TEMPORAL PATTERNS OF GRAZERS AND VEGETATION IN A TEMPERATE SEAGRASS SYSTEM

The densities of benthic vegetation and invertebrate grazers were moni tored in the seagrass system dominated by Zostera marina L. and Zoster a japonica Aschers. & Graebn. in Padilla Bay, Washington. The primary invertebrate grazers included the isopod Idotea resecata Stimpson, cap rellid amphipods and the gastropod Lacuna variegata Carpenter. Densiti es of Idotea and caprellids peaked in summer, and Lacuna density reach ed a maximum in winter. Spatial variation in densities was great, and was in the range of four orders of magnitude during some seasons. The mean, experimentally determined, grazing rate by Idotea was 0.95 mg dr y wt. m-2 day-1. At this rate, Idotea populations alone could remove u p to 8 g dry wt. eelgrass m-2 day-1. Based upon (1) high grazer densit ies, (2) the results of grazing experiments, and (3) experiments and o bservations by others showing the significance of caprellids and Idote a, we conclude that herbivory is an important process in Padilla Bay.