EXPANSION OF SEAGRASS HABITAT BY THE EXOTIC ZOSTERA-JAPONICA, AND ITS USE BY DABBLING DUCKS AND BRANT IN BOUNDARY BAY, BRITISH-COLUMBIA

The exotic seagrass Zostera japonica was first documented on the Pacific Coast of North America in the late 1950s, and has extensively colonized formerly unvegetated tidal flats and dramatically altered the habitat structure. In Boundary Bay, British Columbia, Canada, there was an almost 17-fold increase in Z. japonica coverage between 1970 and 1991. In Boundary Bay's 6385 ha of intertidal and shallow subtidal area (tidal range 4.7 m), Z. japonica occurred mostly from 0 to -1.8 m Mean Water Level (MWL) and covered 3845 ha in October 1991. The native Z. marina occurred mostly from -0.9 to -5.5 m MWL, and covered 3444 ha including 1684 ha of overlap with Z. japonica. Standing stock of Z. japonica, mostly in formerly unvegetated areas, measured 520 metric tonnes (t) above ground and 235 t below ground in October 1991. This introduced species provides an important feeding habitat for many migratory waterfowl. Percent dry mass of Z. japonica in esophagus contents of birds collected in Boundary Bay was 57.2 % (n = 62) in brant Branta bernicla, 84.8 % (n = 45) in American wigeon Anas americana, 72.3 % (n = 20) in mallard A. platyrhynchos, 48.3 % (n = 54) in northern pintail A. acuta, and 1.7 % (n = 14) in green-winged teal A. crecca. Percent dry mass of the native Z. marina was 41.2 % in esophagi of brant but only 0.1 to 4.6 % in the other species. Grazing by brant and dabbling ducks, with peak numbers of about 80 000 in early December, removed 50 % (262 t) of the above-ground biomass and 43 % (100 t) of the below-ground biomass of Z. japonica. This exotic seagrass thereby supported almost 4.6 million use days by these birds.