A MULTIDISCIPLINARY APPROACH TO EVALUATING IMPACTS OF SHELLFISH AQUACULTURE ON BENTHIC COMMUNITIES

The impact of suspended mussel culture (Mytilus edulis, M. trossulus) on the benthos of a small Nova Scotia cove (7 m depth) was assessed us ing methods involving both benthic metabolism and community structure. Due to deposition of mussel feces and pseudofeces, sedimentation rate was higher under the mussel culture lines than at an adjacent referen ce site of similar sediment texture. Porewater profiles of sediment su lfate and sulfide indicated greater anaerobic metabolism at the mussel site than at the reference site, but sulfide tvas absent from the upp er centimeters of sediments under the mussels. Seasonal measures of se diment oxygen-demand showed little change between sites, but maximum r ates of ammonium release at the mussel site were twice the highest rat es measured at the reference site. Abundance of benthic macrofauna was higher at the reference site, but biomass was generally lower. Biomas s at the mussel site was dominated by molluscs (llyanassa spp. and Nuc ula tenuisulcata), that were attracted to mussels fallen from the cult ure and/or enriched organic matter due to biodeposition. Species diver sity was lower at the reference site due to the dominance of the polyc haete Nephtys neotena. Abundance-biomass comparisons (ABC method) of f aunal analysis did not indicate any impact of biodeposition at this si te; however, disturbance did not result in a typical assemblage of sma ll opportunistic species anticipated with this method. Cluster analysi s of macrofauna usually provided a clear separation between the sites. Since the construction of a causeway (1968), foraminifera species com position showed a temporal response to temperature changes in the cove by shifting toward calcareous species, but assemblages downcore showe d little or no relationship to aquaculture impacts. Although there is a shift toward anaerobic metabolism at the mussel lines, the impact of mussels falling to the sediments was more noticeable in benthic commu nity structure than was any impact due to organic sedimentation or hyp oxia. In general the impact of aquaculture on the benthos appeared to be minor. Further assesment of these consequences may mandate both tax onomic and energetic approaches to impact assessment.