Contiguous Low Oxygen Waters between the Continental Shelf Hypoxia Zone and Nearshore Coastal Waters of Louisiana, USA: Interpreting 30 Years of Profiling Data and Three-Dimensional Ecosystem Modeling.

The multidecadal expansion of northern Gulf of Mexico continental shelf hypoxia is a striking example of the adverse effects of anthropogenic nutrient enrichment on coastal oceans. Increased nutrient inputs and widespread shelf hypoxia have resulted in numerous dissolved oxygen (DO) water quality problems in nearshore coastal waters of Louisiana. A large hydrographic dataset compiled from research programs spanning 30 years and the three-dimensional hydrodynamic-biogeochemical model CGEM (Coastal Generalized Ecosystem Model) were integrated to explore the interconnections of low DO waters across the continental shelf to nearshore coastal waters of Louisiana. Cross-shelf vertical profiles showed contiguous low DO bottom waters extending from the shelf to coastal waters nearly every year in the 30+ year time series, which were concurrent with strong cross-shelf pycnoclines. A threshold Brunt-Väisälä frequency of 40 cycles h-1 was critical to maintaining the cross-shelf subpycnocline layers and facilitating the formation of a contiguous low DO water mass. Field observations and model simulations identified periods of wind-driven bottom water upwelling lasting between several days to several weeks, resulting in both physical advection of oxygen-depleted offshore waters to the nearshore and enhanced nearshore stratification. Both the upwelling of low DO bottom waters and in situ respiration were of sufficient temporal and spatial extent to drive DO below Louisiana's DO water quality criteria. Basin-wide nutrient management strategies aimed at reducing nutrient inputs and shelf hypoxia remain essential to improving the nearshore coastal water quality across the northern Gulf of Mexico.

Modeling Spatiotemporal Patterns of Ecosystem Metabolism and Organic Carbon Dynamics Affecting Hypoxia on the Louisiana Continental Shelf.

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. In this study we present results from a three-dimensional hydrodynamic-biogeochemical model named CGEM (Coastal Generalized Ecosystem Model) applied 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 successfully reproduced spatial and temporal patterns of carbon production, respiration, and bottom-water oxygen gradients compared to field observations. We then used interannual simulations to identify transport of particulate organic carbon (POC) from nearshore areas where riverine organic matter and phytoplankton carbon production are greatest. The spatial disconnect between carbon production and respiration in our simulations was driven by westward and offshore POC flux, a pattern that supported heterotrophic respiration on the middle shelf where hypoxia is frequently observed. These results validate the importance of offshore carbon flux to hypoxia formation, particularly on the west shelf where hypoxic conditions are more variable.

Modeling the Relative Importance of Nutrient and Carbon Loads, Boundary Fluxes, and Sediment Fluxes on Gulf of Mexico Hypoxia.

The Louisiana continental shelf in the northern Gulf of Mexico experiences bottom water hypoxia in the summer. In this study, we applied a biogeochemical model that simulates dissolved oxygen concentrations on the shelf in response to varying riverine nutrient and organic carbon loads, boundary fluxes, and sediment fluxes. Five-year model simulations demonstrated that midsummer hypoxic areas were most sensitive to riverine nutrient loads and sediment oxygen demand from settled organic carbon. Hypoxic area predictions were also sensitive to nutrient and organic carbon fluxes from lateral boundaries. The predicted hypoxic area decreased with decreases in nutrient loads, but the extent of change was influenced by the method used to estimate model boundary concentrations. We demonstrated that modeling efforts to predict changes in hypoxic area on the continental shelf in relationship to changes in nutrients should include representative boundary nutrient and organic carbon concentrations and functions for estimating sediment oxygen demand that are linked to settled organic carbon derived from water-column primary production. On the basis of our model analyses using the most representative boundary concentrations, nutrient loads would need to be reduced by 69% to achieve the Gulf of Mexico Nutrient Task Force Action Plan target hypoxic area of 5000 km(2).

The formation and fate of internal waves in the South China Sea.

Internal gravity waves, the subsurface analogue of the familiar surface gravity waves that break on beaches, are ubiquitous in the ocean. Because of their strong vertical and horizontal currents, and the turbulent mixing caused by their breaking, they affect a panoply of ocean processes, such as the supply of nutrients for photosynthesis, sediment and pollutant transport and acoustic transmission; they also pose hazards for man-made structures in the ocean. Generated primarily by the wind and the tides, internal waves can travel thousands of kilometres from their sources before breaking, making it challenging to observe them and to include them in numerical climate models, which are sensitive to their effects. For over a decade, studies have targeted the South China Sea, where the oceans' most powerful known internal waves are generated in the Luzon Strait and steepen dramatically as they propagate west. Confusion has persisted regarding their mechanism of generation, variability and energy budget, however, owing to the lack of in situ data from the Luzon Strait, where extreme flow conditions make measurements difficult. Here we use new observations and numerical models to (1) show that the waves begin as sinusoidal disturbances rather than arising from sharp hydraulic phenomena, (2) reveal the existence of >200-metre-high breaking internal waves in the region of generation that give rise to turbulence levels >10,000 times that in the open ocean, (3) determine that the Kuroshio western boundary current noticeably refracts the internal wave field emanating from the Luzon Strait, and (4) demonstrate a factor-of-two agreement between modelled and observed energy fluxes, which allows us to produce an observationally supported energy budget of the region. Together, these findings give a cradle-to-grave picture of internal waves on a basin scale, which will support further improvements of their representation in numerical climate predictions.

Short-term Influences on Suspended Particulate Matter Distribution in the Northern Gulf of Mexico: Satellite and Model Observations.

Energetic meteorological events such as frontal passages and hurricanes often impact coastal regions in the northern Gulf of Mexico that influence geochemical processes in the region. Satellite remote sensing data such as winds from QuikSCAT, suspended particulate matter (SPM) concentrations derived from SeaWiFS and the outputs (sea level and surface ocean currents) of a nested navy coastal ocean model (NCOM) were combined to assess the effects of frontal passages between 23-28 March 2005 on the physical properties and the SPM characteristics in the northern Gulf of Mexico. Typical changes in wind speed and direction associated with frontal passages were observed in the latest 12.5 km wind product from QuikSCAT with easterly winds before the frontal passage undergoing systematic shifts in direction and speed and turning northerly, northwesterly during a weak and a strong front on 23 and 27 March, respectively. A quantitative comparison of model sea level results with tide gauge observations suggest better correlations near the delta than in the western part of the Gulf with elevated sea levels along the coast before the frontal passage and a large drop in sea level following the frontal passage on 27 March. Model results of surface currents suggested strong response to wind forcing with westward and onshore currents before the frontal passage reversing into eastward, southeastward direction over a six day period from 23 to 28 March 2005. Surface SPM distribution derived from SeaWiFS ocean color data for two clear days on 23 and 28 March 2005 indicated SPM plumes to be oriented with the current field with increasing concentrations in nearshore waters due to resuspension and discharge from the rivers and bays and its seaward transport following the frontal passage. The backscattering spectral slope γ, a parameter sensitive to particle size distribution also indicated lower γ values (larger particles) in nearshore waters that decreased offshore (smaller particles). The use of both satellite and model results revealed the strong interactions between physical processes and the surface particulate field in response to the frontal passage in a large riverdominated coastal margin.