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The Gulf of Mexico Coastal Hypoxia Glider Experiment was designed to assess the feasibility of using ocean glider technology in the coastal hypoxic zone of the northern Gulf of Mexico in Summer/Fall 2014. The objectives were (1) to coordinate and operate multiple autonomous buoyancy ocean gliders in depths less than 50 m and (2) to determine how close to the bottom gliders can reliably reach without making contact. Strong vertical and horizontal stratification gradients, strong coastal currents, and the low-oxygen conditions that occur within the lower water column characterize the coastal area of the northern Gulf of Mexico. These environmental conditions combine with the presence of more than 5,000 surface piercing oil/gas structures to make piloting and navigation in the region challenging. We quantify glider performance to assess the usefulness of buoyancy gliders to address the National Oceanic and Atmospheric Administration Action Plan goal to monitor the spatial extent, duration, and severity of the Gulf hypoxic zone. We find that the gliders, despite the operational challenges, were consistently able to travel from the surface to the oxygen-depleted depths of subpycnocline waters, that is, within 2 m of the ocean bottom. Our assessment is that gliders are able to provide real-time observations suitable to monitor coastal hypoxia. [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]

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]

The spring-to-summer seasonal transition in the spatial and vertical distribution of chromophoric dissolved organic matter (CDOM) optical properties was examined during surveys conducted in March, May, July, and August of 2005 in the waters influenced by the Mississippi-Atchafalaya River system of the northwestern Gulf of Mexico. Strong seasonal river influences and a well-correlated inverse relationship between CDOM absorption at 412 nm and salinity during spring of 2005 suggests conservative mixing in the water column between the riverine freshwater sources and the oceanic end members. Trends in the CDOM absorption and the spectral absorption slope (S) and deviations from the end-member conservative mixing line in surface and bottom waters during the summer suggest apparent sources and sinks of CDOM. Elevated CDOM in lower-salinity surface waters at many stations appears to be linked to high concentrations of dissolved oxygen (DO; >10 mg L−1) and elevated chlorophyll concentrations. Effects of photo-oxidation in surface waters were observed as increasing S and enhanced CDOM loss in the ~27-32 salinity range and were attributed to seasonal stratification and the increased solar radiation during the summer. In the bottom waters, elevated CDOM levels in the ~32-36 salinity range were associated with low DO and relatively higher bacterial abundance; this suggests a potential CDOM source due to microbial remineralization of organic matter in the hypoxic zone in bottom waters. Seasonal dynamics associated with CDOM loss in surface waters and CDOM gains in bottom waters may be another pathway influencing hypoxia in coastal waters. [ABSTRACT FROM AUTHOR]

Abstract: A realistic hydrodynamic model of the Texas–Louisiana shelf is configured with various simple oxygen respiration models to isolate the effects of stratification and circulation on the formation and maintenance of hypoxia. Biological activity is parameterized through various forms of respiration rather than using a complex biogeochemical model. The model domain covers the region that has historically been observed to be affected by seasonal hypoxia, and is forced with observed fresh water fluxes from the Mississippi and Atchafalaya Rivers and winds. Three simple parameterizations of biological respiration are compared. Results of the numerical simulations indicate that water column respiration (dependent on the position of the two regional fresh water plumes) reproduces observed temporal and spatial structures of seasonal hypoxia in Louisiana Bight, whereas benthic respiration (dependent on local temperature and oxygen concentrations) reproduces the temporal and spatial structure of hypoxia west of Terrebonne Bay. The differences in the structure of hypoxia are related to the differences in vertical stratification east and west of Terrebonne Bay, which are controlled by the outflow characteristics from the two major river plumes. The model shows two dynamically distinct plumes. The Mississippi River plume enters the shelf near the shelf edge, forms a recirculating gyre in Louisiana Bight, and typically does not interact directly with topography. Conversely, the Atchafalaya River plume enters a broad shelf at the coast, is more diffuse, and interacts more with the shallow coastal topography. Both plumes are strongly affected by winds, and tend offshore during the mean summer conditions of upwelling winds. The principle conclusion of this study is that the biological processes responsible for producing hypoxia change from east to west, with the shelf region south of Terrebone Bay being the approximate dividing line between water column respiration (predominantly causing hypoxia to the east) and benthic respiration (causing hypoxia to the west). Also, the formation and destruction of hypoxia is a primarily vertical process; hypoxia formed in one region is not advected laterally to different shelf regions. [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]

Abstract: Over the past two decades China has become the largest global consumer of fertilizers, which has enhanced river nutrient fluxes and caused eutrophication and hypoxia in the Yangtze (Changjiang) large river delta-front estuary (LDE). In this study, we utilized plant pigments, lignin-phenols, stable isotopes (δ13C and δ15N) and foraminiferal microfossils in 210Pb dated cores to examine the history of hypoxia in the Changjiang LDE. Two sediment cores were collected onboard R/V Dong Fang Hong 2 using a stainless-steel box-corer; one at a water depth of 24.7m on Jun. 15, 2006 and the other at 52m on Nov. 20, 2007, both in the hypoxic region off the Changjiang LDE. There has been a significant increase in the abundance of plant pigments after 1979 that are indicators of enhanced diatom and cyanobacterial abundance, which agrees with post-1980 record of increasing nutrient loads in the Changjiang River. The increased inputs of terrestrially derived materials to the LDE are largely woody plant sources and most likely due to deforestation that began in the early 1950s. However, post-1960 lignin data did not reflect enhanced loading of woody materials despite continued deforestation possibly due to increased trapping from greater dam construction, a reduction of deforestation in the drainage basin since the last 1990s, and soil conservation practices. The lack of linkages between bulk indices (stable isotopes, % OC, molar C/N ratios) and microfossil/chemical biomarkers may reflect relative differences in the amount of carbon tracked by these different proxies. Although NO3 − is likely responsible for most of the changes in phytoplankton production (post 1970s), historical changes in N loading from the watershed and hypoxia on the LDE shelf may not be as well linked in East China Sea (ECS) sediments due to possible denitrification/ammonification processes; finally, increases in low-oxygen tolerant foraminiferal microfossils indicate there has been an increase in the number of hypoxic bottom water events on the Changjiang LDE over the past 60years. [Copyright &y& Elsevier]