Establishing cause-effect relationships in hydrocarbon-contaminated sediments using a sublethal response of the benthic marine alga, Entomoneis cf punctulata

A sublethal whole-sediment toxicity test that uses flow cytometry to measure inhibition of esterase activity in the marine microalga Entomoneis cf punctulata was applied to the assessment of hydrocarbon-contaminated sediments and toxicity identification and evaluation (TIE). Concentration-response relationships were developed, and a 20% effect concentration for total polycyclic aromatic hydrocarbons (PAHs) of 60 mg/kg normalized to 1% total organic carbon was calculated. Relationships between toxic effects and sediment organic carbon concentrations, organic carbon forms (e.g., black carbon), and sediment particle size indicated that further normalization of hydrocarbon concentrations to sediment particle size may improve concentration-response relationships. The algal toxicity test was applied as a rapid whole-sediment TIE procedure that involved the addition to sediment of powdered coconut charcoal (PCC), a hydrophobic, carbon-based material that strongly adsorbs PAHs and decreases the pore-water exposure pathway. Sediments with PCC concentrations of up to 15% (w/w) provided acceptable responses in control sediments. For six sediments with total PAH concentrations of 1,060, 4,060, 5,120, 9,150, 9,900, and 15,900 mg/kg, inhibition of E. cf punctulata esterase activity (% of control) was 75, 97, 94, 93, 100, and 97%, respectively. Following a 15% PCC amendment to these sediments, inhibition of esterase activity was 0, 1, 11, 69, 32, and 68%, respectively, indicating a decrease in toxicity in all sediments. Because the alga E. cf punctulata is exposed to toxicants via both pore water and overlying water, the reduction in toxicity achieved by 15% PCC additions can be related to the efficient removal of dissolved hydrocarbons released from sediment particles. The sediment-PCC manipulations coupled with algal whole-sediment toxicity tests provided an effective and rapid TIE method to determine whether hydrocarbon contaminants are responsible for toxicity in sediments.