A comparison of benthic macroinvertebrate assemblages among kryal and rhithral lake outlets in the North Cascade Mountains

This study compares the physico-chemical 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. Non-metric, non-parametric cluster and association analysis (NMCAA) 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 representing 93 macroinvertebrate taxa were collected. Rhithral lake outlets had significantly higher densities and supported more taxa than kryal sites (9,049 ind./m2 and 77 taxa versus 821 ind./m2 and 35 taxa). Chironomidae were the dominant taxon amongst all sites, although densities and taxa richness were one-third in the kryal lake outlets when compared to rhithral sites. 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 variables most strongly correlated to density and taxa richness. My results suggest that glacial presence was the dominant factor influencing instream environmental conditions and subsequently macroinvertebrate assemblages of alpine lake outlet streams.