Chemical characterization of dissolved organic material in Pony Lake, a saline coastal pond in Antarctica

Abstract

Pony Lake is a saline and hypereutrophic coastal pond located on Cape Royds in the McMurdo Sound area in Antarctica. Previous studies have shown that Pony Lake typically has high concentrations of dissolved organic carbon (DOC) concentrations (e.g., 9.2 mM) associated with dense blooms of a chlorophyte, Chlamydomonas intermedia Chodat. We studied different classes of dissolved organic material (DOM) isolated from Pony Lake on two dates that bracketed the transition from ice-covered to ice-free conditions. We used tangential flow ultrafiltration followed by XAD column chromatography to isolate fulvic acids and transphilic acids in three molecular size ranges. The small and large size fractions (passing through a 10,000-amu filter and being retained by a 100,000-amu filter, respectively) accounted for most of the total DOM and fulvic acid pools, whereas most of the transphilic acid was in the small size fraction. Measurement of molecular weight using high-pressure size exclusion chromatography (HPSEC) showed that the fulvic acid and transphilic acid in the medium and large size fractions had molecular weights corresponding to a range of 1260–1470 amu, and those in the small size fraction had molecular weights of about 1000 amu. The 13C nuclear magnetic resonance (NMR) spectra of both the fulvic and transphilic acids showed a predominance of aliphatic and carbohydrate carbons and were similar among size fractions and between the two dates. Although a large decrease in the DOC concentration occurred during the transition to ice-free conditions (from 7.7 to 2.7 mM), the changes in the DOM fractionation were relatively small. The percentage of the DOM accounted for by transphilic acid increased from 11% to 18%, and the peak in the HPSEC chromatograms corresponding to the smaller molecular weight range decreased for all fulvic acid and transphilic acid fractions. The nitrogen content of all fulvic acid and transphilic acid fractions decreased significantly during the transition and the δ 15N values became heavier for all fractions. The only discernible change in the 13C NMR spectra of all the fractions was a decrease in a peak in the region corresponding to amide carbons. These results suggest that preferential loss of N-containing molecules of fulvic acid and transphilic acids occurred in association with the considerable loss of DOM in midsummer.