Macroinvertebrates of glacial and snowmelt-fed lake outlets in the North Cascade Mountains

Our study compares the physico-chemical conditions and composition of benthic macroinvertebrates from 5 rhithral (snowmelt-fed) and 5 kryal (glacially-fed) lake outlet streams in the North Cascade Mountains, WA.  Non-metric, non-parametric cluster analysis (RIFFLE) clearly separated outlet streams of kryal and rhithral origin based on physico-chemical and taxon variables.  Kryal lake outlets were characterized by lower water temperatures, unstable in-stream channels and higher turbidity, discharge and fine substrates than rhithral sites.  A total of 24,985 specimens represented by 90 macroinvertebrate taxa were collected.  Rhithral lake outlets had significantly higher densities (P < 0.02) and supported more taxa (P < 0.01) than kryal sites (9,049 ind./m² and 77 taxa versus 821 ind./m² and 35 taxa).  Chironomidae were the dominant taxa amongst all sites, though their densities and taxon richness were 3 times lower in the kryal lake outlets.  Rhithral lake outlets contained higher densities of  non-insect taxa such as Acari, Oligochaeta, Nemathelminthes, Planariidae and crustaceans.  Water temperature, stream discharge and turbidity were the habitat variables most strongly correlated to density and taxon richness.  Our results suggest that glacial presence was the dominant factor influencing in-stream environmental conditions and subsequently macroinvertebrate assemblages of alpine lake outlet streams.