Lake Bonney (McMurdo Dry Valleys, east Antarctica) represents a year-round refugia for life adapted to extreme conditions. Lake level has risen by more than 3 m since 2004, but impacts of rapid lake level rise on phytoplankton community structure is also poorly understood. From 2004 to 2015, in concert with the summer limnological samplings, an in situ submersible spectrofluorometer (bbe FluoroProbe) was used to profile phytoplankton throughout the water columns of east and west lobes Lake Bonney to quantify the vertical structure of four functional algal groups (green algae, brown/mixed algae, cryptophytes, cyanobacteria). Our findings showed that phytoplankton communities were differentially impacted by physical and chemical factors over long-term vs. seasonal time scales. Following a summer of rapid lake level rise (2010-11), an increase in depth integrated chlorophyll a (chl-a) occurred in Lake Bonney caused by stimulation of photoautotrophic green algae. Collectively our data reveal that phytoplankton groups possessing variable trophic abilities are differentially competitive during seasonal and long-term time scales owing to periods of higher nutrients (photoautotrophs) vs. light/energy limitation (mixotrophs).
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A submersible spectrofluorometer (bbe Moldaenke Fluoroprobe; Beutler et al., 2002; Li et al., 2019) was lowered through the water columns in Lake Bonney beginning in the 2004-2005 field season. This instrument employs five light emitting diodes (LEDs) calibrated to the known excitation spectra of light harvesting antennae pigments of the following specific algal groups: 470 nm = green algae; 525 = brown/mixed algae; 570 nm = cryptophytes; 610 nm = cyanobacteria. An additional LED excited at 370 nm to estimate colored dissolved organic matter (CDOM), which was used to correct chl-a fluorescence. The Fluoroprobe collected data continuously as it was lowered through the water column at approximately 20 cm s-1 yielding measurements at ~10 cm depth intervals.