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The hypoxic zone on the Louisiana Continental Shelf (LCS) forms each summer due to nutrient‐enhanced primary production and seasonal stratification associated with freshwater discharges from the Mississippi/Atchafalaya River Basin (MARB). Recent field studies have identified highly productive shallow nearshore waters as an important component of shelf‐wide carbon production contributing to hypoxia formation. This study applied a three‐dimensional hydrodynamic‐biogeochemical model named CGEM (Coastal Generalized Ecosystem Model) to quantify the spatial and temporal patterns of hypoxia, carbon production, respiration, and transport between nearshore and middle shelf regions where hypoxia is most prevalent. We first demonstrate that our simulations reproduced spatial and temporal patterns of carbon production, respiration, and bottom‐water oxygen gradients compared to field observations. We used multiyear simulations to quantify transport of particulate organic carbon (POC) from nearshore areas where riverine organic matter and phytoplankton carbon production are greatest. The spatial displacement of carbon production and respiration in our simulations was created by westward and offshore POC flux via phytoplankton carbon flux in the surface layer and POC flux in the bottom layer, supporting heterotrophic respiration on the middle shelf where hypoxia is frequently observed. These results support existing studies suggesting the importance of offshore carbon flux to hypoxia formation, particularly on the west shelf where hypoxic conditions are most variable. Plain Language Summary: Formation of hypoxia, or low dissolved oxygen, is a seasonal occurrence on the Louisiana Continental Shelf associated with stratification of the water column and excess nutrient loads delivered via the Mississippi and Atchafalaya River systems. Frequently referred to as "dead zones," bottom‐water hypoxia results in stress or death of aquatic organisms, especially those that cannot move to areas with more oxygen. To study the sources and distribution of organic matter that supports oxygen consumption, we applied a three‐dimensional hydrodynamic‐biogeochemical model named CGEM (Coastal Generalized Ecosystem Model). CGEM simulations between 2003 and 2007 successfully simulated spatial and temporal patterns of hypoxia and important biological processes that control its formation. Our simulations revealed that highly productive nearshore waters serve as a source of organic matter that supports oxygen consumption offshore. We identified seasonal bottom‐layer currents that transport organic matter offshore and interannual variations in river discharge that influence biological production and bottom‐layer oxygen consumption offshore. The ecological processes described in this study increase our understanding of how nearshore processes affect the development and maintenance of offshore hypoxia. Key Points: Simulation models describe ecological processes controlling spatial patterns in production and respiration and effect on hypoxiaOffshore organic matter flux from highly productive nearshore areas supports increased respiration offshoreOffshore carbon flux is an important driver of oxygen demand and resulting hypoxia at intermediate depths where hypoxia commonly occurs [ABSTRACT FROM AUTHOR]

Abstract: We examined the effects of freshwater flow and light availability on phytoplankton biomass and production along the Louisiana continental shelf in the region characterized by persistent spring–summer stratification and widespread summer hypoxia. Data were collected on 7 cruises from 2005 to 2007, and spatially-averaged estimates of phytoplankton and light variables were calculated for the study area using Voronoi polygon normalization. Shelf-wide phytoplankton production ranged from 0.47 to 1.75mg Cm−2 d−1 across the 7 cruises. Shelf-wide average light attenuation (kd ) ranged from 0.19–1.01m−1 and strongly covaried with freshwater discharge from the Mississippi and Atchafalaya Rivers (R 2=0.67). Interestingly, we observed that the euphotic zone (as defined by the 1% light depth) extended well below the pycnocline and to the bottom across much of the shelf. Shelf-wide average chlorophyll a (chl a) concentrations ranged from 1.4 to 5.9mgm−3 and, similar to kd , covaried with river discharge (R 2=0.83). Also, chl a concentrations were significantly higher in plume versus non-plume regions of the shelf. When integrated through the water-column, shelf-wide average chl a ranged from 26.3 to 47.6mgm−2, but did not covary with river discharge, nor were plume versus non-plume averages statistically different. The high integrated chl a in the non-plume waters resulted from frequent sub-pycnocline chl a maxima. Phytoplankton production rates were highest in the vicinity of the Mississippi River bird''s foot delta, but as with integrated chl a were not statistically different in plume versus non-plume waters across the rest of the shelf. Based on the vertical distribution of light and chl a, a substantial fraction of phytoplankton production occurred below the pycnocline, averaging from 25% to 50% among cruises. These results suggest that freshwater and nutrient inputs regulate shelf-wide kd and, consequently, the vertical distribution of primary production. The substantial below-pycnocline primary production we observed has not been previously quantified for this region, but has important implications about the formation and persistence of hypoxia on the Louisiana continental shelf. [Copyright &y& Elsevier]

Abstract: The extent of hypoxia and the physical factors affecting development and maintenance of hypoxia were examined for Pensacola Bay, Florida (USA) by conducting monthly water quality surveys for 3years (2002–2004) and by constructing salt-and-water balance box models using the resulting data. We also analyzed data from earlier summer probabilistic water quality surveys (1996–1999). Hypoxia (O2 <2.0mgL−1) affected an average of 24% (range=16–36%) of the Bay bottom during 1996–1999 summer surveys; similar results were obtained using the 2002–2004 monthly survey data. The water column in Pensacola Bay was usually well-stratified, apparently as a result of the low amplitude (<50cm) diurnal tide, which provides low mixing energy. Vertical diffusivity at the pycnocline was between 0.002 and 0.02cm2 s−1, 10-fold less than comparable estimates for Chesapeake Bay, Maryland/Virginia. Residual (sub-tidal) estuarine circulation was sluggish, with landward velocity in the bottom layer between 1 and 4cms−1 during summer. The observed physical transport regime severely limits exchange of bottom waters and is very conducive to development of hypoxia. Net non-conservative O2 fluxes and physical O2 inputs were generally in a dynamic balance during summer, such that the median imbalance, the accumulation or depletion of O2, was only 11% of the non-conservative flux. Computed net non-conservative O2 fluxes for the lower water column and sediments were generally low relative to other estuaries (<0.5g O2 m−2 d−1), indicating an absence of eutrophic conditions. This suggests that the current extent of hypoxia in Pensacola Bay may be largely attributable to natural susceptibility to hypoxia resulting from physical factors. Balanced O2 metabolism or net autotrophy below the pycnocline was observed for some segments of the Bay. We hypothesize that O2 production resulting from photosynthesis below the pycnocline sometimes offset O2 consumption in Pensacola Bay bottom waters, potentially reducing development of hypoxia. [Copyright &y& Elsevier]

Abstract: The Sea-viewing Wide Field-of-View Sensor (SeaWiFS) derived diffuse light attenuation along the Louisiana continental shelf (LCS) was examined at monthly scales from 1998 to 2007 to characterize temporal and spatial patterns, and responsible physical forcing conditions. The SeaWiFS diffuse light attenuation ranged from 0.10 to 2.64m−1. Stepwise multiple linear regression analysis suggested that spatial and temporal patterns in diffuse light attenuation were influenced by wind speed, nutrient loading, and river discharge from the Mississippi and Atchafalaya River Basin. SeaWiFS daily integrated surface photosynthetically active radiation (PAR, 400–700nm) and diffuse light attenuation were used to calculate the absolute PAR and percentage of surface PAR that reached the sediment water interface (SWI) on the LCS. Large portions of the LCS were euphotic to the SWI especially during April and May. This finding implied that significant primary production was possible beneath the pycnocline during spring and early summer. In addition, this study was the first to demonstrate that the euphotic depth was correlated to the depth at which the water column turned hypoxic on the LCS. The development of hypoxic waters may be influenced by decreased light availability below the pycnocline in addition to aforementioned physical forcing. [Copyright &y& Elsevier]