Pelagic bacterial and phytoplankton production in a subtropical marine environment exposed to chronic oil contamination

The abundance and production of pelagic bacteria, phytoplankton primary production and chlorophyll content were studied in coastal waters receiving the effluent from an oil refinery in the Arabian Gulf. The area also receives unknown amounts of other effluents rich in organic matter and nutrients. The abundance of bacteria was measured by epifluorescent direct counts, and productivity was estimated by ³H-thymidine uptake measurements. The results showed a clear stimulation of the primary productivity as well as elevated amounts of chlorophyll a in the area receiving the effluent. Both bacterial abundances and production were an order of magnitude higher in a small area close to the refinery outlet, but dropped rapidly and reached background values outside an impacted area of c 10 km². The increased bacterial production in this area corresponded to a substrate demand of 4 to 11 tonnes of carbon per day, 4 to 12 times the daily discharge of some 0·9 tonnes of carbon in the form of petroleum hydrocarbons from the oil refinery. These data, plus the low petroleum hydrocarbon concentrations found in the sediments and in bivalves outside the impacted area, suggest that bacterial degradation of the petroleum hydrocarbons from the refinery could be a major process restricting the area impacted by oil pollution.