Studies Explore the Dynamics of How Offshore Oil Spills Affect Coastal Environments

– AUGUST 17, 2020

(Click to enlarge) Students Jessica Diller (bottom) and Kamala Earl (top) prepare enclosed treatment plots in oiled marshes of Barataria Bay, La. (Photo by Gabriel Kasozi)

Oil from the 2010 Deepwater Horizon incident moved from deep waters to coastal shorelines, overwhelming their natural defenses which, in turn, slowed or prevented their recovery. Scientists with the Gulf of Mexico Research Initiative (GoMRI) have been assessing the health of these complex environments that experience stressors from multiple sources, providing information that can inform response decisions during future disturbances.

Here are brief recaps of five recent studies exploring coastal hydrodynamics that drive transport processes, the indirect and sub-lethal effects on coastal fishes and marsh snails and blue crabs, and the replanting of native grasses in oil-affected areas.

The Barataria Basin protects the Louisiana coast against wind and waves; however, these areas suffered heavy oiling following Deepwater Horizon that exacerbated erosion there and increased vulnerability to physical forces. Researchers sought to better understand wave and current dynamics affecting water and material transport at six inlets connecting Barataria Bay with the Gulf of Mexico using a 3D high-resolution wave-current coupled numerical model (FVCOM-SWAVE) that they validated against observational data during April–June 2010. Simulations showed that at the inlets’ shelf side, where opposing and following currents amplify wave blocking and stretching effects, the spectral energy of waves and swells from the Gulf dropped by an order of magnitude when approaching the inlets, and southeasterly winds caused the highest wave height. There was a distinct change in wave characteristics on the inlets’ bay side and the basin’s middle area, where the wave field is strongly dependent on the local wind intensity and direction. Inside the basin’s middle area, a cold front proved to be a more energetic event than persistent southeasterly winds and enhanced wave height and changed wave direction. The researchers published their findings in Ocean Modelling: Wave dynamics near Barataria Bay tidal inlets during spring-summer time. Data are publicly available through the Gulf of Mexico Research Initiative Information and Data Cooperative (GRIIDC) at doi: 10.7266/n7-e9x3-6136.

Upwelling (when cold water from below replaces warm wind-dispersed surface water) and downwelling (when wind forces surface water downward) are coastal processes that influence biogeochemical pathways and material transport from deeper waters into estuaries. Researchers sought to improve our understanding about the connectivity between the nearshore region and Mobile Bay using data collected during downwelling and upwelling events in the summer of 2016. They observed that it took ~5 days for an offshore cold-water mass to travel the 48-km estuary length, which drove a 7°C swing in temperature over a 17-day period, with the warmest temperatures at the bottom of the estuary’s 12-meter deep shipping channel. Upwelling events brought higher salinities, colder temperatures, and low levels of dissolved oxygen, while downwelling generated relatively less variability in these metrics. The temperature and salinity changes can generate variability in the processes that drive subtidal circulation, which may increase how long upwelled materials like pollutants might remain in the estuary. They published their findings in Journal of Geophysical Research: Oceans: Effects of Coastal Upwelling and Downwelling on Hydrographic Variability and Dissolved Oxygen in Mobile Bay. Data are publicly available through the GRIIDC at doi: 10.7266/n7-s40s-3785.

Researchers compared fisheries-independent nekton community data from two decades (1997-2001 and 2007-2009) in coastal Alabama and Mississippi waters to data collected immediately after the spill (2010-2012) and when most surface oil had degraded (2014-2017). They found that immediately after the spill, there was increased catch abundance, decreased diversity, and altered community composition. These changes may have resulted from a combination of oil-related variables such as oil avoidance (such as fish emigrating eastward from Louisiana), altered freshwater discharge, the death of higher order consumers (such as fishes, dolphins, and birds) and ensuing food web alterations due to differential oil toxicities, and the NOAA-mandated fisheries closures. The increased abundance in fishes and macroinvertebrates after the spill contradicted the authors’ expectations of decreased abundance. They offered the possibility that trophic release and other indirect mechanisms stemming from the oil spill may have caused some species to increase (such as Atlantic Bumper), which they plan to explore using mass balanced ecosystem models. The researchers published their findings in Marine Pollution Bulletin: Disturbance-driven changes to northern Gulf of Mexico nekton communities following the Deepwater Horizon oil spill. Data are publicly available through the GRIIDC at doi: 10.7266/N78W3BSF, 10.7266/N72R3PNJ, 10.7266/N7BZ648P, 10.7266/N7H70CWP.

Researchers conducted mesocosm studies to examine the sub-lethal effect of Macondo oil (buried in sand in the mesocosms) on predator-prey interactions of blue crabs (predator) and periwinkle snails (prey). When no blue crab was present, the snails’ survival was the same in oil and no oil treatments; however, when a blue crab was present, oil reduced snail survival by 15%. Blue crabs consumed 43% of the snails in oiled treatments compared to 28% in unoiled conditions. The oil-exposed snails’ climbing height on Spartina alterniflora (planted in the mesocosms) was reduced (45.7 cm to 40.9 cm) regardless of blue crab presence, and that reduction is likely what caused the snails increased mortality. The authors said that immediate field research after an oil spill would help verify the results of this study and, more importantly, provide additional insight into how oil affects predator-prey interactions and possible food web alterations. The researchers published their findings in Journal of Experimental Marine Biology and Ecology: The effects of oil on blue crab and periwinkle snail interactions: A mesocosm study. Data are publicly available through the GRIIDC at doi: 10.7266/n7-2hgq-jh13.

Planting and fertilization are common wetland habitat restoration methods, but how these treatments impact the native soil microbial community when implemented in oiled salt marshes is not well understood. Using DNA sequencing, researchers helped fill that knowledge gap by determining how the soil microbial community responds to these restoration efforts. The team conducted a two-year field experiment to evaluate how planting Spartina alterniflora, fertilization, and their interaction affect the soil microbial community at five sites with minimal vegetation regrowth in heavily oiled Barataria Bay, Louisiana. Compared to control sites, replanted S. alterniflora rapidly and significantly altered the soil microbial community composition, accelerating transition of the soil microbial community from an unvegetated to a vegetated microbiome type that included greater abundances of typical sulfate-reducing bacteria. Fertilizer alone had a significant effect on the soil microbial community structure in the absence of transplants between the first 2 to 13 months until vegetation took hold naturally in unplanted sites. The team’s finding builds on the authors’ previous work suggesting that the recovery of the benthic community is linked to the regrowth of S. alterniflora after oil-induced mortality.  The study also supports previous findings that showed the total microbial community structure in vegetated salt marshes is resistant to fertilizer effects. These results indicate that transplanted S. alterniflora promotes a rapid shift in the soil microbial community composition with concurrent establishment of a diverse sulfate-reducing bacterial community and mediates the fertilizer’s effect on the soil microbial community in previously oiled salt marsh systems. The researchers published their findings in Ecological Engineering: Planting Spartina alterniflora in a salt marsh denuded of vegetation by an oil spill induces a rapid response in the soil microbial community. Data are publicly available through the GRIIDC at doi: 10.7266/N7MP51NQ and 10.7266/n7-q0rj-2170.

These studies highlight the diverse and complex responses that economically and ecologically important coastal environments can have to oil spills. Two studies described coastal water hydrodynamics that drive transport processes: Wave dynamics in the Barataria Basin are influenced by variable water properties and conditions, such as riverine flows, local winds, and cold fronts; and Mobile Bay observations highlight how offshore changes impact shallow estuaries, the extent and pathways of intrusion in these systems, and suggests that shipping channels are important facilitating transport pathways. Two studies examined the complexities of effects stemming from the oiling of coastal waters and shorelines: It is likely that oil exposure influenced the abundance, diversity, and composition of nekton communities, though confounding variables during Deepwater Horizon may have obscured any negative effects; and sublethal oil effects can include alterations to nekton behavior and predator-prey interactions which may have broader food web implications. One study was forward looking, providing evidence that replanting S. alterniflora could help accelerate habitat restoration following an oil spill. All of these studies helped improve our understanding of the coastal environment’s responses following an oil spill and can inform science-based sustainable management framework to protect and restore coastal and estuarine systems.

Here are related studies on oil spill dynamics and coastal environments for further reading:

• Recovery of Saltmarsh Meiofauna Six Years after the Deepwater Horizon Oil Spill
• Study Links Recovery of Oiled Marsh with Return of Salt Marsh Vegetation
• Study Identifies Positive Influencers on Marsh Infauna Recovery after Oiling
• Study Examines How Multiple Plant Species Affect Marsh Response to Oiling
• Study Reveals Mix of Resilience and Vulnerability of Oiled Marsh Grass
• Study Estimates Reduced Nitrogen Removal Capacity in Marshes with Vegetation Loss
• Study Finds Hurricane Isaac Prolonged Oil Spill Impacts on Some Marsh Insects and Spiders
• Study Uses Fiddler Crabs and Periwinkle Snails to Monitor Long-Term Marsh Health after Oil Spill
• Five-Year Study Finds Deepwater Horizon Negatively Affected Periwinkle Snails
• Study Finds Oil Impacts on Fiddler Crabs May Also Affect Broader Marsh Health

By Nilde Maggie Dannreuther and Stephanie Ellis. Contact maggied@ngi.msstate.edu with questions or comments.

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This research was made possible in part by grants from the Gulf of Mexico Research Initiative (GoMRI) to the Coastal Waters Consortium III (CWC III), the Consortium for Oil Spill Exposure Pathways in Coastal River-Dominated Ecosystems (CONCORDE), the Alabama Center for Ecological Resilience (ACER), and to Louisiana State University for the project A Decade-Long Study on Impact, Recovery, and Resilience in Louisiana Salt Marshes: The Evolution of the Oil Transformation Compounds and Plant-Soil-Microbial Responses to the Deepwater Horizon Oil Spill.

The Gulf of Mexico Research Initiative (GoMRI) is a 10-year independent research program established to study the effect, and the potential associated impact, of hydrocarbon releases on the environment and public health, as well as to develop improved spill mitigation, oil detection, characterization and remediation technologies. An independent and academic 20-member Research Board makes the funding and research direction decisions to ensure the intellectual quality, effectiveness and academic independence of the GoMRI research. All research data, findings and publications will be made publicly available. The program was established through a $500 million financial commitment from BP. For more information, visit https://gulfresearchinitiative.org/.

© Copyright 2010-2020 Gulf of Mexico Research Initiative (GoMRI) – All Rights Reserved. Redistribution is encouraged with acknowledgement to the Gulf of Mexico Research Initiative (GoMRI). Please credit images and/or videos as done in each article. Questions? Contact web-content editor Nilde “Maggie” Dannreuther, Northern Gulf Institute, Mississippi State University (maggied@ngi.msstate.edu).