Quantitative examination of macrobenthic community changes along an organic enrichment gradient

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Abstract
Organic enrichment, 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 mariculture facility were examined as a general model for enrichment, without the confounding effects of toxicants often associated with anthropogenic inputs. Stations nearest the facility were subject to continuous input of fish feed and fecal matter, and gross structural changes in the macrofaunal community (e.g. reduced species richness, dominance of opportunistic species) were similar to those commonly reported for other enriched sites. More complex community and population responses were indicated by changes in body size, vertical distribution of infauna and patterns of trophic dominance. Enriched areas are generally assumed to be characterized by macrofauna with small body sizes, and mean individual size did, in fact, decrease with proximity to the farm. At stations farthest from the farm, however, size distributions were skewed by a few large individuals, indicating that trends in mean individual size rare susceptible to the methodological limitations inherent in adequately sampling are individuals. While interspecific measures of animal size decreased with increasing enrichment, intraspecific measures indicated a tendency for larger individuals to occur at the most enriched sites. Enriched areas may represent increased food resources, and thus increased potential for growth and attainment of larger body size in those species capable of exploiting such habitats. Organically enriched areas are generally considered to be characterized by infauna living at or near the sediment-water interface. Increasing organic enrichment 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 vertical abundance profiles. Changes in trophic dominance did occur along the enrichment gradient, but the general lack of autecological information limits current efforts to define trophic groups and assign species to them. The utility of trophic grouping aproaches in identification of enrichment-induced disturbances is thus limited.