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The extent of hypoxia on the Louisiana shelf has been measured during July since 1985. The measured area was assumed to represent the seasonal maximum each year and was related to the Mississippi‐Atchafalaya riverine May NO2+3 loading and May–June total nitrogen loading, for planning management strategies. In this study, we analyze 25 years of simulations from a coupled physical‐biogeochemical model. An empirical orthogonal function analysis of the hypoxia frequency reveals that the dominant pattern is east‐west, with the seasonal maximum occurring in June, July, or August. This indicates that the July hypoxic area may or may not always be the largest of the year. A simple linear regression model was constructed to examine the explained variance of hypoxia attributable to nutrients. Results reveal that the May NO2+3 (May–June total nitrogen) loading can explain 35% ± 7% (54% ± 7%) of the maximum cumulative hypoxic area in contrast to 22% ± 9% (48% ± 12%) of the July cumulative hypoxic area. Our results suggest that the current nitrogen loading reduction suggested by the Hypoxia Task Force 2013 is not sufficient to reduce the 5‐year moving average Gulf's hypoxic zone to less than 5,000 km2. A reduction of at least 66% (48%) of May NO2+3 (May–June total N) loading is needed when using July hypoxic area as the criterion, while a reduction of 77% (60%) of May NO2+3 (May–June total N) loading is needed when using maximum hypoxic area. Plain Language Summary: Shelf‐wide surveys of hypoxia have been conducted in July since 1985 over the Texas‐Louisiana shelf. The measured hypoxic area has been used as the metric for hypoxia severity. Management practices have been designed based on this metric to control the amount of nitrogen loading to the Mississippi River Basin. Our analysis of 25 years of model output reveals that the reported measured area may not always be the largest of the year. Therefore, using July hypoxic area will underestimate the reduction in nutrient loading required to achieve a 5,000‐km2 area required reduction in nitrogen. Key Points: Monthly hypoxia frequency is predominantly east‐west, with seasonal maximum occurring in June, July, or AugustSatellite sea surface salinity, Chl a, and wind can be used with empirical relations to map spatiotemporally continuous bottom dissolved oxygen fieldsMore nitrogen reduction will be needed to reduce hypoxic area to 5,000‐km2 target once the maximum hypoxic area could be known [ABSTRACT FROM AUTHOR]

An integrated observational field effort that makes simultaneous and collocated measurements of turbulence and fine-scale parameters has been conducted near the Deepwater Horizon oil spill site in the northern Gulf of Mexico (GOM). Full water column profiles are collected across the continental slope in July 2013. The observational results suggest that strong turbulence is patchy and mostly measured in the thermocline and deepwater when using the buoyancy Reynolds number, Re b =200 criterion, the boundary between weak and strong turbulence. Bottom enhanced turbulence is often seen on the continental slope. Using the ratio of the turbulent velocity scale and the oil droplets rising velocity, we develop criteria for when turbulence will dominate the movement of oil droplets and when turbulence can be ignored. Based on the data collected, for oil droplets with rising velocity greater than 6×10 −3 m s −1 , the turbulence effect can be ignored on the continental slope of the northern GOM. For oil droplets with rising speed less than 10 −4 m s −1 , their motions will be affected by the turbulent flow at all depths. For oil droplets with rising speed between 10 −4 and 6×10 −3 m s −1 , the role of turbulence will depend on the strength of the local turbulence and water stratification. We also relate turbulent velocity to the size and density of oil droplets by estimating the rising velocity of different size oil droplets due to balance between buoyancy and drag force. Droplet size and density difference are the two critical parameters in determining the role of turbulence. [ABSTRACT FROM AUTHOR]

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]

Abstract: The circulation and distribution of water properties in the water column of the Gulf of Mexico influence the flux of carbon to the benthic environment. The eddy field of the upper 1000m creates environmental conditions that are favorable for biological productivity in an otherwise oligotrophic subtropical ocean. This eddy field results in the transport of nutrients and organic matter into the photic zone through cross-margin flow of shelf waters, upwelling in cyclones, and uplift from the interaction of anticyclones with bathymetry. These conditions then allow the productivity that becomes a possible source of carbon to the benthos. Data from four cruises during summers of 2000–2002 are used to describe the currents and water property distributions in the deepwater Gulf of Mexico, which consists of water depths greater than 400m. Comparisons are made to historical data sets to provide an understanding of the persistence of the characteristics of the Gulf and the processes that occur there. The currents in the Gulf are surface intensified, have minimum in 800–1000m depths, and also exhibit bottom intensification, especially near sloping topography. Historical time series records show current speeds near-bottom reach 50–100cms−1. At basin scales, these currents tend to flow cyclonically (counter-clockwise) along the bathymetry. These near-bottom, episodic, high-speed currents provide a mechanism for the transport of organic material, in both large and small particle sizes, from one benthic area to another. The distributions of temperature, salinity, nutrients, and dissolved oxygen during the study appear to be unchanged from historical findings. The source waters for the deep Gulf are the water masses brought into the Gulf by the Loop Current system. The properties in the upper 100–200m are the most variable of the water column, consistent with their proximity to wind mixing, river discharge mixing, and atmospheric influences. Below 1500m, there are no major horizontal variations in these water properties. [Copyright &y& Elsevier]

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]

Abstract: The hypoxic region in the northern Gulf of Mexico, one of the largest man-made hypoxic zones in the world, has received extensive scientific study and management interest. A previous statistical study has concluded that in addition to anthropogenic nitrogen loading, the observed hypoxic extent is correlated to the duration of upwelling favorable (westerly) wind without elucidating the underlying mechanism. In this study, we use a three-dimensional, coupled hydrological–biogeochemical model to mechanistically examine how variations of the hypoxic area are related to the duration of upwelling-favorable wind. We performed scenario experiments with different durations of upwelling-favorable wind using realistic winds from summer 2002 (when upwelling-favorable winds were present only for about 1month) and summer 2009 (when upwelling-favorable conditions started early and persisted for about 2months). While the maximum simulated hypoxic area is approximately 15,000km2 in both cases, the evolutions of the hypoxic area and the dates when its maximum extent are reached are different. With an early start of persistently upwelling-favorable wind in 2009, the hypoxic area reached its maximum in early summer and decreased afterwards. By contrast, the hypoxic area was small in early summer of 2002 and peaked during the short period of upwelling-favorable wind in late summer. The model revealed that the wind influences the evolution of the hypoxic area by changing the vertical and horizontal distributions of the low salinity, high chlorophyll water on the shelf. [Copyright &y& Elsevier]