INTRODUCTION

High-elevation lakes in the Pacific Northwest of the United States typically are capped each year by ice and snow covers from late fall to early summer. After the ice and snow covers disappear (the open-water period), the lakes undergo substantial changes in physical, chemical, and biological characteristics (Larson 1973; Hall 1973; Larson et al. 1994; and Larson et al. 1995). Densities and taxonomic structures of the phytoplankton assemblages are influenced by environmental conditions such as climate, duration of the open-water period, lake morphology, water quality, and geographical location (Larson et al. 1994; Larson et al. 1995). For example, the taxonomic structure of phytoplankton assemblages in high-elevation lakes along the east side of Olympic National Park (OLYM) is related to lake depth and the concentration of nitrate-N. For chain lakes connected by streams in the park, phytoplankton densities increase with decreasing lake elevation. Phytoplankton densities also increase in isolated lakes and chains of lakes as the duration of the open-water period increases (Larson et al. 1995). At Mount Rainier National Park (MORA), phytoplankton densities and the taxonomic structure of assemblages in high-elevation lakes are associated with the duration of the open-water period and geographical location, the latter corresponding to a west-east climate gradient in the park (Larson et al. 1994).

High-elevation lakes in North Cascades National Park Service Complex (NOCA), another national park in the Pacific Northwest, exhibit a range of physical and chemical characteristics depending on their location in alpine, subalpine, high-forest, or low-forest vegetation zones (Lomnicky et al. 1989; Liss et al. 1995). During the open-water period, alpine lakes have low water temperature and high concentrations of nitrate-N relative to low-forest lakes. Decreasing elevation and associated changes in vegetation zone correspond to increases of water temperature, pH, alkalinity, conductivity, the concentration of Kjeldahl-N, and the concentration of ammonia-N (Liss et al. 1995). Given this array of lake characteristics and our knowledge of phytoplankton assemblages in OLYM and MORA, a reasonable hypothesis is that characteristics of phytoplankton assemblages in NOCA lakes correspond to the physical and chemical characteristics of lakes in different vegetation zones. The objective of this paper is to characterize the taxonomic structure of phytoplankton assemblages in NOCA lakes in relation to the physical and chemical characteristics of the lakes.