Prospects For Gulf of Mexico Environmental Recovery and Restoration

Previous oil spills provide clear evidence that ecosystem restoration efforts are challenging, and recovery can take decades. Similar to the Ixtoc 1 well blowout in 1979, the Deepwater Horizon (DWH) oil spill was enormous both in volume of oil spilled and duration, resulting in environmental impacts from the deep ocean to the Gulf of Mexico coastline. Data collected during the National Resource Damage Assessment showed significant damage to coastal areas (especially marshes), marine organisms, and deep-sea habitat. Previous spills have shown that disparate regions recover at different rates, with especially long-term effects in salt marshes and deepsea habitat. Environmental recovery and restoration in the northern Gulf of Mexico are dependent upon fundamental knowledge of ecosystem processes in the region. PostDWH research data provide a starting point for better understanding baselines and ecosystem processes. It is imperative to use the best science available to fully understand DWH environmental impacts and determine the appropriate means to ameliorate those impacts through restoration. Filling data gaps will be necessary to make better restoration decisions, and establishing new baselines will require long-term studies. Future research, especially via NOAA’s RESTORE Science Program and the state-based Centers of Excellence, should provide a path to understanding the potential for restoration and recovery of this vital marine ecosystem

Influences of River Discharge on Biological Production in the Inner Shelf: A Coupled Biological and Physical Model of the Louisiana-Texas Shelf

A coupled biological and physical model was applied to study the influence of river discharge on biological variability on the Louisiana-Texas (LATEX) continental shelf. The physical part included a primitive-equation turbulent closure model, and the biological part was a simple phytoplankton (P), zooplankton (Z), and nutrient (N) model. The model was forced by freshwater discharge from the river and ran prognostically under initial conditions of springtime water stratification and a steady-state solution of the P-Z-N model with no horizontal dependence. A nutrient source was included at the mouth of the river. The model predicted a well-defined density frontal zone on the inner shelf. The biological field showed a region of high phytoplankton biomass in the whole water column near the coast and a moderately high biomass patch in the upper 10 m at the outer edge of the frontal zone. A high concentration dome of nutrients was found near the bottom within the frontal zone. New production of nutrients was high throughout the whole water column near the coast and in the upper 10 m at the outer edge of the density front, but lower in the frontal zone. The model results were in reasonable agreement with observational data taken from a May 1993 interdisciplinary survey on the LATEX shelf. Cross-shelf distribution of biological production varied significantly with direction of wind stress but not with the diurnal tide. The model results suggested that the bottom-rich nutrient distribution within the frontal zone was caused by the interaction of physical and biological processes. Physical processes caused the formation of an area of high nutrient concentration in the weak current region within the frontal zone. Subsequent biological processes limited the increase of nutrients in the upper euphotic zone and hence led to the bottom-rich nutrient pattern

Distribution and Controlling Mechanisms of Primary Production on the Louisiana-Texas Continental Shelf

The northwest (NW) Gulf of Mexico is marked by strong seasonal patterns in regional and mesoscale circulation and variable effects of riverine/estuarine discharge, which influence distributions of nutrients, phytoplankton biomass and primary production. During a series of five cruises in the NW Gulf of Mexico in 1993 and 1994, an extensive data set was collected including nutrients, phytoplankton biomass (chlorophyll a), and photosynthesis-irradiance (P-E) parameters. Primary production was estimated using P-E parameters in conjunction with profiles of biomass and irradiance. Relatively high biomass and primary production were observed in inner shelf waters during spring conditions of high river discharge. This was attributed to the retention of biomass and nutrients on the shelf by the combination of high river outflow and a westward flow along the inner shelf with consequent onshore Ekman component. During summer, when surface currents shifted towards the north and east, values of nutrients, biomass and primary production were relatively high east of Galveston Bay and decreased outward from the coast. This pattern was apparently a consequence of nutrient inputs from riverine, upwelling and benthic sources. Nutrients, biomass and productivity in the western portion of the study area in summer were generally lower as a result of the upcoast Row of oligotrophic offshore water. Inter-annual variability was observed between November 1993 and 1994 with higher biomass and productivity occurring in November 1993. This was partially attributed to higher river discharge prior to November 1993, retention of biomass and nutrients by the downcoast flow along the inner shelf, and possibly, injection of nutrients onto the shelf at the shelf break. Our findings demonstrate that the interaction of circulation and availability of Light and nutrients are largely responsible for variations in primary production. Nitrogen appeared to be the primary limiting nutrient, however, a potential for phosphate limitation was also observed particularly during periods of higher river discharge. Light availability was a critical variable during the fall and winter months, when higher primary production was restricted to shallow waters where vertical mixing was constrained by bottom topography. In deep waters, counteractive changes in nutrient and light availability apparently resulted in minor temporal variation between seasons. The annual carbon production in the Louisiana-Texas (LATEX) continental shelf region was estimated to be 159 g C m(-2) year(-1), which is within the range of prior estimates for this region. Given that the area of the study region was approximately 140,000 km(2), this would be equivalent to an areal carbon production of about 22.2 million metric tons. (C) 2000 Elsevier Science B.V. All rights reserved

Interactions of Mesoscale Features with Texas Shelf and Slope Waters

During April and May 1993, the typical south and southwestward directed flow over the Texas shelf interacted with current rings located just off the Texas shelf in the Gulf of Mexico. The rings, a cyclone/anticyclone eddy pair, were oriented such that water was transported onto the shelf between the rings and water was drawn from the shelf on the rings\u27 offshore-directed limbs. A shelf convergence zone transported water to the shelf edge where some of the water became entrained between the cyclone/anticyclone pair. Active shelf-slope exchange such as that seen in the April-May 1993 episode may be a more typical example of synoptic shelf circulation tl;an the general circulation pattern suggested by averaging multiannual observations. Nutrient and particle distributions were influenced by the interactions of the shelf current with the rings. Where the cyclonic and anticyclonic eddies directed flow off the shelf, particles were transported off the shelf: Additionally, particles in the bottom nepheloid layer were transported off the shelf in the shoreward limb of the cyclone, even though currents in this limb were directed along the isobaths. Nutrient concentrations, hence nutrient fluxes, were generally low in near-surface waters everywhere in the study area. At mid-depth and in near-bottom waters, nutrient fluxes were highest near the cyclone because of higher current speeds in this feature and higher nutrient concentrations at depth. Copyright (C) 1996 Elsevier Science Ltd

A Comparison of In-Situ and Simulated In-Situ Methods for Estimating Oceanic Primary Production

Primary production data measured by in situ (IS) and \u27simulated\u27 in situ (SIS) incubations were compared. To minimize differences between the two types of incubations, SIS experiments were conducted in temperature-controlled incubators in which the spectral distribution and irradiance were adjusted to approximate IS conditions. IS available irradiance (I(IS)) was computed from vertical attenuation of integrated surface irradiance. Vertical attenuation was estimated using a spectral irradiance model, validated by measured profiles of the vertical attenuation coefficient. IS incubations were carried out using two methods. The first involved deployment of bottles on a drifting array for whole-day (dawn to dusk) incubations. The second method employed an autonomous submersible incubation device that performed short term (\u3c 1 h) incubations at multiple depths. Differences between whole-day IS and SIS incubation estimates were attributed partially to differences between I(IS) and SIS-available irradiance (I(SIS)). Photosynthesis-irradiance (P-I) properties of IS and SIS populations from the whole-day incubations were not significantly different. P-I properties of the short-term IS and SIS populations were significantly different, although estimates of P(B) (mg C mg Chl-1 h-1) from contemporaneous IS and SIS incubations did not differ by \u3e 40%. Integrated water-column primary production (IPP) estimated using P-I models derived from SIS data were within 15% of IS estimates of IPP

Geochemical Patterns in Sediments Near Offshore Production Platforms

Patterns of the geochemical characteristics of sediments adjacent to three production platforms (22-150 m water depths) in the northwestern Gulf of Mexico were determined y the presence of the structure, the amount and type of materials discharged from the structure, and the local hydrographic setting. Sediments close to platforms (\u3c500m) were enhanced in coarse-grain materials primarily derived from discharged muds and cuttings. Hydrocarbon and trace metal (Ag, Ba, Cd, Hg, Pb, and Zn) contaminants were associated with these coarse-grain sediments. Contaminants were asymmetrically distributed around each platform in response to the prevailing currents. Contaminant concentrations t most locations were below levels thought to induce biological responses. At a few loctions close to one platform, trace metal (i.e., Cd, Hg) concentrations exceeded levels thought to induce biological effects. In deep water (\u3e80 m), sediment trace metal contaminant patterns were stable over time frames of years. A few metals (Pb, Cd) exhibited evidence of continued accumulation in sediments over the history of the platform