Abstract
The densities of benthic vegetation and invertebrate grazers were monitored in the seagrass system dominated by Zostera marina L. and Zostera japonica Aschers. & Graebn. in Padilla Bay, Washington. The primary invertebrate grazers included the isopod Idotea resecata Stimpson, caprellid amphipods and the gastropod Lacuna variegata Carpenter. Densities of Idotea and caprellids peaked in summer, and Lacuna density reached 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 dry wt. m −2 day −1. At this rate, Idotea populations alone could remove up to 8 g dry wt. eelgrass m −2 day −1. Based upon (1) high grazer densities, (2) the results of grazing experiments, and (3) experiments and observations by others showing the significance of caprellids and Idotea, we conclude that herbivory is an important process in Padilla Bay.
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