We used a combination of approaches to measure primary production and plankton photophysiology in oligotrophic Flathead Lake, Montana (USA). Radiocarbon (¹⁴C) assimilation incubations and photophysiological parameters revealed seasonal patterns in both ¹⁴C-primary production and photophysiology. Rates of ¹⁴C-primary production were highly seasonal, with peak rates in the summer and lower rates in the winter. Photophysiology was similarly seasonal, with phytoplankton demonstrating both increased adaptation to low light and increased susceptibility to photoinhibition during the winter. Throughout the water column, observed photophysiology diverged when the lake stratified, indicating functionally distinct communities of phytoplankton within the water column. Photosynthesis-irradiance curves indicated that planktonic productivity in the subsurface chlorophyll maximum was light-limited year-round, while planktonic production in the near-surface waters was light-saturated during the summer. However, diel variability in incoming radiation drove daily variation in light limitation, as organisms at a given depth can experience both photoinhibition and light limitation during a single day. Additionally, we found that, despite physiological evidence of photoinhibition during the summer, this process appears to play a minor role in constraining primary production in Flathead Lake. Overall, this work highlights the importance of quantifying the variability in photophysiology and primary production across a variety of time scales.

Primary Presenter: Kate Evans, University of Montana (katherine1.evans@umontana.edu)

Authors:

Kate Evans, University of Montana  (katherine1.evans@umontana.edu)

Tyler Tappenbeck, University of Montana  (tyler.tappenbeck@flbs.umt.edu)

Logan Peoples, University of Montana  (logan.peoples@flbs.umt.edu)

Matthew Church, University of Montana  (matt.church@flbs.umt.edu)