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 spatial distribution of early bivalve recruits, a correlative held study was carried out on a highly dynamic intertidal sandflat in the Westerschelde estuary, 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 Macoma 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 higher part of this range of intertidal levels. The strong gradient in densities from the lower towards the higher intertidal was significantly negatively correlated with sediment dynamics. No significant correlations of early-recruit densities were found with silt content, or with densities of adult benthos. The relationship between early recruitment and bed-level height differed from that observed in Wadden Sea studies of recruits of similar size, where maximum early recruitment occurred in the lower intertidal. It is suggested that in highly dynamic environments, sediment dynamics may have an important influence on passive resuspension of early recruits and on spatial patterns of early recruitment. Based on field and model data, it is discussed which processes could cause the difference in early recruitment patterns in low and highly dynamic intertidal environments. It is concluded that the presence of low-dynamic areas is essential for the success of early recruitment, and thus for the maintenance of bivalve populations. [KEYWORDS: early recruitment; settlement; resuspension; sediment dynamics; bed-level height Wadden sea; mya-arenaria; tidal flats; habitat selection; bivalve larvae; flume flow; settlement; transport; invertebrates; juveniles