Recent anecdotal evidence suggests increased algal productivity in many lakes of the western US. Causal factors might include atmospheric nitrogen (N) deposition, dust inputs that may introduce phosphorus (P), and warming air and lake temperatures, acting either singly or interactively. In the Loch Vale Watershed of Rocky Mountain National Park we observed increased benthic and planktonic algae since 2010 in alpine Sky Pond (3,300 m) and The Loch (3,048 m), a subalpine lake downstream. Since 2010 benthic algae in both lakes form prominent patches and mats dominated by common green algae including Zygnema spp. and Spirogyra spp. In Sky Pond phytoplankton chlorophyll a ranged 6-11 μg/L in 2015-2017, compared to 3-4 μg/L in 1984-1985. The Loch is less productive than Sky Pond, but chlorophyll a values also were twice as high (2-4 μg/L) in 2015-2017 than in 1984-1985. We present new analyses of sediment cores relating to the origin, magnitude and potential causes of changes in primary producers through time. Laboratory experiments with nutrient and temperature treatments provide insight into environmental drivers of algal growth.
Results/Conclusions
Sediment records reveal productivity rising from mid-20th century to present day. This trend in higher productivity affirms measurements in contemporary samples compared with those of the 1980s. Changes in the two lakes were asynchronous, but the increase in total production in both lakes was driven by an increase in benthic green algae. In Sky Pond, nitrophilic Asterionella formosa increased in relative abundance compared to other pelagic fossil diatoms, but pigment-based estimates of abundance indicated an overall decrease in diatoms in Sky Pond over the last few decades. Colonial cyanobacteria pigments increased in The Loch, but were undetectable in Sky Pond. Overall, alpine Sky Pond displayed stronger rates of increase in productivity and in benthic green algae.
While N deposition triggered the onset of higher productivity in mid-20th century, other drivers now contribute. Laboratory assays revealed two distinct and interactive responses of green algae to warming and nutrient additions: 1) N assimilation by algae responded strongly to temperature; 2) N uptake increased significantly with P additions, regardless of temperature. Thus, both warming and P appear to separately enhance N uptake, possibly providing the stimulus for increasing benthic and pelagic algal growth in alpine