SEASONAL-VARIATION IN GROWTH AND BIOMASS OF AN INTERTIDAL ZOSTERA-NOLTII STAND IN THE DUTCH WADDEN SEA

To assess relationships between the life cycle of the seagrass Zostera noltii and light conditions in its habitat, the seasonal dynamics of a seagrass-dominated community on a tidal Rat off Terschelling were studied. The main components of this community were seagrass, periphyton and the periphyton grazing mudsnail Hydrobia ulvae. Total biomass of the seagrass stand showed a unimodal curve with a maximum of more than 110 gADW . m(-2) in August and a minimum of less than 10 gADW . m(-2) in January. Chlorophyll density of periphyton on the seagrass leaves followed a more or less similar pattern, ranging from 0.4 mu g chlorophyll . cm(-2) in May to more than 3 mu g chlorophyll . cm(-2) at the end of August. Periphyton biomass was, however, already maximal in May with almost 1.6 mgADW . cm(-2) and subsequently decreased to less than 0.6 mgADW . cm(-2) in August. The total weight of H. ulvae was more or less stable, varying between more than 150 and less than 400 gDW . m(-2), although significant changes were observed within size classes. Light is assumed to be the primacy limiting factor for seagrass distribution in the Wadden Sea. The light conditions of seagrass in the study area were influenced by periphyton and mudsnails. Leaf growth rates and biomass development appeared to be related with light conditions in the seagrass habitat. Shading caused by periphyton during the growing season was estimated at 10 to 90% of incident light, resulting in a reduction of about 2 to 80% of the yearly period during which the light compensation point (LCP) of the seagrass is exceeded. The mudsnails were found to be theoretically able to ingest daily 25 to 100% of the standing stock of periphyton and microphytobenthos. It is concluded that shading by periphyton and grazing by mudsnails play an important role in the seasonal biomass development and survival of Z. noltii in the seagrass-dominated community on a tidal flat off Terschelling.