BIOGEOCHEMICAL DYNAMICS OF DISSOLVED SI IN TAYLOR VALLEY, ANTARCTICA

Silicon is an important biogeochemical element as aquatic microorganisms, such as diatoms, need it for growth. In this study, we quantified the flux of reactive Si in a series of closed-basin lakes in the McMurdo Dry Valleys, Antarctica (~78° S). In addition, we have evaluated the potential processes, including biological uptake, controlling the distribution of dissolved Si in the lakes.

Water samples were collected at the beginning of the summer (Nov. 1999) and then again in mid-summer (Jan. 2000) from Lakes Bonney, Fryxell and Hoare in the Taylor Valley. Stream samples from each of the lake basins were also collected during the summer months. For this work, a colorimetric method was used to analyze the concentrations of reactive Si in the water samples.

Streams of the Lake Fryxell basin had the highest average Si concentrations, 1.32mg/L. Streams that flow into Lake Hoare had the lowest average concentrations, 0.2mg/L. This difference is most likely a result of the length of the streams. The streams in Lake Fryxell basin are longer than those of Lake Hoare, and therefore the water is in contact with the underlying sediment for a longer period of time, resulting in more Si being dissolved. The annual flux of Si into the lakes was determined by calculating the average Si concentrations in the streams and multiplying by the volume of stream flow for each lake basin. The highest reactive Si fluxes occurred in the Lake Bonney basin followed by Lake Fryxell and Lake Hoare.

Si concentrations in the lakes vary over time due to certain physical, chemical and biological processes. The concentrations in the lakes could be influenced by dilution with stream water of lower Si concentrations, precipitation and dissolution of Si within the water column, or biological uptake of Si by diatoms. For example, a decrease in Si concentration occurred in the upper part of the water column of Lake Hoare between the early and midsummer sample collections. The change in Si is much greater than the expected uptake by diatoms. Approximately 3% of the Si decrease can be attributed to biological uptake, based on primary productivity measurements. Therefore, other physical processes such as dilution must control the Si variations in Lake Hoare.