PHYTOPLANKTON ASSEMBLAGES IN HIGH-ELEVATION LAKES IN THE NORTHERN CASCADE MOUNTAINS, WASHINGTON-STATE USA

Phytoplankton assemblages in high-elevation lakes of North Cascades Na tional Park Service Complex were studied during the open-water period in 1989. Collectively, 93 taxa were identified in 55 samples from 51 l akes. Based on cell densities, cyanobacteria had the highest relative abundance (36.7 %), followed by chlorophytes (29.8 %), and chrysophyte s (19.6 %). Aphanocapsa delicatissima had the highest proportional abu ndance (14.0 %). Only 15.1 % percent of the taxa occurred in more than 20 samples. Phytoplankton cell densities increased following a gradie nt of increasing lake-water temperature, alkalinity, and concentration of total Kjeldahl-N with decreasing lake elevation. Chrysophytes and cyanobacteria were quantitatively (relative abundance) the most import ant taxa in alpine and subalpine lakes, whereas cyanobacteria had the highest relative abundances in high-forest and low-forest lakes. Chlor ophytes had their highest relative abundance in high-forest lakes. Alt hough low in relative abundance, diatoms and dinoflagellates were most abundant in alpine lakes. An ordination by correspondence analysis in dicated that most alpine, subalpine, and high-forest lakes had similar floras. Although a few subalpine lakes exhibited deviations from this pattern, the main differences in phytoplankton composition were found in a group of low-forest and high-forest lakes. Canonical corresponde nce analysis (CCA) provided evidence that the distribution of samples and taxa in ordination space was correlated with a gradient of decreas ing lake elevation and increasing water temperature, alkalinity, and c oncentration of nitrogen. When CCA was used to examine relationships a mong phytoplankton taxa and vegetation zones, a continuous distributio n of taxa was found from the low-forest zone to the subalpine zone, wi th a large number of taxa occurring primarily in the subalpine and hig h-forest zones. Three phytoplankton taxa occurred primarily in alpine lakes, whereas five taxa co-occurred in alpine, subalpine, and high fo rest zones. Collectively, lake elevation and associated changes in wat er quality and concentrations of nutrients, especially nitrogen, appea red to be the primary physical and chemical factors influencing the ta xonomic structures of phytoplankton assemblages.