FACTORS CONTROLLING THE UPPER AND LOWER LIMITS OF THE INTERTIDAL DISTRIBUTION OF 2 COROPHIUM SPECIES IN THE WADDEN SEA

On the tidal flats of the Dutch Wadden Sea, high densities of Corophium volutator (in silty areas) and of C. arenarium (in sandy areas) were limited to the upper parts of the intertidal zone. The upper Limit was always situated close to the local level of high water at neap tides. After periods of higher- or lower-than-average water levels (caused by prevailing westerly or easterly winds, respectively), the zone of Corophium volutator distribution had shifted upward or downward, respectively. Conversely, the lower limits of high Corophium spp, densities did not show a consistent relation with intertidal level or duration of immersion but coincided with the upper level of high (>10 m(-2)) densities of adult lugworms Arenicola marina. In areas with sandy sediments (silt content < ca 15%), this limit was situated about halfway between mean high water and mean tide level. In more muddy sediments, lugworms were scarce and high C. volutator densities extended to lower intertidal levels, It is concluded that zonation in the soft-bottom intertidal may generally be governed by the same processes as are prevalent in the rocky intertidal, viz. abiotic factors setting the upper limits and biotic interrelations setting the lower Limits of species.