Dissolved organic carbon characteristics in an acidified groundwater-dependent ecosystem

Abstract

Quantifying and characterising dissolved organic carbon (DOC) is critical to understanding its role in aquatic ecosystems. This is particularly challenging in acidic groundwater-dependent ecosystems, where low pH and high concentrations of Fe affect DOC characterisation. We investigated the variability in DOC concentrations and chemical structure in an acidic wetland, using UV visible spectrophotometry, a range of digestion methods and subsequent TOC analysis, high-pressure size exclusion chromatography (HPSEC) and rapid fractionation techniques. HPSEC results showed that increasing the pH from an original pH 2.3 to a neutral pH reduced the column adsorption of organic carbon, but did not change molecular weight distributions. Principal component analysis suggested that iron concentrations had a more direct effect on molecular structure than pH. The pH, Fe concentrations and DOC characteristics were highly dynamic and spatially variable, and were linked to surface water–groundwater connectivity, as well as horizontal connectivity of surface ponding. The changing pH and Fe concentrations affected DOC concentration and molecular structure with expected effects on bioavailability of DOC.