Responses of selected Scleractinian corals to drilling fluids used in the marine environment

Abstract

Eleven speices of Scleractinian corals (Agaricia agaricities, Acropora vervicornis, Porites astreoides, P. divaricate, P. furcate, Madracis mirabilis, M. decatis, Montastrea annularis, M. cavernosa, Diploria strigose, and Dichocoenia stokesii) have been exposed to several layered and suspended drilling fluids collected from exploratory drilling platforms in the Gulf of Mexico, or to individual drilling fluid components. Quantified response parameters include rate of sediment removal and polyp contraction. Other stress responses have been qualitatively measured. Close-up time lapse movie photography has been used in the laboratory and underwater in conjunction with still photography and direct observation to record the appropriate responses. Time lapse movies in particular have proven useful in providing a large, readily quantifiable data base which permits small shifts in coral polyp behavior to be discerned. A combination of field and laboratory experiments have revealed that: 1) very high concentrations of some drilling fluids can impair the sediment removal ability of some corals; 2) average daily polyp expansion may be reduced by exposure to low concentrations of drilling fluid in seawater ranging from less than 1 μ1/1 to greater than 1,000 μ1/1, depending on the species of coral being exposed and the particular drilling fluid used; 3) death (to some or all individuals) occurred within 96 hours when 3 species (Montastrea annularis, Agricia agaricities, and Acropora cervicornis) were exposed to 1,000 μ1/1 of one drill mud. Four other species (Porites devaricata, P. furcate, P. astreoides, and Dichocoenia stokesii) experienced no morality at this concentration, which was the highest used in any of the suspended drilling mud experiments. Effects of chronic (greater than 30 days) exposure to drilling fluids have not been investigated. Sensitivity of different species of coral was seen to be at least partially related to their normal ecological niches. Those species normally found in area which experience considerable exposure to suspended sediments were generally also found to be more tolerant to suspended drilling fluids. For example, Dichocoenia stokesii was not significantly affected by 1,000 μ1/1 of drilling mud, while Montastrea annularis was affected by 100 μ1/1 of the same mud, and killed within 96 hours by 1,000 μ1/1 mud...