EFFECTS OF DISPERSANT AND OIL ON SUBTROPICAL AND TROPICAL SEAGRASSES

ABSTRACT Preliminary experiments, using the subtropical/tropical coastal and estuarine seagrasses Thalassia testudinum, Halodule wrightii, and Syringodium filiforme, were carried out to examine the effects of dispersants. Experiments exposed seagrasses in vitro to concentrations of Louisiana crude oil ranging from 7.5 to 500 milliliters (mL) in 105 mL seawater at exposure times of 5 to 100 hours (seagrass not in contact with oil slick). In other experiments, the seagrasses were exposed to the dispersant Corexit 9527, which was combined with the oil in a ratio of 1 part dispersant to 10 parts oil with dispersant concentrations ranging from 0.75 to 50 mL in 105 mL seawater (dispersant plus oil forming a cloud of the substance in contact with seagrasses). The oil or oil with dispersant treatment was removed from the seagrasses after the designated exposure periods. Thereafter, the seagrasses were monitored for 14 days. Blade length was measured as a factor of growth. Thalassia showed the greatest tolerance to dispersant plus oil of the three species tested. It was not substantially affected by any oil concentration alone; however, when exposed to oil and dispersant, growth significantly decreased with concentrations of 125 mL oil and 12.5 mL dispersant in 105 mL seawater at longer periods of exposure (100 hours), and also at much decreased exposure times (5 hours) for 500 mL oil and 50 mL dispersant in 105 mL sea water. Syringodium and Halodule were generally less tolerant than Thalassia, particularly to oil. For example, at 75 mL oil/105 mL sea water and an exposure of 100 hours, growth decreased significantly and mortality increased to 53 percent. Growth and mortality of Syringodium and Halodule were further affected by the addition of dispersant.