Nutrient and trophic conditions in high-elevation lakes of the Sierra Nevada and Rocky Mountains.
Sickman, James*,1, Melack, John2, Clow, David3, 1 Department of Geology and Geophysics, New Orleans, LA, USA2 Department of Ecology, Evolution and Marine Biology, Santa Barbara, CA, USA3 United States Geological Survey, Denver, CO, USA
ABSTRACT- Nutrient and seston conditions in western high elevation lakes indicate they are oligotrophic, thus, even small changes in nutrient supply may impact both algal biomass and species composition. Using long-term monitoring data (Emerald Lake) and synoptic surveys of wilderness lakes we validated nutrient limitation indices (NLIs), (based on seston and nutrient supply ratios) and then applied them a population of lakes (n=97) in order to assess the current trophic status of high-elevation lakes of the Sierra Nevada and Rocky Mountains. The Redfield ratio (PN:PP) was accurate in predicting the limiting-nutrient in 12 out of 18 lakes (67%) where bioassay data were available, while an index based on nutrient supply ratios (DIN/TP) had an accuracy of 88%. When applied to the 1999 synoptic lake data, the NLIs estimate that 20% of Rocky Mountain Lakes (n=59) and 70% of Sierra Nevada Lakes (n=38) had phytoplankton populations that were N-limited. In the Rocky Mountains, the lack of wide-spread phytoplankton N-limitation during the growing season is suggestive of more advanced N-saturation of terrestrial watersheds. We hypothesize that differences in the trophic status between Rocky Mountain and Sierra Nevada lakes are driven by higher N deposition in the Rocky Mountains (mean annual deposition: 3.6 kg-N/ha/yr vs. 1.8 kg-N/ha/yr) and resultant higher DIN concentrations in Rocky Mountain Lakes (1999 lake-survey means: 7.0 M vs. 1.4 M). While phytoplankton biomass is similar between the two regions (mean PC = 32 M), the C:N ratio of seston in the Rocky Mountains (14) is significantly lower than in the Sierra Nevada (20) indicating higher DIN loading rates. In contrast, nitrate levels in Sierra Nevada Lakes are declining in response to P loading and the release of phytoplankton from P limitation. Compared to the 1980s, Emerald Lake experiences more frequent N limitation and has 2-3x more phytoplankton in the late 1990s.
Key words: Rocky Mountains, Nutrient, Sierra Nevada, Lake
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