DEVELOPMENT OF A BENTHIC THRESHOLD INDEX USING EVENNESS OF RAREFACTION LOG SERIES AND DIVERSITY OF FUNCTIONAL GROUPS
Blake, J. A. 1; Gallagher, E. D. 2 & Hecker, B. 3
1ENSR, 89 Water Street, Woods Hole, Massachusetts, 02543 USA
2University of Massachusetts, Environmental Coastal and Oceani Sciences Department, Boston, MA 02125 USA
3Hecker Enivronmental consulting, Falmouth, MA USA
The new MWRA sewage outfall in Massachusetts Bay is scheduled to begin discharges in late 1998. Soft-bottom benthic communities have been monitored annually sunce 19992. Due to the heterogeneous nature of the sediments, natural disturbance from storms, and trawling, the sediments and benthic biota vary from year to year and the development of an index or metric that would be indicative of change has been difficult to establish. Long-term trends in species diversity, species richness, and faunal composition are variable. Species diversity has been re-examined with regard to evenness which might be more informative than richness. Because rarefaction curves are log series, any departure from predictions might indicate lower evenness and a possible disturbance. Results suggest that 30% of the stations are already disturbed from natural causes. Diversity was also examined from the standpoint of functional groups. The 50 most abundant taxa, dominated by polychaetes, were classified according to eight functional groups: suspension feeders, interface feeders, surface deposit feeders, reverse conveyor-belt feeders, subsurface deposit feeders, top-down conveyor-belt feeders, omnivores/scavengers, and predators. Ordination analysis using PCA-H resulted in high correlation with functional groups. When species were plotted against functional evenness, high correlations suggested that Caution and Warning levels could be developed as indicators of change. Caution levels would include: reduced species evenness, changes in community composition, and changes in functional group diversity. Warning levels would include faunal shifts to nearshore assemblages, loss of species richness, and dveelopment of an " inner-harbour" evenness pattern. These results suggest that diversity when analyzed for departure from log series and the diversity of functional groups can serve as powerful new metrics to assess change in benthic communities.