Analysis of Long Term Photosynthetically Active Radiation Data from McMurdo Dry Valley Lakes to Identify Turbidity Stratification Patterns
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The current study focused on four perennially ice covered, meromictic, fresh water lakes in the Taylor Valley of the McMurdo Dry Valley region: West and East Lobe Bonney, Fryxell, and Hoare. Data from these lakes have been collected annually for more than 25 years, and are cataloged in the McMurdo Dry Valley Long Term Ecological Research database. The objectives of the current study were to determine whether turbidity stratification patterns could be identified within the four McMurdo Dry Valley Lakes, to look for seasonal and annual changes in those stratification patterns, and to correlate environmental data to identify potential causes of the turbidity. To identify patterns in turbidity, extinction coefficient depth profiles were calculated using photosynthetically active radiation data collected annually during the past 29 years. Averaging the profiles revealed distinct stratification of turbidity layers that were shown to be relatively stable across all recorded years. To examine potential causes of turbidity revealed by the extinction coefficient data, chlorophyll-a profiles were compiled for the same years, and compared to those of the extinction coefficients. Within the photic zones of Lake Hoare and East and West Lobe Bonney, chlorophyll-a was significantly correlated with turbidity. A linear plot of extinction coefficients against time revealed that later season average turbidity is much higher than that of early season turbidity, and that turbidity increases with depth in the photic zones of all lakes. Future research focusing on stream flow data could help identify the causes of the increase in late season turbidity observed in this study, and analysis of environmental data other than chlorophyll could help to determine what other phenomena control turbidity gradients in Lakes Fryxell and Hoare.