SUBTIDAL BENTHIC COMMUNITY RESPIRATION AND PRODUCTION NEAR THE HEAVILY OILED GULF-COAST OF SAUDI-ARABIA

As part of the NOAA/ROPME/MSRC sponsored assessment of the coast of Sa udi Arabia, heavily impacted by the large oil spills of the 1991 Gulf War, we measured benthic community respiration (R) and primary product ion (P) rates in shallow subtidal basins near heavily oiled coastlines . Study sites were located in zones predicted to be basins of maximum deposition of any oiled sediments likely to wash off the adjacent coas t and in reference bays predicted to be clean and unaffected. We measu red oxygen exchange using in situ benthic respirometers and analysed p etroleum hydrocarbon (PHC) levels in the near surface sediments at fiv e stations. We concentrated our efforts in mud habitats to complement related studies in seagrass habitats. Measured community R rates range d from a low of 700 muM m2 h-1 in coarse sand sediments with total org anic carbon content (TOC) of only 0.16% of dry wt to the highest rate of 2184 mum m-2 h-1 in finer mud-sand habitats with 0.39% TOC. All mea sured rates were in the range of literature values for shallow marine sediments at temperatures of 17 to 19-degrees-C. Sediment oil content was 13-540 mug g-1 dry wt by ultraviolet fluorescence (UVF) analysis a nd 0.5-103 mug g-1 by gas chromatography (GC). Benthic P rates, calcul ated as light minus dark changes in dissolved oxygen, ranged from a lo w of 1162 mum m-2 h-1 at the most heavily oiled site to a high of 5216 mum m-2 h-1 at less oiled sites. While a weak inverse relationship be tween benthic P and sediment oil content was not significant statistic ally due to the small number of samples, a significant inverse relatio nship was found between the ratio of production to respiration (P/R) a nd the total petroleum content of the sediments by UVF. The effect app ears to be driven more by differences in production than an effect on respiration which showed little relationship with either oil content o r productivity. We conclude that within 1 year after the oil grounded on the intertidal sands of Saudi Arabia, the levels of oil in subtidal benthic sediments had decreased in most habitats to levels that did n ot show community stress by our oxygen measurements. Rather, long term damage to benthic subtidal habitats was limited only to enclosed bays adjacent to the most heavily oiled coastlines. Preliminary estimates of yearly P converted to carbon units indicates that the shallow mud s ediments of the Gulf are at least as productive as most oligotrophic w ater columns. Since benthic habitats covered by seagrass, algae beds o r coral reefs are likely to have even higher production rates, we conc lude that benthic processes contribute significantly to the overall ca rbon flux in the Gulf ecosystem.