Autonomous year-round sampling sheds light on Antarctica’s polar night

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The perennially ice-covered lakes of the McMurdo Dry Valleys (MDV), Antarctica have been the focus of intense ecological research as part of the MCM LTER for over 20 years. Research in the MDV is typically restricted to the austral summer and, consequently, the biogeochemical and phytoplankton community responses to the darkness of winter are poorly understood. In December of 2013 we deployed autonomous lake profiling systems (ALPS) in the east and west lobes of Lake Bonney. Tethered to the surface of the ice cover, these autonomous instrumentation systems made year-round measurements of physicochemical and biological parameters, providing the most complete annual MDV lake dataset in the history of the MCM LTER. Along with the physicochemical data, the total chlorophyll-a concentration (μg L-1) and the relative contribution of chlorophyll-a by four functional groups of microalgae (chlorophytes, cyanophytes, cryptophytes, and chrysophytes) was recorded with a submersible spectrofluorometer. Our data show a shift in the phytoplankton community dominated by chlorophytes during the austral summer to a greater relative abundance of chrysophytes during the darkness of winter in east Bonney (15-22m). Also highlighted by these data is the surprising length of time that the total chlorophyll-a remains stable in both east and west Bonney before declining in August. These data provide a link between the fall and spring sampling which occurs annually as part of the MCM LTER project, and begin to fill our knowledge gap regarding organisms responsible for primary production in a microbially-dominated aquatic ecosystem during extended periods of complete darkness.