Changes in midge (Diptera: Chironomidae) populations of sub-arctic supratidal vernal ponds in response to goose foraging

Abstract

Grubbing and grazing by lesser snow geese (Anser caerulescens caerulescens) have led to loss of terrestrial vegetation and changes in the hydrology, chemistry and biology of shallow vernal ponds in a supratidal marsh at La Pérouse Bay, Manitoba, on the Hudson Bay coast. Chironomid populations in ponds have been examined because they are an important food source for ducks and shore birds, and are also widely used as ecological indicators of change. Seventy ponds were sampled during the summers of 1996 and 1997. They were located within a densely vegetated freshwater area, a relatively undamaged salt marsh and a degraded salt marsh where loss of vegetation had occurred as a result of goose foraging. Principal Components Analysis indicated that of 21 environmental descriptors, salinity was the primary gradient distinguishing ponds. Ponds in the degraded marsh that dried out early were the most saline (8 to 30‰) and those in the freshwater area the least saline (< 2‰). Densities of chironomids approached 22 000 m-2 with the highest numbers recorded from salt-marsh ponds. Estimated chironomid biomass in all ponds ranged between 0.7 and 9.0 g dry wt m-2 which reflected differences in species composition of ponds. Freshwater ponds contained seven species from six genera of which five did not occur in the other two habitats. Ponds in the undamaged salt marsh contained five species from five genera of which two were restricted to the area, while only the large-bodied Cricotopus (Isocladius) sp., most likely ornatus, was representative of ponds in the degraded salt marsh.