Investigating the sublethal effects of oil exposure on infaunal behavior, bioturbation, and sediment oxygen consumption

Anthropogenic disturbances such as oil spills can cause mortality in benthic infaunal communities, reducing diversity and abundance and impeding sediment ecosystem functions. Sublethal effects of oil exposure have received less attention, however. We conducted a mesocosm experiment exposing 2 infaunal taxa, the polychaete Owenia fusiformis and the brittle star Hemipholis elongata, to sublethal concentrations of the water-accommodated fraction (WAF) of oil. We evaluated the effects of WAF on animal behavior, bioturbation, and sediment oxygen demand (SOD) in infaunal assemblages of both mixed and single species. WAF exposure did not affect O. fusiformis feeding behavior, nor did it influence bioturbation. Compared to O. fusiformis, the brittle star H. elongata mixed more surface sediments to greater depths and to a greater extent horizontally. Bioturbation in mesocosms with both taxa was consistent with predictions from monocultures for substitutive densities, but lower than predicted for additive densities. This indicates that taxa interacted (negatively) only at higher densities. SOD was higher in oiled than unoiled treatments initially (at 1-3 d), but this difference disappeared after the first sampling, consistent with a decrease in total petroleum hydrocarbons in the WAF treatment over the same time period. Higher SOD in WAF-exposed faunal treatments than sediments with no fauna suggested that faunal activities may enhance microbial degradation of hydrocarbons. These findings suggest that exposure to WAF stimulated microbial metabolism in the first few days of the experiment, but did not affect macrofaunal behavior/function, nor have lasting effects on sediment ecosystem functions.