Indicators of Nutrient Limited Plankton Growth in Lakes Near Mount Saint Helens, Washington

Several lakes located in the blast zone of the 1980 eruption of Mount St. Helens were studied to determine if the plankton in the lakes were limited in their growth by nitrogen or phosphorus availability. Long term nutrient enrichment experiments were performed on lake water from five lakes and measures of chlorophyll-a, carbon fixation, and nutrient uptake were used to evaluate the extent of limitation. Nutrient concentrations, ratios, and uptake from 14 lakes provided additional evidence for limitation by nitrogen and/or phosphorus. The physical, chemical, and biological characteristics of the lakes were also examined to monitor the return of these lakes to pre-eruption conditions. Lakes heavily impacted by the 1980 eruption (Ryan, Fawn, and Hanaford Lakes) and newly formed Castle Lake produced positive responses to nitrogen additions, reflecting the continuing importance of nitrogen in these lakes. Evidence for colimitation by phosphorous was apparent in some of the lakes. Venus Lake, located near the fringe of the blast zone received less organic debris and responded only with phosphorus additions. Evaluations of nutrient depletion from the photic zones of these lakes during the growing season supported the results from the bioassays. Two measures of growth (chlorophyll-a and carbon fixation) often responded differently to enrichment, suggesting active algal and bacterial communities in some of the lakes. Qualitative measures of alkaline phosphatase activity normalized to chlorophyll-a indicated that those lakes which contained the highest dissolved organic carbon and bacterial counts following the eruption had higher alkaline phosphatase activities than less impacted lakes, suggesting bacterial enzyme production. Alkaline phosphatase activity confirmed that phosphorus limitation was more severe in lakes having less soluble reactive phosphorus. Most of the lakes appear to be similar to other Cascade Mountain lakes, although many have reduced dissolved oxygen concentrations in the bottom waters during stratification. Incomplete mixing of these lakes during periods of circulation may regulate the decomposition of the organic material and hence, full recovery of these lakes.