Experimental evidence of complex interactions between biotic and abiotic factors in the dynamics of an intertidal population of the bivalve Cerastoderma edule

Densities of the suspension-feeding bivalve Cerastoderma edule (L.) were manipulated inside field enclosures at two tidal elevations (low water level, LWL, and mid-tide level, MTL) on an intertidal sand flat in Arcachon Bay to test the influence of both adult densities and emersion time on (1) individual growth rate, (2) settlement rate, and (3) survival rate of cockles. These experiments were conducted during two consecutive years, in plots with ambient (Ix) and modified (1/3x, 3x, or 10x) densities of adult cockles. Growth rate and condition index of both adults and juveniles were significantly higher at the lowest tidal elevation (LWL), which is in accordance with the feeding mode of the species. The highest juvenile growth rate was recorded in the low-density treatments (160-200 adults m(-2)), which suggests a competitive interaction with adults; for the latter, growth rate was depressed only at the highest density (2000 m(-2)). survival of-adults was affected neither by immersion time nor by densities. Contrasted results were found for the settlement rate and the survival of recruits. During the first year of-experiment, density-treatments had no effect on settlement at MTL, whereas high adult densities negatively influenced settlement at LWL, but only during the period of high settlement; during the second year, a significant effect of-adult densities on settlement was found on a single occasion at MTL. However, at the end of-both experiments (195 and 252 d, respectively), recruit densities did not significantly differ between density-treatments. Although the difference in tidal elevation between the two experimental sites was only 1.0 m, settlement peaks were clearly distinct: they occurred in April (approximate to 4000 m(-2)) at LWL but not earlier than July (approximate to 12000 m(-2)) at MTL; however, the final recruitment was higher at LWL than at MTL. Preliminary results of a laboratory flume study show that, in conditions of smooth turbulent flow (u* = 0.51 cm s(-1), Re* = 1.8), juvenile cockles are able to leave the substratum, to migrate by byssus drifting into the water column over several metres and to avoid resettlement in areas with high densities of conspecific adults. Although competition with adults may occur during larval settlement, subsequent migration of juveniles between different tidal levels is likely to affect significantly the growth and recruitment of intertidal cockle populations.