Effects of a used drilling fluid on an experimental seagrass community (Thalassia testudinum Konig et Sims) were measured by exposing the community to the suspended particulate phase (SPP) in laboratory microcosms. Structure of the macroinvertebrate assemblage, growth and chlorophyll content of grass and associated epiphytes, and rates of decomposition as indicated by weight loss of grass leaves in treated and untreated microcosms were compared. Health of the plants and structure of the macroinvertebrate assemblage maintained in the laboratory were compared periodically with the seagrass community from which the plants and attendant sediment were taken.
Treated microcosms were exposed to either 190 parts per million (ppm), volume to volume, of SPP or an equivalent amount of montmorillonite clay. Untreated microcosms received only flowing water from Santa Rosa Sound. Sixteen replicates were provided for each treated and untreated set.
There were statistically significant differences in community structure and function among untreated microcosms and those receiving the clay and drilling fluid. For example, drilling fluid and clay caused a significant decrease in the numbers of the ten most numerically abundant (dominant) macroinvertebrates, and drilling fluid decreased the rate at which Thalassia leaves decomposed.