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

Grubbing and grazing by lesser snow geese (Anser caerulescens caerulescens) have led to loss of terrestrial vegetation and changes in the hydrology. c hemistry and biology of shallow vernal ponds in a supratidal marsh at La Pe rouse Bar. Manitoba. on the Hudson Bay coast. Chirdnomid populations in pon ds have been examined because they are an important food source for ducks a nd shore birds, and are also widely used as ecological indicators of change . Seventy ponds were sampled during the summers of 1996 and 1997. They; wor e located within a densely vegetated freshwater area, a relatively undamage d salt marsh and a degraded salt marsh where loss of vegetation had occurre d as a result of goose foraging. Principal Components Analysis indicated th at of 21 environmental descriptors, salinity was the primary gradient disti nguishing ponds. Ponds in the degraded marsh that dried our early were the most saline (8 to 30 parts per thousand and those in the freshwater area th e least saline (< 2<parts per thousand>). Densities of chironomids approach ed 22 000 m(-2) with the highest numbers recorded from salt-marsh ponds. Es timated chironomid biomass in all ponds ranged between 0.7 and 9.0 dry wt m (-2) which reflected differences in species composition of ponds. Freshwate r ponds contained seven species fi sm six genera of which five did not occu r in the other two habitats. Funds in the undamaged salt marsh contained fi ve species h from five genera of which two were restricted to the area, whi le only the large-bodied Cricotopus (Isocladius) sp.. mast likely ornatus, was representative of ponds in the degraded salt marsh.