Seasonal Variation in Dissolved Organic Matter Composition and Photoreactivity within a Small Sub-arctic Stream.

Abstract:

Dissolved organic matter (DOM) is a complex heterogeneous mixture, ubiquitous to all natural surface waters, uniquely composed of source inputs specific to spatial, temporal, and ecological circumstances. In arctic and sub-arctic regions, elucidating DOM composition and reactivity is complicated by seasonal variations. These include changes in productivity and source inputs to the water column, as well as winter overflow events that may contribute allochthonous organic material. DOM from a small boreal stream in a watershed of discontinuous permafrost in the Goldstream Valley of interior Alaska was isolated by solid-phase extraction (PPL) at multiple points during the year – late spring, late summer, and in the winter during an active overflow event. Compositional characteristics of each of the isolates were characterized by SPR-W5-WATERGATE ¹H NMR spectroscopy, specific UV-Vis absorbance, and excitation emission matrix (EEM) fluorescence spectroscopy and compared against end-member reference DOM isolates. Kinetics of photobleaching experiments reveal the influence of compositional differences among the isolated DOMs on their chemical reactivity, and offer insight into potential differences in their source materials and ecological function throughout the year. Photobleaching studies were conducted using a variety of reactive species quenchers or sensitizers in order to assess susceptibility of oxidative transformation mechanisms on the different DOM isolates, which were then analyzed by ¹H NMR, UV-Vis degradation kinetics, and parallel factor analysis (PARAFAC) of fluorescence EEMs. Better understanding of the seasonal variations of boreal DOM character and function on a molecular level is critical to assessing alterations in its ecological role and cycling in the face of current and future ecosystem perturbations in arctic and sub-arctic regions.