Community structure of macroinvertebrates inhabiting the rocky subtidal zone in the Gulf of Maine: seasonal and bathymetric distribution

Abstract

ABSTRACT: Community structure of the macroinvertebrate fauna inhabiting a rocky subtidal habitat at Pemaquid Point, Maine, USA, was studied, using qualitative and quantitative descriptions of the distribution, diversity and abundance of benthic species, as well as their spatial (bathymetric) and temporal (seasonal) changes. A total of 60 species of macroinvertebrates representing 9 phyla were found in the 133 (0.25 m2) disruptive benthic samplings obtained by SCUBA between August 1984 and October 1986. Crustaceans, mollusks and polychaetes were best represented accounting for ca 77 % of the total number of species identified. Green sea urchln Strongylocentrotus droebachiensis and horse mussel Modiolus modiolus were consistently the most important (in biomass and density) assemblage components. Bathymetrically, there were clear patterns in the composition and abundance of macroinvertebrates. Sea urchins decreased in abundance with depth (from 5 to 18 m), while the opposlte was obsemed in horse mussels. Distribution and abundance patterns of remaining benthic macroinvertebrates were strongly influenced by the spatial distrlbuhons of sea urchins (at 5 and 10 m depth) and horse mussels (at 18 m). Species richness was higher In the deepest zone (18 m; 41 species) than In the shallower zones (5 and 10 m; 34 and 31 species respectively), due to the presence of clumps of A4. modiolus Comparative analysis at 18 m depth showed that the invertebrate fauna within I\~o~~o~us beds is significantly more abundant, dense, and dlverse than that outside pointing out the functional importance of Modolus beds in providing spatial refuges from predators, and suitable and stable mlcrohabitats for numerous invertebrates. No significant temporal changes were observed in the biomass and density of the invertebrate community. Number of species, however, showed marked seasonal vanations. Maximum values occurred during summer, intermediate values in fall and spring, and mlnimum values in winter, probably related to migration or changes in activlty of some species. Results of this and other studies indicate that coralline communities, despite their low primary productivity, are ecological systems with relatively high species diversity and secondary productivity, sometimes comparable to systems dominated by kelps. INTRODUCTION