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Abstract: A numerical hydrodynamic simulation of the Texas–Louisiana continental shelf circulation is compared with hydrographic and moored observations of temperature, salinity, and current velocity. The spatial and temporal structure of the model error suggests that the model is able to reproduce the observed broad-scale features and seasonal patterns faithfully. However, there are energetic features that have small spatial and temporal scales, 30–50km and 5–12days, that are not reproduced in the model. These small-scale features are not well resolved concurrently in both space and time by most modern hydrographic and moored measurements, and thus appear as observational noise when comparing observations with numerical simulations. We demonstrate that these features are reproduced statistically within the model in terms of mean and variance, but are not reproduced exactly. Thus, these features create a substantial noise floor that constrain assessments of model skill to large spatial and temporal scales. [Copyright &y& Elsevier]

Abstract: Hypoxia of the northern Gulf of Mexico is a recurrent summer phenomenon that is principally driven by the combined effects of nutrient loading and stratification introduced onto the continental shelf mainly from effluents derived from the Mississippi and Atchafalaya Rivers. Spatially detailed observations along and across the continental shelf in the central and western areas of coastal Louisiana, where hypoxia is most frequent and severe, reveal that the spatial variability (horizontal and vertical) of dissolved oxygen concentration is closely linked to physical processes including the spatial geometry of local topographic features. The along shelf spatial variability of dissolved oxygen is estimated to have a principal wavelength of approximately 50km that is phase-locked with the location of three shallow shoals along the coastline. Based on a comparison with the historical record, the area between these shoals coincides with areas of more frequent hypoxia. These results may have important implications for coastal management strategies by identifying physically controlled hotspots in the central and western Louisiana shelf where monitoring efforts can be most efficiently targeted. [Copyright &y& Elsevier]

Abstract: Here we report on spatial and temporal changes in the concentration and composition of total dissolved organic carbon (DOC) and dissolved lignin-phenols (sum 8) in surface and bottom waters in estuarine and inner shelf off the Louisiana coast (USA). DOC samples were collected at 6 stations on 3 cruises (April 2008, July 2008) for organic analyses along a transect that spanned from inside Terrebonne–Timbalier Bay estuary, Louisiana (7m water depth) to the outer-most station on the inner Louisiana shelf (18.5m water depth). An additional set of samples was taken for DOC and dissolved lignin-phenols in April 2009 in the upper marshes of the estuary in water depths typically ca. 1m. Hydrographic properties of shelf waters in April 2008 were drastically different than in July 2008 because of the higher river discharge from the Mississippi River (MR) and the opening of man-made water diversion structures in Louisiana in April. There were no significant differences in DOC concentrations between the inner bay and shelf stations, likely due to extensive mixing between the bay and shelf waters. However, there were significantly higher concentrations of dissolved lignin (sum 8) and Λ8 at the marsh stations (x̄ =0.28±0.15mgL−1 and x̄ =0.69±0.35), compared to the inner shelf (x̄ =0.16±0.06mgL−1 and x̄ =0.24±0.12) stations, respectively. Low S/V and C/V ratios observed in Terrebonne–Timbalier Bay estuary were likely the result of selective degradation of syringyl and cinnamyl over vanillyl lignin-phenols, and not inputs of woody gymnosperms. A gradient of decreasing S/V ratios along a transect from the upper marshes to the shelf was likely the result of photochemical breakdown of lignin and dilution effects from marine waters near the outer reaches of the estuary. Inputs of photochemically-altered DOC from estuaries such as Terrebonne–Timbalier Bay to the inner Louisiana shelf may provide an additional source of OC for microbial food webs in the northern Gulf of Mexico. [Copyright &y& Elsevier]

Since systematic measurements of Louisiana continental-shelf waters were initiated in 1985, hypoxia (oxygen content <2 mg L−1) has increased considerably in an area termed the dead zone. Monitoring and modeling studies have concluded that the expansion of the Louisiana shelf dead zone is related to increased anthropogenically derived nutrient delivery from the Mississippi River drainage basin, physical and hydrographical changes of the Louisiana Shelf, and possibly coastal erosion of wetlands in southern Louisiana. In order to track the development and expansion of seasonal low-oxygen conditions on the Louisiana shelf prior to 1985, we used a specific low-oxygen foraminiferal faunal proxy, the PEB index, which has been shown statistically to represent the modern Louisiana hypoxia zone. We constructed a network of 13 PEB records with excess 210Pb-derived chronologies to establish the development of low-oxygen and hypoxic conditions over a large portion of the modern dead zone for the last 100 years. The PEB index record indicates that areas of low-oxygen bottom water began to appear in the early 1910s in isolated hotspots near the Mississippi Delta and rapidly expanded across the entire Louisiana shelf beginning in the 1950s. Since ~1950, the percentage of PEB species has steadily increased over a large portion of the modern dead zone. By 1960, subsurface low-oxygen conditions were occurring seasonally over a large part of the geographic area now known as the dead zone. The long-term trends in the PEB index are consistent with the 20th-century observational and proxy data for low oxygen and hypoxia. [ABSTRACT FROM AUTHOR]

Abstract: To examine the sources of freshwater and carbon cycling associated with Louisiana shelf hypoxia, we measured δ18O and δD of water, δ13C of dissolved inorganic carbon (DIC), salinity and dissolved oxygen (DO) in waters from 37 stations during April and July of 2008. Seafloor δ18O values resemble typical Gulf of Mexico seawater (≈1.1%) while surface waters values are substantially lower (e.g., <−2.0%) due to mixing with river-sourced freshwater. Salinity-δ18O regressions for 2008 surface waters show the δ18O of discharge to average −6.8% in April and −5.1% in July. Salinity-δD regressions show the δD of discharge to be −38% in April and −28% in July. Together these regressions suggest Mississippi discharge was the dominant freshwater source in the study area in April followed by a shift to nearly total Atchafalaya discharge during July, a trend that coincides with summer coastal current reversals. The δ13CDIC of July surface water varies from −5.0 to 1.2% and correlates with salinity indicating mixing of seawater and river water. April surface water shows no relationship with salinity because of the influence of primary productivity, which enriches certain waters in δ13CDIC and DO. The δ13CDIC of sub-pycnocline water ranges from −2.3 to 0.3%, with lower values reflecting increased respiration. The inshore (10m depth) δ13CDIC-DO relationship yields a lower y-intercept relative to offshore (20m depth) bottom waters, possibly indicating a terrestrial source of OC being respired. Mass balance estimations of respired OC do not have the accuracy to quantify any difference between nearshore and offshore locations. Regardless, the δ13CDIC-DO relationships suggest that the δ13C of biogenic carbonates may provide a valuable tool for paleo-redox studies in this region. [Copyright &y& Elsevier]

Stratification has been proved to be essential to the development and sustainment of bottom hypoxia off the Texas-Louisiana shelf. Observations indicate that bottom hypoxia does not always arise under strong stratification, especially in spring/winter season when the strong stratification is transient. A simple oxygen model is coupled with a high-resolution hydrodynamic model to further investigate linkage between stratification and oxygen dynamics. We put forward a definition of stratification when vertical stratification maxima exceeds some critical value. This duration time is longest in summer and shortest in winter, and is identified regulating the horizontal and vertical extensions of bottom hypoxia. A long duration of strong stratification expands over the shelf, defining a stratification envelope within which bottom hypoxia occurs with high probability. Persistent strong stratification also acts to reserve local oxygen depletion, facilitating the growth of the hypoxic layer thickness. The main pycnocline is identified to be the upper limit of the top reach of bottom hypoxia. • Stratification duration is longest in summer and shortest in winter. • The duration regulates the horizontal and vertical extents of bottom hypoxia. • The main pycnocline is identified to be the upper limit of top reach of bottom hypoxia. [ABSTRACT FROM AUTHOR]