Benthic response index for assessing infaunal communities on the southern California mainland shelf

Although benthic infaunal communities are commonly measured to assess the e ffectiveness of environmental management in protecting biological resources , the tools used to interpret the resulting data are often subjective or si te specific. We present an objective, quantitative index for application th roughout the southern California coastal shelf environment that measures th e condition of a benthic assemblage, with defined thresholds for levels of environmental disturbance. The index was calculated using a two-step proces s in which ordination analysis was employed to quantify a pollution gradien t within a 717-sample calibration data set. The pollution tolerance of each species was determined based upon its distribution of abundance along the gradient. The index is calculated as the abundance-weighted average polluti on tolerance of species in a sample. Thresholds were established for refere nce condition as well as for four levels of biological response. Reference condition was established as the index value in samples taken distant from areas of anthropogenic activity and for which no contaminants exceeded the effects range low (ERL) screening levels. The four response levels were est ablished as the index values at which key community attributes were lost. I ndependent data sets were used to validate the index in three ways. First, index sensitivity to a spatial gradient of exposure to a discharge from a p oint source was tested. Second, index response to a temporal gradient of ex posure to a discharge from a point source was examined, testing index robus tness to natural temporal variation. Third, the effect of changes in natura l habitat (e.g., substrate, depth, and latitude) on index sensitivity was t ested by evaluating the ability of the index to segregate samples taken in areas with high and low chemical exposure, across a gradient of physical ha bitats.