On April 20, 2010 the Deepwater Horizon oil drilling platform (DWH) exploded in the Gulf of Mexico causing a massive oil spill that has continued to date, largely unabated. The scale of this oil spill, as well as the application of chemical dispersants to control it, is unprecedented; and its ecological impacts on the Gulf of Mexico are completely unknown. One issue of particular concern is the effect of surface oil slicks on the mortality rate of pelagic larvae of important fish and invertebrates, such as the blue crab, Callinectes sapidus. Blue crabs spawn in coastal waters, but larvae move offshore into the open marine environment where they pass through several larval stages before migrating back into the nearshore zone as megalopae to transform into juvenile crabs and settle into coastal marsh habitat. The pelagic larvae stages (i.e., zoea) are very sensitive to environmental stress and exposure to surface oil slicks and dispersant are highly likely to result in wide-spread mortality of blue crab larvae present. The location and size of the DpH oil slick currently in the northern Gulf of Mexico makes it highly likely that blue crab zoea will be exposed to either oil, dispersant or both and the objective of this RAPID project is to examine the impact of the DWH oil spill on blue crab settlement rates in coastal estuaries.
Dr. Fulford and colleagues will measure the settlement rates of blue crab megalopae along the Mississippi coast and compare settlement in 2010 to historical patterns of settlement at the same sites. In addition, a predictive hydrodynamic model will be used to correct observed settlement for hydrodynamic forcing so that settlement rates in 2010 can be compared to historical patterns with a focus on changes in larval mortality rather than larval dispersal. The project will also make use of blue crab zoea and megalopae from the Gulf Coast Research Laboratory (GCRL) blue crab hatchery to conduct a series of experimental assays to assess the mortality rate of blue crab zoea and megalopae in response to crude oil and dispersant. These assays will allow for a stronger interpretation of field data, as the PIs can compare observed concentrations of crude oil and dispersant in the open marine environment to assay results to state clearly whether any observed changes in megalopal settlement might be due to oil-induced mortality. The results of this project will be important as an assessment of the ecosystem impact of the DpH oil spill, as well as important baseline data for measuring ecosystem recovery.
Broader Impacts:
This project will also have important broader impacts as the data will be used to calibrate food web model s of the coastal ecosystem to impacts of the oil spill. These models will allow for a broader examination of the impacts on specific species at the scale of the entire ecosystem. Further, the results of this project will be combined with the results of a spatial examination of blue crab settlement being conducted at Tulane University by Taylor and the two projects will form the nexus of a larger project to examine regional patterns of blue crab recruitment in support of ecosystem-based management. Finally, the PIs will make use of a long-term dataset from GCRL on blue crab settlement in coastal marshes that has not been previously used or published in its entirety, thereby making the data more available to the scientific community. This project is being implemented by several early-career researchers who will be able to build on project findings to build their research programs.
The goal of our research was to understand the potential for the Deepwater Horizon oil spill to affect the survival and settlement of larval blue crabs in the northern Gulf of Mexico. To do this, we used a combination of laboratory exposures, and field collection surveys. Blue crab are important residents of the northern Gulf of Mexico ecosystem both ecologically and economically, so understanding the impacts on this species is very important. Understanding the effect of oil spills on natural ecosystems is difficult, because exogenous contaminants such as released oil are only one stressor that may affect the organism. However, oil spills have been shown to alter habitat usage, reduce growth and development, impair reproduction, and change behavior, including predator prey interactions. We observed a number of responses that indicated potential for an effect of the Deepwater horizon oil spill on blue crab populations in the northern Gulf of Mexico. First, laboratory exposures showed increased mortality of both zoea and megalopae stages when exposed to increasing concentrations of oil. Secondly, larval collections indicated that blue crab larvae (which tend to settle in marshes in waves, or pulses), were settling in pulses that were shorter (approximately 1-2 days long), and had a longer time between settlement events (15-17 days). These data did not appear to be attributable to environmental factors.
