The fate and biodegradation of petroleum hydrocarbons and dispersant in sediments of coastal ecosystems can be influenced significantly by the presence of organisms inhabiting the sediment. This is especially so for benthic animals that are active bioturbators. Effectively serving as ecosystem engineers, they greatly modify physical and chemical conditions in the sediment. Burrowing, feeding and other activities by these organisms move sediment between surface and subsurface zones, increase sediment surface area, increase pore-water movement within the sediment, and increase the rate of water and chemical exchange between the water column and sedimentary pore-waters. These activities consequently change sediment characteristics such as redox potential, organic content, and particle size. These changes in turn affect microbial activity and abundance. In the case of oiled sediment, bioturbators can have a direct effect on the distribution of the oil within the sediment, as well as exchange between the water column and sediment. Indirectly, their effects on sediment characteristics, microbial abundance, and the amount of sediment/water interface are likely to affect the rate of microbial degradation of oil.
This project adopted experimental approaches for assessing the influence of bioturbators on biodegradation of petroleum. The research was mainly conducted with two abundant and widely distributed groups of bioturbators; ghost shrimp and razor clams (some research was also conducted with fiddler crabs). These groups differ in burrowing depth and various other bioturbation characteristics, though all are ecosystem engineers and common inhabitants of inshore Gulf of Mexico intertidal to shallow subtidal zones. Experiments were conducted in greenhouse mesocosms containing natural sediment with oil typically introduced at the sediment surface. Experiments were also conducted with a single aromatic hydrocarbon (pyrene). Hydrocarbon levels were measured in the water column and in surface sediment (typically at the beginning and end of an experiment) and in the subsurface sediment (at the end of an experiment). Environmental variables (water column temperature, turbidity, and oxygen levels; sediment temperature and redox potential) were monitored during the experiments in order to assess whether the presence of the bioturbators and the oil affected these variables. Monitoring these variables also helped assess the potential for the bioturbators to indirectly affect microbial hydrocarbon degradation. Effects on the microbial community’s capacity for hydrocarbon degradation were quantified directly using incubations of mesocosm sediment with radiolabelled naphthalene. In addition, effects of the presence of oil and/or bioturbators were assessed for the microbial community composition and for the activity of specific microbial genes known to be involved in petroleum hydrocarbon degradation. The project also assessed whether the microbial population that developed in the greenhouse mesocosms was comparable to that at Gulf of Mexico sites where sediment and animals were collected, and whether there is a distinct microbial community associated with the burrows of the bioturbators in there natural habitat.
This project used an interdisciplinary approach (combining the expertise of an environmental toxicologist, a benthic ecologist, a marine microbiologist, and a chemist) to address research theme 2 “Chemical evolution and biological degradation of the petroleum/dispersant systems and subsequent interaction with coastal, open-ocean, and deepwater ecosystems”. Specifically, the project investigated whether animals inhabiting shallow subtidal sediments and shorelines have an effect on where the oil is found and on the rate at which the oil is broken down by microbes. It sheds light on such questions as: “Will oil that has seeped into sediment be redistributed to the surface by the activity of animals burrowing in the sediment? And: “Will the presence of active burrowing animals impact the rate at which microbes break down the oil?” Insight into the influence of bioturbators on the fate and biodegradation of oil in sediment improves the ability for assessing long-term impacts of oil spills. The scientific impact of this study is an increased understanding of processes that affect oil distribution between sediment and the water column, the rate of microbial degradation in bioturbated environments, and the potential role of bioturbators in these processes. Societally, the project potentially educates a concerned public on the topic of risks from oil that has washed onto shores or soaked into nearshore sediment, while also providing insight into potential roles played by healthy infaunal communities in the course of oil degradation. The project also contributed to the training of young scientists.
Dr. Klerks’ research, which included 4 outreach products and activities, resulted in 8 data sets submitted to the GoMRI Information and Data Cooperative (GRIIDC), and 23 presentations at scientific meetings. Since the goal for publications on this research is to provide a comprehensive overview of the effect of the bioturbators on oil fate and degradation, preparation of manuscripts had to await completion of the research and is currently underway. Significant outcomes of their research (all related to GoMRI Research Theme 2) are highlighted below.
- The presence of both ghost shrimp and razor clams affect the environmental distribution of oil (especially by lowering the amount of oil present on the sediment surface). Their presence also seems to enhance the rate of oil biodegradation. It appears that ghost shrimp have a much bigger influence on the distribution and biodegradation of crude oil and individual PAHs than is the case for the razor clam. This is of relevance, as ghost shrimp (unlike razor clams) are very abundant in many of the intertidal systems on the northern GoM. This, therefore,makes the ghost shrimp bioturbation effect important from an overall ecological and ecotoxicological perspective.
- The ghost shrimp are relatively tolerant to oiling of sediment. This is still a preliminary finding (research in progress as part of a Ph.D. student’s dissertation). Implications: the influence of ghost shrimp bioturbation on the distribution and degradation of oil would be a moot point if the initial oiling would kill of the ghost shrimp populations. Such mass mortality does now seem unlikely. The Ph.D. student is conducting further research on their oil tolerance, not only with respect to survival of adults and larva but also for their rate of bioturbation.
- For our mesocosms, neither the addition of relatively low amounts of oil nor the presence of the ghost shrimp resulted in drastic changes in community composition of the microbial community. Further research (by a second Ph.D. student) is looking in more detail at changes in microbial community activity and changes in the expression of specific genes that may explain the observed changes in microbial biodegradation of the oil. Implications: while other research has documented changes in microbial community composition following exposure to oil, this may be limited to situations where heavy oiling occurred.
- For experiments with fiddler crabs, research demonstrated that the burrowing of fiddler crabs is affected by sublethal levels of oil exposure, in both of the two fiddler crab species studied. Both burrow diameter and burrow depth were negatively affected. Implications: shoreline oiling on beaches, even at low levels, will affect fiddler crab burrowing and thereby the many ecological processes that are affected by this burrowing.