Spatial variation in growth of the bivalve Macoma balthica (L.) on a tidal flat: effects of environmental factors and intraspecific competition

Abstract

A descriptive field study was conducted on a tidal flat of the Lower St. Lawrence Estuary to examine spatial variations in shell growth of the infaunal bivalve Macoma balthica (L.) in relation to tidal level, sediment characteristics, and local density. This study was based on 83 0.3 × 0.3 m 2 sediment samples randomly located on a 1.6 km 2 tidal flat where maximal amplitude of tide was 4.8 m. Mean density of Macoma balthica for the whole sampling area was 1254 m −2 ( S x = 87 m −2 ) and densities were higher and more variable near the mean water level. Variations in the effects of density and environmental factors along the tidal gradient were studied after a stratification of the stations by immersion time. Overall, shell growth rate increased with immersion time, the latter accounting for about 5 to 63% of the spatial variation of mean shell length for the six youngest generations. Fifty-two to 61% of the adjusted annual shell growth was explained by a linear relationship with tidal level. A significant but weaker linear relationship was obtained between growth and density, and between growth and sediment grain size. For the whole sampling area, a linear model with tidal level and density as independent variables explained 37 to 80% of the spatial variability of mean shell length for each generation and 59 to 77% of the spatial variability in adjusted annual growth rates. The effect of density was not significant in the lower part of the tidal flat, however, it became significant when immersion time was ≈ 50%, and had a maximum effect when immersion time was about 40%. About 65 to 85% of the spatial variability of the growth rate along the tidal gradient can be explained by stepwise multiple regressions involving density as the first independent variable. Overall, the results suggested that intraspecific competition played a major role in ≈ 67% of the population of Macoma balthica within the study area.