Hydrogeochemical controls on the variations in chemical characteristics of natural organic matter at a small freshwater wetland

This study investigated the spatiotemporal variability in dissolved organic carbon concentration ([DOC]), natural organic matter (NOM) weight average molecular weight (M_w), and absorptivity at 280 nm (ε₂₈₀, an estimator of aromaticity) at McDonalds Branch, a first-order stream that is a fen wetland. When ground-water discharge to the stream was predominant, the [DOC], M_w, and ε₂₈₀ were all relatively low. When soil pore water represented an important contribution to streamflow, not only was the [DOC] higher, but also the M_w and ε₂₈₀. Hence, the contribution of soil pore water relative to ground water controlled not only the concentration but also the average physicochemical characteristics of the NOM. Combined field and laboratory experiments suggested that preferential adsorption of higher M_w more aromatic NOM components to mineral surfaces, most likely within the lower soil horizons, resulted in a lower M_w more aliphatic ground-water NOM pool. This process ultimately affected surface water as ground water discharged to the lower reaches of the stream. Within the stream, higher molecular weight, more aromatic components were susceptible to photo-aggregation and photo-degradation, in the presence of Fe. Results from this small watershed study provide insight into climatic effects on surface-water NOM characteristics in a small freshwater fen. Low flow periods resulted in lower M_w more aliphatic NOM derived primarily from ground-water discharge to the stream, whereas higher periods resulted in a higher M_w (by 150-500 Da, i.e., increased by as much as 25%), more aromatic downstream surface-water NOM pool. Hence, during future summer drought periods, as suggested by climate-change models for much of North America, surface-water NOM likely will be lower molecular weight, more aliphatic, and more hydrophilic with lesser metal binding and hydrophobic organic carbon (HOC) uptake abilities [Environ. Sci. Technol. 34 (2000) 1103], along with decreased ability to attenuate UV radiation. Because NOM plays an important role in attenuating UV radiation, research on climate-induced effects on aquatic ecosystems needs to consider not only decreases in NOM concentration and photochemistry but also hydrology-induced changes to the NOM physicochemical characteristics.