Quantitative examination of macrobenthic community changes along an organic enrichment gradient

Orgaruc ennchment, both natural and anthropogenic, is one of the most common forms of disturbance in the benthos The effects of organic enrichment on the benthos in the vicinity of a large manculture facihty were examined as a general model for enrichment, without the confoundmg effects of toxlcants often associated with anthropogenic inputs Stations nearest the fac~lity were sublect to continuous input of fish feed and fecal matter and gross structural changes in the macrofaunal community (e g reduced species richness, dominance of opportunislc species) were sim~lar to those commonly reported for other ennched sites More complex community and population responses were Indicated by changes in body size, vertical distnbution of infauna and patterns of trophic dominance Enriched areas are generally assumed to be charactenzed by macrofauna small body sizes and mean individual size did, in fact decrease m t h proximity to the farm At stations farthest froin the farm however, size distribut~ons were skewed by a few large individuals indicating that trends in mean indimdual size are susceptible to the methodological limitations inherent in adequately sampling rare indimduals Whde lnterspeafic measures of a n ~m a l size decreased with increasing enrlchment lntras p e a f ~c measures indicated a tendency for larger ind~viduals to occur at the most ennched sites Ennched areas may represent increased food resources, and thus Increased potential for growth and attainment of larger body sizes in those species capable of exploiting such habitats Organ~cally ennched areas are generally considered to b e charactenzed by infauna living at or near the sedimentwater interface Increasing organic enr~chment resulted in the loss of large, deep-dwelling species and dramatically altered the vertical biomass profiles, but because these individuals were numerically few had little effect on the verbcal abundance proflles Changes In trophic dominance did occur along the ennchment gradient, but the general lack of autecological informahon limits current efforts to define trophic groups and assign species to them The utility of trophic grouping approaches in ident~fication of enrichment-induced disturbances is thus limited