Biomonitoring and risk assessment tools to manage impact of diesel oil in tropical coastal habitats

Abstract

The focus of this work is in developing biology–based tools for environmental monitoring and risk assessment associated with diesel oil contamination in tropical coastal habitats. Prediction of impacts is generally conducted via environmental monitoring, in which environmental quality over time and space is assessed by repeated observations. Prediction of risk is included within the risk assessment process, which is the procedure that estimates the likelihood or the actual adverse effects caused by anthropogenic activities on ecosystems. During the past decades, oil production has increased, and so has the risk of oil pollution, either through produced water discharges, accidents, or other diffuse sources. This risk is notably high in tropical and subtropical areas, which represent around 60% of total global oil production. Petroleum is composed of a mixture of various mono– and polycyclic–aromatic hydrocarbons, toxic chemicals consisting of two or more fused benzene rings. The mode of action of PAHs is classified as narcotic, meaning that PAHs are expected to penetrate cell membranes and alter the lipid bilayer, ultimately disturbing the normal function of cells. On average, nearly 85% of the total petrogenic PAH input to the marine environment origins from petroleum consumption or diffuse sources. Among marine coastal habitats, tropical and subtropical coastal regions are home to speciose and highly productive ecosystems. Estuaries are among the most productive of marine ecosystems and are areas with high economic and ecological importance. Being economic centers for coastal communities that harvest biotic resources, tropical and subtropical estuarine intertidal environments (i.e. mangroves, salt marshes, and unvegetated tidal flats) are particularly susceptible to anthropogenic disturbance. Specifically, chronic diesel oil contamination that leaks from marine vessels poses a real risk to the species inhabiting the Paranaguá Estuarine System (PES) in southern Brazil, which host the third largest harbor of Brazil, and receives around 200 ships per month. Oil contamination from such diffuse sources, is an untraceable chronic source of contamination that can occur anywhere a ship travels and may have different effects, depending on the physical–chemical characteristics of the environment into which the oil is released. Therefore, tools for biomonitoring the effects of short and long–term exposure to diffuse oil contamination are much needed. The general objectives of this work are to validate the use of antioxidant biomarkers as tools for biomonitoring coastal estuarine habitats in Brazil, as also to compare the sensitivity and risk assessment metrics from species distributed from subtropical, temperate and Arctic regions exposed to a toxic PAH.

Biomarkers are defined as measures of exposure or effect expressed at the sub–organism level (i.e. biochemical, cellular, physiological or behavioral) in taxa under environmental stress. We proposed the use of antioxidant biomarkers as sub-lethal measures of exposure at the sub-organism level. Before implementing antioxidant biomarkers in biomonitoring programs, several conceptual and methodological issues needed to be addressed. Namely, it is important to determine their basal levels of activity, to select an appropriate sentinel species for their measurement, and to determine the best group of biomarkers for a multi–biomarker approach. Also, it is necessary to establish a correlation between the presence of diesel oil contamination and the activity of selected biomarkers. This work addresses these points, first by conducting a seasonal baseline of biomarker values, and then by performing experimental manipulations both in the lab and the field. Because the activity of antioxidant enzymes is involved in cell homeostasis, they are expected to vary in relation to reproductive cycles, food availability, and environmental drivers. Thus an initial screening in the activity of 5 different subtropical species was conducted at two seasons (austral winter and austral summer) at two different locations that have different levels of organic and PAH contamination. Then, experimental manipulations that tested the correlation between the antioxidant response and diesel oil exposure were conducted. The first experiment characterized the antioxidant biomarker response in two common species under laboratory conditions; while in the second experiment, the antioxidant biomarker response in the clam species Anomalocardia flexuosa was evaluated after chronic exposure to diesel oil in situ. The significant changes in the biomarkers activities following exposure suggested a causal relationship between biomarkers and diesel oil contamination, with the activities of GST and SOD being the most sensitive to experimental manipulations. These cause–effect relationships indicate that it is possible to use these biomarkers as tools in biomonitoring programs at PES. However, it was noticeable that natural variability is a major confounding source of variation, which in our experiments was handled by including appropriate control treatments for comparing the response from the experimental treatment with that from natural conditions. As part of the outcomes of this work, a guiding framework for selecting biomarkers and testing their causal relationship to contamination and specific recommendations for designing experiments for biomonitoring purposes are provided. Briefly, well–designed experiments have a clear hypothesis to test, for which the measurement of environmental parameters at an adequate sampling intensity is feasible, given financial and logistic constraints. The statistical power of the design must be

considered before starting sampling and the design should include spatial and temporal variability. Regarding differences in risk assessment metrics following the exposure to 2– Methylnaphthalene, our results indicate that No–Effect Concentration (NEC) values — concentration thresholds use to assess species sensitivity to toxic exposure— were not significantly different among the studied species and differences among regions were not identified. However, when defining sensitivity as the time to observe an effect —a metric that includes the NEC and a toxicokinetic parameter like the elimination rate— differences in sensitivity among regions were detected. In summary, species from Arctic to subtropical regions have similar NEC thresholds, but the time they need to reach that threshold varies, and this variation is related to taxonomy and trophic level. Arctic species had on average shorter times for starting to show an effect, followed by subtropical and finally temperate species. Our results suggest that assuming that species sensitivities from Arctic, and temperate regions is sufficiently similar to those from subtropical regions might be incorrect. We suggest that in in the search for metrics for safeguarding the marine ecosystem, attention should not be given only to concentration thresholds. Concentration thresholds might be providing assessors an inaccurate metric for species sensitivity, which is ultimately underestimating the risk to marine and estuarine ecosystems.