Spatial pattern of early recruitment of Macoma balthica (L.) and Cerastoderma edule (L.) in relation to sediment dynamics on a highly dynamic intertidal sandflat

To investigate the possible relationship between sediment dynamics and spat ial distribution of early bivalve recruits, a correlative held study was ca rried out on a highly dynamic intertidal sandflat in the Westerschelde estu ary, SW Netherlands. On a spatial grid, 43 plots over an area of 700 x 800 m(2), early recruits (300-1000 mum mesh fraction) of the tellinid clam Maco ma balthica (L.) and the edible cockle Ceratoderma edule (L.) were sampled during the spatfall period (May-June) in 1997. Data were also collected on bed-level height, sediment dynamics and -composition and abundance of adult benthos. The grid covered a range of -50 to + 140 cm with respect to mean- tide level. In both species, maximum early recruitment was Found at the hig her part of this range of intertidal levels. The strong gradient in densiti es from the lower towards the higher intertidal was significantly negativel y correlated with sediment dynamics. No significant correlations of early-r ecruit densities were found with silt content, or with densities of adult b enthos. The relationship between early recruitment and bed-level height dif fered from that observed in Wadden Sea studies of recruits of similar size, where maximum early recruitment occurred in the lower intertidal. It is su ggested that in highly dynamic environments, sediment dynamics may have an important influence on passive resuspension of early recruits and on spatia l patterns of early recruitment. Based on field and model data, it is discu ssed which processes could cause the difference in early recruitment patter ns in low and highly dynamic intertidal environments. It is concluded that the presence of low-dynamic areas is essential for the success of early rec ruitment, and thus for the maintenance of bivalve populations. (C) 2001 Els evier Science B.V. All rights reserved.