Explaining mesoscale variation in intertidal mussel community structure

We examined how the recruitment and structure of intertidal mussel Mytilus edulis communities and a number of variables (phytoplankton concentration, mean and maximal wave forces, water flux, abrasion by ice, along-shore distance) vary among 4 size classes of embayments (straight coastlines, and embayments with mouth openings of 100, 250 and 400 m) at 12 sites over an apparently homogeneous 15.6 km stretch of coastline in the St. Lawrence estuary, Canada. Only species richness was influenced by embayment size such that mean richness was highest along linear coastlines. Despite larger within-site than among-site Variation in community structure indices, significant correlations were found for community structure, recruitment and a number of other physical and biological variables (r(2) = 0.76, 0.20, 0.06, and 0.13 for recruitment, Shannon-Weiner diversity calculated from abundance and biomass data, and richness, respectively), highlighting the importance of each of these factors to intertidal community structure. Overall, alongshore distance explained the highest proportion of variance for both recruitment (partial r(2) = 0.34) and diversity calculated from abundance data (partial r(2) = 0.11). The other variables explained varying portions of the total variance of these and other indices of community structure. Possible mechanisms relating the observed patterns in community structure to the various indices measured are discussed.