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Abstract: Moored observations in the northern Arabian Sea (NAS) show substantial velocity, temperature and dissolved oxygen fluctuations, accompanied by episodic high salinity intrusions with maximum on time scales of 2–10 days after the passage of Cyclone Gonu in 2007. These events are characterized by a rapid increase in temperature, salinity and dissolved oxygen followed by an abrupt decline. The mechanisms behind these high salinity events are investigated using a comprehensive dataset of temperature and salinity profiles from ARGO floats and sea surface height anomaly maps. The spatial and temporal distribution of the Persian/Arabian Gulf outflow to the Sea of Oman is also studied using ARGO profiles. Persian Gulf water (PGW) is mainly measured close to the Strait of Hormuz or along the Oman coast on the continental slope in the Sea of Oman. Both mooring and ARGO data show that high salinity PGW can be advected off the slope and into the interior. More high salinity water is measured in the interior of the Sea of Oman within three months after the Gonu passage in summer 2007, which is caused by the combination effect of the oceanic responses to Cyclone Gonu and a clockwise eddy circulation located at northern Ras al Hadd. At other times, the high salinity water appears more in isolated patches and rare in the interior. This study provides a first look at the high salinity events appearing after Gonu and the properties and dynamics of the PGW in the northern Arabian Sea and Sea of Oman. [Copyright &y& Elsevier]

The article discusses the results of a study that examined the potential for gastropod and bivalve shells to serve as proxies for benthic respiration and hypoxia in the Gulf of Mexico during three periods from 1964 to 2009. The mollusks shells were analyzed for oxygen and carbon stable isotopes. The study showed a correlation between carbon stable isotopes of dissolved inorganic carbon and dissolved oxygen.

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: 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]