Mats of the attached alga Zygnema spp. have been observed since about 2010 in the long-term research lakes in Loch Vale watershed, Rocky Mountain National Park. Zygnema spp. are common green algae in nutrient-rich waters, and Loch Vale watershed has received chronic nitrogen (N) enrichment from atmospheric deposition since mid-20th century. The fact that the algal mats are only recently observed suggests some other forcing factor is facilitating attached algal growth. Summer, especially July, mean air temperatures have increased by 2 °C since 1983, and July mean water temperatures at The Loch outlet have risen by 3 °C over the same period, from 8.5 °C in 1983 to 11.5 °C in 2014. We hypothesize warming is enhancing access to nutrients. We initiated a field study in 2015 to characterize the physical, chemical, and biological characteristics of the pelagic and benthic environments in alpine Sky Pond and subalpine The Loch in Loch Vale. We report here on initial results.
Results/Conclusions
Phytoplankton chlorophyll a values peaked in alpine Sky Pond in August, and showed no temporal trend in the subalpine Loch; by August chlorophyll a values in surface waters of Sky Pond were fourfold greater, with values up to 18 μg L-1 compared with ~3 μg L-1 in The Loch. Chlorophyll a did not vary with depth in The Loch. For the two warmest weeks in August surface chlorophyll a values in Sky Pond were much higher than in hypolimnetic waters and at the lake outlet. Periphyton chlorophyll a and ash-free dry mass from the dominant algae, Zygnema spp. increased June-August commensurate with water temperatures in alpine Sky Pond, but were not correlated with nutrients. Chlorophyll a values were 2-3x lower in samples collected in the 1980s from Sky Pond and The Loch, but our 2015 values were not greater than what has been reported from other Rocky Mountain lakes. Measured nutrient concentrations are also within ranges reported from other lakes in the Colorado Front Range. Field sampling will continue, and be accompanied by laboratory and field experiments to test whether nutrient uptake rates and biomass are coupled to temperature for phytoplankton and especially periphyton.