1 The effects of scouring by ice and of grazing by the greater snow go
ose (Chen caerulescens atlantica) on the substrate and vegetation of a
brackish tidal marsh in the St Lawrence estuary were studied from 198
5 to 1987. During spring and fall migratory stopovers, the birds fed m
ainly on Scirpus americanus rhizomes. 2 The presence of ice-made depre
ssions was related mainly to the physical characteristics of Scirpus m
arshes. Goose grubbing has no significant effect on the annual dynamic
s of these depressions. 3 In exclosures, i.e. in the absence of geese,
ice drift has little influence on species abundance and net above-gro
und primary production of the marsh except at high levels of disturban
ce. 4 In ice-made depressions, goose grazing reduced net primary produ
ction by approximately 45% and facilitated colonization by competitive
species such as Zizania aquatica, Sagittaria spp. & Scirpus torreyi.
History or frequency of disturbance seems to play an important role in
this process. 5 By exploiting ice-made depressions, snow geese contri
bute to an increase in the heterogeneity but a decrease in the primary
production of the Scirpus marsh.