Seasonal Monitoring of Hydrocarbon Degraders in Alabama Marine Ecosystems Following the Deepwater Horizon Oil Spill

Following the Deepwater Horizon explosion and crude oil contamination of a marsh ecosystem in AL in June 2010, hydrocarbon-degrader microbial abundances of aerobic alkane, total hydrocarbon, and polycyclic aromatic hydrocarbon (PAH) degraders were enumerated seasonally. Surface sediment samples were collected in October and December of 2010 and in April and July of 2011 along 40?C70-m transects from the high tide to the intertidal zone including Spartina alterniflora-vegetated marsh, seagrass (Ruppia maritima)-dominated sediments, and nonvegetated sediments. Alkane and total hydrocarbon degraders in the sediment were detected, while PAH degraders were below detection limit at all locations examined during the sampling periods. The highest counts for microbial alkane degraders were observed at the high tide line in April and averaged to 8.65?×?10⁵ of cells/g dry weight (dw) sediment. The abundance of alkane degraders during other months ranged from 9.49?×?10³ to 3.87?×?10⁴, while for total hydrocarbon degraders, it ranged between 5.62?×?10³ and 1.14?×?10⁵ of cells/g dw sediment. Pore water nutrient concentrations (NH ₄ ⁺ , NO ₃ ^? , NO ₂ ^? , and PO ₄ ^3? ) showed seasonal changes with minimum values observed in December and April and maximum values in October and July. Concentrations of total petroleum hydrocarbons in sediments averaged 100.4?±?52.4 and 141.9?±?57.5?mg/kg in January and July, 2011, respectively. The presence of aerobic microbial communities during all seasons in these nearshore ecosystems suggests that an active and resident microbial community is capable of mineralizing a fraction of petroleum hydrocarbons.     

Modelling the impact of climate change on sediment yield in a highly erodible Mediterranean catchment

Purpose

The assessment of climate change impacts on the sediment cycle is currently a primary concern for environmental policy analysts in Mediterranean areas. Nevertheless, quantitative assessment of climate change impacts is still a complex task. The aim of this study was to implement a sediment model by taking advantage of sediment proxy information provided by reservoir bottom deposits and to use it for climate change assessment in a Mediterranean catchment.

Materials and methods

The sediment model was utilised in a catchment that drains into a large reservoir. The depositional history of the reservoir was reconstructed and used for sediment sub-model implementation. The model results were compared with gauged suspended sediment data in order to verify model robustness. Then, the model was coupled with future precipitation and temperature scenarios obtained from climate models. Climatological model outputs for two emission scenarios (A2 and B2) were simulated and the results compared with a reference scenario.

Results and discussion

Model results showed a general decrease in soil moisture and water discharge. Large floods, which are responsible for the majority of sediment mobilisation, also showed a general decrease. Sediment yield showed a clear reduction under the A2 scenario but increased under the B2 scenario. The computed specific sediment yield for the control period was 6.33 Mg ha^?1 year^?1, while for the A2 and B2 scenarios, it was 3.62 and 7.04 Mg ha^?1 year^?1, respectively. Furthermore, sediment transport showed an increase in its time compression, i.e. a stronger dependence of total sediment yield from the largest event contributions.

Conclusions

This study shows a methodology for implementing a distributed sediment model by exploiting reservoir sedimentation volumes. This methodology can be applied to a wide range of catchments, given the high availability of reservoir sedimentation data. Moreover, this study showed how such a model can be used in the framework of a climate change study, providing a measure of the impact of climate change on soil erosion and sediment yields.