Large-scale Shoreline Configuration Influences Phytoplankton Concentration and Mussel Growth

Abstract

This study examines the relationship between large-scale shoreline configuration (≥100m), phytoplankton concentration and mussel growth in the St Lawrence Estuary, Canada. Cages containing mussels were moored in zones inside and external to embayments of different sizes (0·15, 1·5, 4 and 7km apertures) and along a straight coastline to evaluate mussel growth. Water samples were collected to determine phytoplankton (chlorophyll a) concentrations in the same zones. In 1993, only the 7-km embayment was studied, but all sites were studied in 1994. In 1995, nine further embayments (3×100, 3×250 and 3×400m apertures) and three sections of straight coastline were studied to examine more precisely the relationship between chla concentration and zones for small embayments. In 1993, mussel growth (shell length and body mass increments) was significantly greater inside the embayment than in zones external to it. Although growth rates for shell length and body mass were similar for 1993 and 1994 for the largest embayment, body mass increments within embayments in 1994 were greater than those observed outside of them only for the 1·5 and 4km embayments. Chlorophyll a concentration inside of small embayments (≤400m) was lower or equal to that outside of them in both 1994 and 1995 and no trends with respect to aperture size were observed. Nevertheless, 44% of the variation in the ratio of chla between zones inside and outside embayments was explained by the size of embayments (100m to 7km opening and the straight coast). Current velocity, water temperature and the flux of chla explained a high proportion of the variation in mussel growth in 1993. Further, the general patterns observed in the ratios between mussel growth within and outside of the differently sized embayments agree rather well with those of chla concentrations. Lower chla concentrations inside small embayments may be due to grazing pressure by zooplankton and bivalves.