Dissolved organic matter biogeochemistry in the McMurdo Dry Valleys, Antarctica: varying chemical quality of microbially-derived DOM in glacial meltwater streams

From a limnological perspective, dissolved organic matter (DOM) can originate from allochthonous sources on the landscape or from autochthonous sources within the water body itself. In many streams and lakes, allochthonous organic materials contributing to the DOM are derived from terrestrial plants, plant litter, and soil organic material, which all include some products of microbial growth and decay. The many streams in the McMurdo Dry Valleys (MDV) provide an opportunity to understand the biogeochemistry of DOM derived solely from microbial phototrophs and heterotrophic bacteria because of the absence of plants on the barren landscape and the abundant perennial microbial mats in the stream channels. Analysis of the 20-year record of dissolved organic carbon concentrations in the streams indicates that biogeochemical processes in microbial mats and the hyporheic zone support chemostasis for DOC in these streams. Even though the stream DOC concentrations are typically quite low, about 1 mg C/L or less, we were able to use fluorescence spectroscopy to chemically characterize the DOM samples from a broad array of meltwater streams.  Many streams had a distinct “humic-like” signature and some presence of an “amino-acid like” signature. In contrast,  a short dilute stream that does not support mats and primarily receives DOM from the surface of the glacier had an “amino-acid like” and only a weak “humic-like” fluorescence signature. The presence of a “humic-like” signature may indicate a source from organic matter pools in the hyporheic zone which accumulate due to advection of microbial mat material from the channel. Autochthonous organic matter pools may also influence DOC concentrations in temperate streams.  In addition, stream DOM may represent a labile DOM source to the lakes that contributes to supporting the mixotrophic phytoplankton communities.