Reprint Series No. 15

TEMPORAL PATTERN OF GRAZERS AND VEGETATION IN A TEMPERATE SEAGRASS SYSTEM

R. M. Thom, B. Miller and M. Kennedy

1991

Bibliographic Citation
Thom, R.M., B. Miller and M. Kennedy. 1991. Temporal pattern of grazers and vegetation in a temperate seagrass system. Report to NOAA/OCRM/SRD by University of Washington, Fisheries Research Institute (FRI-UW-9122). 28pp. Seattle, Washington. Padilla Bay National Estuarine Research Reserve Reprint Series No. 15

Abstract
The density of grazers and the biomass of benthic vegetation were monitored for 10 months in the seagrass system in Padilla Bay, Washington. Experiments were conducted in the laboratory to evaluate the importance of grazing in the Zostera marina L. and Zostera japonica Aschers. & Graebn. subsystems in the Bay. The primary grazers included the isopod Idotea resecata, the caprellid amphipod Caprella laeviuscula, the gastropod Lacuna variegata, black brant geese (Branta bernicula), and several species of dabbling ducks. Invertebrate herbivore density peaked in summer and autumn, in conjunction with peak plant biomass. The isopod probably removed more plant material than the other invertebrates. Spatial variation in grazing effect was substantial. Grazing pressure calculated from the experiments was always greater in the Z. japonica subsystem and exceeded a previous estimate of net annual primary productivity (NPP). The caprellids and isopods were prominent in the guts of shiner (Cymatogaster aggregata) and striped (Embiotoca lateralis) perch, two species commonly associated with seagrass systems in the Pacific Northwest. The presence of herbivorous birds, which are generally found in the system in autumn and winter, coincided with the period of most rapid loss of epiphyte biomass and eelgrass standing stock. However, birds appeared to remove less of the NPP as compared with invertebrates. We have concluded that herbivory is an important process in Padilla Bay in terms of controlling plant dynamics and may be a major pathway for the transfer of energy to higher trophic levels in this system.