Research Product

Concurrent 12-week laboratory and field studies were conducted to determine toxicity of the suspended particulate phase (SPP) of drilling fluid to Thalassia testudinum and its epiphytes. Test systems were treated once per week to achieve nominal concentrations of 100 mg/L SPP . Chlorophyll content of Thalassia leaves and epiphyte biomass and chlorophyll content were monitored during each test. Laboratory exposures were conducted in 7-L, flow-through (7 L/h) microcosms consisting of Plexiglass cylinders containing intact cores of Thalassia from a local seagrass bed. Field exposures were conducted in water-tight plexiglass chambers (2 m x 2 m x 1.5 m) placed over test plots in a seagrass bed for 24 h during SPP additions. The chamber base was buried several cm into the sediment to minimize water exchange. Drilling fluid exposure had no significant effect on chlorophyll a or b content of Thalassia leaves in laboratory or field tests. Epiphyte biomass was reduced after 6 weeks of intermittent exposure to SPP in laboratory and field tests. After 12 weeks, epiphyte biomass had increased to densities similar to control values. Tributyltin chloride (TBT-Cl) was tested only in laboratory systems, using weekly treatments for six weeks. Nominal test concentrations ranged from 0.2 to 50 µg/L. Leaf protein and rhizome carbohydrate content of Thalassia were employed as effect measures in the TBT-Cl test. Leaf concentrations of chlorophyll a and b were not affected by exposure to TBT-Cl at nominal concentrations < or = 50 µg/L. Leaf protein and rhizome carbohydrate concentrations were reduced by exposure to 50 µg/L TBT-Cl. Epiphyte biomass was reduced after exposure to 50 µg/L TBT-Cl for 6 weeks; concentrations