EFFECT OF PERIPHYTON GRAZING BY HYDROBIA-ULVAE ON THE GROWTH OF ZOSTERA-NOLTII ON A TIDAL FLAT IN THE DUTCH WADDEN SEA

The decrease of the intertidal seagrass Zostera noltii in the Dutch Wa dden Sea may have been the result of enhanced periphyton load due to e utrophication. Decrease of this seagrass species coincided with an inc rease in the mudsnail Hydrobia ulvae. Feeding of this mudsnail on peri phyton may have partly counteracted an increase of periphyton biomass. Exclosure experiments on seagrass stands in the Dutch Wadden Sea in 1 987 showed that density of periphyton on leaves of Z. noltii decreased significantly with increasing density of grazers. An increased densit y of mudsnails significantly enhanced the density and biomass of seagr ass, in particular of the below ground parts. Since this seagrass spec ies survives winter in temperate climate zones mainly by means of rhiz omes, grazing may also influence the seagrass dynamics in the long ter m. Results of the experiment were in agreement with field observations on coinciding low densities of mudsnails and high densities of foulin g of seagrass stands on the tidal flats of western Wadden Sea in the l ate 1970s.