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An analytical multiphase plume model, combined with time-varying flow and hydrographic fields generated by the 3-D South Atlantic Bight and Gulf of Mexico model (SABGOM) hydrodynamic model, were used as input to a Lagrangian transport model (LTRANS), to simulate transport of oil droplets dispersed at depth from the recent Deepwater Horizon MC 252 oil spill. The plume model predicts a stratification-dominated near field, in which small oil droplets detrain from the central plume containing faster rising large oil droplets and gas bubbles and become trapped by density stratification. Simulated intrusion (trap) heights of~310-370 m agree well with the midrange of conductivity-temperature-depth observations, though the simulated variation in trap height was lower than observed, presumably in part due to unresolved variability in source composition (percentage oil versus gas) and location (multiple leaks during first half of spill). Simulated droplet trajectories by the SABGOM-LTRANS modeling system showed that droplets with diameters between 10 and 50 μm formed a distinct subsurface plume, which was transported horizontally and remained in the subsurface for >1 month. In contrast, droplets with diameters ≥90 μm rose rapidly to the surface. Simulated trajectories of droplets ≤50 μm in diameter were found to be consistent with field observations of a southwest-tending subsurface plume in late June 2010 reported by Camilli et al. [2010]. Model results suggest that the subsurface plume looped around to the east, with potential subsurface oil transport to the northeast and southeast. Ongoing work is focusing on adding degradation processes to the model to constrain droplet dispersal.

A better understanding of oil droplet formation, degradation, and dispersal in deep waters is needed to enhance prediction of the fate and transport of subsurface oil spills. This research evaluates the influence of initial droplet size and rates of biodegradation on the subsurface transport of oil droplets, specifically those from the Deepwater Horizon oil spill. A three-dimensional coupled model was employed with components that included analytical multiphase plume, hydrodynamic and Lagrangian models. Oil droplet biodegradation was simulated based on first order decay rates of alkanes. The initial diameter of droplets (10-300 μm) spanned a range of sizes expected from dispersant-treated oil. Results indicate that model predictions are sensitive to biodegradation processes, with depth distributions deepening by hundreds of meters, horizontal distributions decreasing by hundreds to thousands of kilometers, and mass decreasing by 92-99% when biodegradation is applied compared to simulations without biodegradation. In addition, there are two-to four-fold changes in the area of the seafloor contacted by oil droplets among scenarios with different biodegradation rates. The spatial distributions of hydrocarbons predicted by the model with biodegradation are similar to those observed in the sediment and water column, although the model predicts hydrocarbons to the northeast and east of the well where no observations were made. This study indicates that improvement in knowledge of droplet sizes and biodegradation processes is important for accurate prediction of subsurface oil spills.

Management of marine and estuarine fish and shellfish would benefit from a numerical approach that quantifies the impacts of climate variability and eutrophication. We present a proof-of-concept habitat volume model that incorporates predictions from a 3-dimensional biophysical model. Using temperature, salinity, and dissolved oxygen, habitat volumes were calculated based on threshold physiological tolerances (fixed criteria) and potential growth (bioenergetics) for Atlantic sturgeon Acipenser oxyrinchus. Simulations from a coupled oxygen and hydrodynamic model of the Chesapeake Bay, USA, were used to estimate habitat volumes of juvenile sturgeon and assess the sensitivity of habitat to environmental factors. In winter, salinity controlled the required (needed for survival) and optimal (needed for highest growth) habitat. Temperature and salinity defined spring and autumn optimal habitat, and a combination of salinity, temperature and dissolved oxygen influenced habitat volumes during summer. Although average summertime oxygen limitation reduced the volumes of juvenile habitat by 3.3−28.0%, the largest reductions in summertime habitat resulted from temperature limitation. The average difference in annual and seasonal volumes between fixed-criteria and bioenergetics methods was approximately 14%, with similar trends over the annual cycle for most life stages and habitat types. We conclude that fixed-criteria habitat volume models would be suitable when bioenergetics information is not available. Both habitat volume models can be used to assess the impacts of climate change and eutrophication on the habitat of fish and shellfish in regions where hydrodynamic models exist and for species for which physiological tolerances are known.KEY WORDS: Habitat · Model · Biophysical · Hypoxia · Atlantic sturgeon · Physiological tolerance · Oxygen model Resale or republication not permitted without written consent of the publisher

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