Alpine aquatic environments are one of the harshest on earth and are sensitive indicators of climate change. In Washington, the most glaciated state in the contiguous United States, relatively little is known about the effect of loss of glacial water sources on macroinvertebrate communities. Our study compares the water quality conditions and composition of benthic macroinvertebrates from five rhithral (snowmelt-fed) and five kryal (glacially-fed) lake outlet streams in the North Cascade Mountains, WA. Kryal streams were characterized by lower water temperatures, higher discharge rates, higher turbidities, and less stable in-stream channels compared to rhithral sites. A total of 24 985 specimens representing 96 macroinvertebrate taxa were collected. Rhithral lake outlet streams had significantly higher densities and supported more taxa than kryal sites (9049 individuals m-2 and 82 taxa versus 821 individuals m-2 and 38 taxa). Chironomidae dominated macroinvertebrate populations at all sites, although densities and taxa richness were significantly lower in kryal sites. Rhithral sites contained higher densities of non-insect taxa such as Acari, Oligochaeta, Nemathelminthes, Planariidae, and crustaceans. Kryal sites were dominated by cold stenotherm or cool eurythermal taxa and supported glacial-tolerant taxa such as chironomids Diamesa and Parorthocladuis, and the mayfly Rhithrogena. Water temperature, stream discharge, and turbidity were most strongly correlated to macroinvertebrate density and taxa richness. Our results suggest that glacial presence was the dominant factor influencing in-stream environmental conditions and macroinvertebrate assemblages of alpine lake outlet streams. Continued glacial retreat due to climate change may result in an overall loss of specialists and decreased regional diversity.
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Vol. 90 • No. 2