Spatial variation in the intertidal bivalve Macoma balthica: biotic variables in relation to density and abiotic factors

Abstract

We studied the spatial variation in biotic variables of a Macoma balthica population. Systematic sampling was carried out over an environmentally heterogeneous intertidal bay of the St. Lawrence Estuary. Several biotic (density, body tissue and shell mass, reproductive effort, shell length and growth) and abiotic (intertidal level, sediment characteristics) variables were concur- rently analysed to quantify the part of the variance accounted for by the abiotic factors and the part accounted for by density (density was also studied as an explanatory variable). Redundancy analysis and multiple regression were used. The variables examined were mapped to show the trends in their spatial variation. Density was highly variable, from 0 to 2700 ind. m -2 , and was mainly positively related to the percentage of sand in the sediment. Most biotic variables were spatially variable, and the variables accounting for most of this variation were shore level and density. Body tissue and shell mass were negatively affected by increasing shore level and density. Shell length decreased with increasing shore level and increased with increasing pore water content. Reproductive effort was enhanced at high density.