtered for particulate matter (PM) and particulate organic carbon (POC) concentration for calibration of optical instruments interfaced with a CTD and a towed, undulating Acrobat system and a shipboard flow-through system during June and August 2012 cruises of the Mechanisms Controlling Hypoxia program. Hydrographic stations and cross-shelf tow sections extended from Galveston, Texas to the Mississippi River. Concentrations, gradients and ratios of optical and compositional values are used to infer generic sources of particles and their relationship to, and influence on, areas of hypoxia. Near the mouths of the Mississippi and Atchafalaya Rivers, most particulate matter flocculates and settles to the seafloor. Further offshore and alongshore, surface primary production fueled initially by nutrient input from the rivers produces organic matter that settles to the seafloor, consuming oxygen along the way and at the seafloor. Further to the west, surface nutrients are depleted, inhibiting surface production of organic matter, but the water is sufficiently clear that light penetrates below the pycnocline, enabling benthic production of oxygen and organic matter until or unless particle concentrations become sufficiently high (through biological production or sediment resuspension/advection) to stop primary benthic production because of light limitation. Oxygen is then consumed through respiration and organic matter remineralization. Hypoxic conditions in these regions become a balance between primary production and respiration/remineralization, and are often patchy and ephemeral.