Evaluating effects of low concentrations of oil in marine benthic communities

Abstract: Large anthropogenic discharges of oil annually enter the marine environment, in the magnitude from large scale discharges to minor leakage, caused by ship collision accidents or seepage from urban areas. Possible detrimental environmental effects from these discharges differ, from acute lethal to chronic sub-lethal. While lethal effects, originating from large oil spill accidents throughout the years, are well studied by numerous studies, less is known regarding effects of small, continuous petroleum discharge effects. Under those circumstances the most toxic part of oil, polycyclic aromatic hydrocarbons (PAHs), contributes largely to the resulting effects. These can manifest themselves in biota as lowered reproduction and growth, cancerogenic effects or less tolerance to other stress. All of which can in the end add up to a negative response in magnitude comparable to a larger spill. Analytical tools to evaluate the consequences of these small, continuous discharges of oil was developed and microbial and meiofaunal communities were used to assess the detrimental effects. Firstly, a method for evaluating effects of PAHs was developed, that includes measurements of microbial inorganic fluxes and a computerized automatic image analysis method for determining abundance of major meiofaunal taxa. Secondly, it was determined that PAH concentrations as low as 130 and 1 300 µg/kg sediment (dw) still produced significant detrimental effects. This resulted in lowered potential nitrification, nitrification flux, silicate flux, phosphate flux and an altered meiofaunal community. Thirdly, it was clarified that the factor bioavailability, the proportion of a toxic substance that is available to organisms for uptake, can modify the produced stress of a PAH addition when it ends up in divergent sediment environments. Fourthly, it was elucidated that microbial communities can develop tolerance to PAHs; tolerance was developed in nitrifying microbes after being exposed to low concentrations of PAHs for 60 days. Moreover, the development entailed an altered community composition. This implicates that bioavailability and the possibility of tolerance must be taken into account when evaluating marine microbial status in areas of long-term PAH exposure with varying sedimentary conditions. Finally, the tools were applied in situ, at an area of long-term oil contamination, to demonstrate their applicability. Induced tolerance to PAHs by nitrifying microbes was detected at the site with highest bioavailable PAH concentrations. Tolerance to PAHs for ammonium oxidisers was introduced at bioavailable PAHs levels greater than 40 µmol/kg (dw) given as silicone equilibrium concentration. In addition, the tolerant microbial community at the site showed a significant lower capability for ammonium oxidizing when not re-exposed to PAHs, indicating a fitness cost of the development of tolerance.