Initial Effects of the Mount St. Helens Eruption on Nitrogen Cycle and Related Chemical Processes in Ryan Lake

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Ryan Lake, a 1.6-hectare basin lake near the periphery of the tree blowdown area in the blast zone 19 km north of Mount St. Helens, was studied from August to October 1980 to determine the microbial and chemical response of the lake to the eruption. Nutrient enrichment through the addition of fresh volcanic material and the organic debris from the surrounding conifer forest stimulated intense microbial activity. Concentrations of such nutrients as phosphorus, sulfur, manganese, iron, and dissolved organic carbon were markedly elevated. Nitrogen cycle activity was especially important to the lake ecosystem in regulating biogeochemical cycling owing to the limiting abundance of nitrogen compounds. Nitrogen fixation, both aerobic and anaerobic, was active from aerobic benthic and planktonic cyanobacteria with rates up to 210 nmol of N2 cm−1 h−1 and 667 nmol of N2 liter−1 h−1, respectively, and from anaerobic bacteria with rates reaching 220 nmol of N2 liter−1 h−1. Nitrification was limited to the aerobic epilimnion and littoral zones where rates were 43 and 261 nmol of NO2 liter−1 day−1, respectively. Potential denitrification rates were as high as 30 μmol of N2O liter−1 day−1 in the anaerobic hypolimnion. Total bacterial numbers ranged from 1 × 106 to 3 × 108 ml−1 with the number of viable sulfur-metal-oxidizing bacteria reaching 2 × 106 ml−1 in the hypolimnion. A general scenario for the microbial cycling of nitrogen, carbon, sulfur, and metals is presented for volcanically impacted lakes. The important role of nitrogen as these lakes recover from the cataclysmic eruption and proceed back towards their prior status as oligotrophic alpine lakes is emphasized.

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