Lake classification in the glacially influenced landscape of the north Cascade mountains, Washington, USA

by Lomnicky, Gregg A.

Factors affecting physical, chemical, and biological characteristics of lakes

were investigated through the development of a lake-classification system for 162

lakes in North Cascades National Park Service Complex. A conceptual view of lake

development and its relationship to the expression of lake and watershed

characteristics was derived. Water quality and biological assemblages of these

primarily glacially formed high-mountain oligotrophic lakes were influenced by

elevation, lake morphology, and certain watershed characteristics: aspect, vegetation,

soils, hydrology, and degree of glacial influence. Lakes continually evolve relative to

changes in their watershed environments. A watershed-based, three-level hierarchical

classification was created to include 1) lake position relative to the hydrologic crest of

the North Cascade Mountain Range, 2) vegetation zone (alpine, subalpine, low

elevation forest, high elevation forest), and 3) basin origin. Hydrologic crest position

differentiated broad-scale climatic differences in precipitation and air temperature.

Vegetation zones reflected the localized geology (soil maturation) and climate

(precipitation, aspect). Morphogenetic class identified differences in lake morphology,

landscape position, and potential for persistence, and were unequally distributed

across vegetation zones with forest zones most diverse. Time of ice-out increased

from low-forest lakes to alpine lakes; eastslope lakes iced-out earlier. Epilimnetic

temperature was warmest in low-forest lakes and coolest in alpine lakes. Classification

did not clearly order lakes relative to chemical characteristics, though westslope low-forest

lakes differed significantly from other lake classes and were most productive.

Little seasonal or annual variation for most chemical characteristics were found.

However, chemical differences did mirror environmental and physical differences

between lakes. High phosphorus levels separated glacially influenced lakes. Total

Kjeldahl-N and total phosphorus concentrations decreased with increasing lake depth.

For a given flushing ratio, Kjeldahl-N decreased from low-forest to alpine zones. Depth

and vegetation class ordered the diversity and composition of phytoplankton,

zooplankton, and benthic macroinvertebrate assemblages. Nutrients, conductivity, pH,

alkalinity, and cations were correlated with phytoplankton and zooplankton

assemblages. Non-native trout presence was associated with large, deep [greater than or equal to] 3 m)

lakes.