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Limnology of Lakes in Gates of the Arctic National Park and Preserve, Alaska

ABSTRACT Limnological reconnaissance data were collected during summers 1992–93 and 1995 from 16 major lakes within Gates of the Arctic National Park and Preserve, Alaska (GAAR) located above the Arctic Circle. In GAAR the southern lakes (~67°N) and those in the Brooks Range foothills are in watersheds with taiga and tundra vegetation. The northern lakes (~68°N) are at higher elevations in the Brooks Range in watersheds that lie wholly beyond the treeline. Average sum of the cations for all lakes matches the world average for fresh waters and the average for drainage from tundra and taiga landscapes. Local lithography explains the measured decrease in calcium equivalents and silica, and an increase in magnesium with altitude because of more calcareous rock in the southern basins at low altitude and shale/conglomerate in northern mountain catchments. In most low altitude lakes Secchi depth was located in sub-surface algal peaks at temperatures averaging ~8.5 C cooler than the surface, typically with double the surface chlorophyll value. Mineral turbidity, and less so color, controlled transparency in several high altitude lakes where turbid inflows were a factor. Lakes were oligotrophic based on nutrients and algal chlorophylli, but a doubling of TP and concurrent halving of TN was measured in GAAR lakes with altitude. Both patterns were correlated with the decreasing density of terrestrial vegetation with altitude, resulting in a sharp decline in the TN:TP ratio. This pattern suggests sources of these nutrients change across the landscape continuum within GAAR described by altitude and vegetation zones. Nitrogen fixation associated with terrestrial vegetation most likely accounts for greater TN in lakes within the taiga and moist or wet tundra, whereas TN levels approximated the N content of regional rainfall in high elevation lakes with predominately barren land and prostrate shrub in the catchments. Ratios of TN:TP and Nutrient Stimulation Bioassays suggest P limitation was likely among low altitude lakes and N limitation increased in importance in lakes at high altitude. The particulate composition ratio (as C:N:P molar ratio) of these lakes averaged ~200:20:1.