Environmental Determinants of Allomyia bifosa Larvae in an Alpine Fen of Glacier National Park, Montana
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The ecology of alpine headwater streams differs drastically from that portrayed by accepted theories on river and stream ecosystems. Alpine streams lack traditional allochthonous energy sources and light is limited by direction of the slope face rather then by canopy. Autotrophy of alpine streams is limited not by light but by nutrients (Ward 1994). Baring Creek, a first-order alpine stream and associated fen, fed by the waters of a permanent snowfield in Glacier National Park, Montana has been recognized as the home of a rare caddisfiy, Allomyia bifosa (Trichoptera) It was found that nitrate in the stream was high at the source and dropped within the first 100 m (the presence of Allomyia bifosa dropped along with the nitrate); this trend differs from the general view that nitrate is gradually added to the water column by the additive action of blue-green algae (Cyanophyta) and other sources of fixed nitrogen. Latrine sites made by small mammals, thought to be the unique source of nitrate, were found in abundance in the headwaters of Baring Creek. It was postulated that Baring Creek would experience a change in periphyton structure from blue-green dominated (blue-greens have the exclusive ability to fix nitrogen in aquatic ecosystems and resistance to extreme situations )to diatoms (Bacillariophyta). As an obligate grazer, Allomyia bifosa, would be restricted to streams with this unique fen association, high nitrate levels, and diatom dominated periphyton. Four streams on a continuum of fen-association were assayed, an upstream and downstream site at each. It was found that nitrate levels were high at the source of all streams and dropped longitudinally; periphyton carbon levels increased, strongly suggesting the periphyton was taking up nitrate. Nitrogen to phosphorus ratios suggest that phosphorus limitation is occurring at some sites. Allomyia bifosa were later found at all sites when in close proximity with groundwater and fen. Algae in Baring Creek were not found and the periphyton is now thought to be primarily microbially mediated by heterotrophs.