Knowledge of the McMurdo Dry Valley (MDV) lakes is limited by the inaccessability of the region during winter, a period which is most relevant in understanding the overall function of these lakes. Nowhere is this gap more evident than in our measurements of metabolic and phylogenetic diversity during the long, dark winter. Given the relevance of the winter period to our understanding of the MDV ecosystem, our goal was to conduct the first year-round investigation of the biogeophysics of these unique lakes.  

We built and deployed autonomous lake profilers and samplers (ALPS) at the primary productivity maxima of the east (ELB) and west lobes (WLB) of Lake Bonney in December 2013.  In November of 2014 and 2015 we redeployed the ALPS systems and retrieved data from the dataloggers.. This instrumentation is comprised of three McLane Laboratories devices: one Phytoplankton Sampler (PPS), one Remote Access Sampler (RAS), and a modified Ice Tethered Profiler (ITP). 

Our research tested the overarching hypothesis that the winter darkness induces a cascade of physiological changes that alters the biodiversity and functional roles of autotrophic and heterotrophic microplankton within the lakes.  For the first time, we have demonstrated that water and microbial communities can be successfully collected and preserved and that physicochemical parameters can be measured in Lake Bonney throughout the year using autonomous samplers.

Sample 16S and 18S rRNA gene diversity was sequenced exhaustively using Illumina sequencing.  Alpha diversity in ELB was lower than in WLB for both Bacteria and Eukarya and was lowest during dark/winter months.  Bacterial and eukaryotic community composition differences among the samples were correlated with photosynthetically available radiation (PAR, R²=0.40, p=0.001), though the eukaryote community structure was more strongly structured by water temp (R²=0.88, compared to R²=0.53 for Bacteria).  

The RAS successfully collected a full suite of samples in the first year, which were analyzed for particulate organic carbon (POC) and nitrogen (PON) levels and major ions.  POC and PON levels at the PPR maxima decreased through the winter, but began to increase in the lakes by early August 2014. Ion and DOC analysis revealed that in general, lake chemistry at this depth remained relatively stable throughout the year. 

Due to the density gradient, we had to pick a depth range of interest and adjust the buoyancy of the profilers to set it at the appropriate depth. We observed saturating levels of PAR through the summer, which then decreased through fall before dropping to zero in the winter.  PAR then increased beginning in August of 2014. 

The depth we were able to sample for the two years of deployment captured a region of the West Lobe Bonney profile that shows the influence of a subglacial discharge from beneath Taylor Glacier. The ITP confirmed the existence of this discharge and comparison with imagery of Blood Falls activity strongly suggested a relationship between Blood Falls discharge and the anomalies.

The BBE Fluoropobe included in the ITP sensor suite allowed us to characterize phytoplankton community dynamics throughout the year in both lobes of Lake Bonney. The fluororoprobe is a submersible spectrofluorometer capable of detecting the chlorophyll-a concentration of 4 spectral classes of phytoplankton using the excitation and emission wavelengths of class specific photosystem antennae pigments. Recent genomic work has provided some insight into the algal species detected in the various spectral classes. During the 2013/2014 winter, the mean total chlorophyll-a concentration in WLB began dropping at a rate of 0.09 µg L^-1 day^-1 (R² = 0.70) in August, but began increasing by 0.60 µg L^-1 day^-1 (R² = 0.60) as UW PAR became available in mid-October. This spring growth period coincided with an increase in chlorophytes and a rapid decrease in cryptophytes. In both the east and west lobes of Lake Bonney the “mixed” group was the dominant phytoplankton class. Many of the species detected in this class of algae are capable of both photosynthetic and heterotrophic metabolism. This is true with the cryptophyte group as well. While less abundant in Lake Bonney, their populations appear to increase during the dark winter months. Strictly phototrophic organisms, like those detected in the “green” and “blue/green” spectral class, either decreased in concentration or remained relatively constant. This suggests alternative metabolic strategies such as retaining their photosynthetic apparatus and surviving off lipid reserves or entering a vegetative state. This project demonstrated that winter community composition varies significantly and winter dynamics must be considered when determining annual productivity and function of the lake ecosystem.

Last Modified: 09/11/2016
Modified by: Cristina D Takacs-Vesbach