Variations in abundance, composition and sources of dissolved organic matter in Green Bay, Lake Michigan

Green Bay is the largest freshwater estuary in the Great Lakes and receives disproportionately high terrestrial runoffs from surrounding watersheds. Although seasonal hypoxic conditions and the formation of “dead zones” in Green Bay have received increasing attention, dynamics of dissolved organic matter (DOM) in the bay and its relation to hypoxia remain understudied. Water samples were collected during summer 2014 from Green Bay, covering stations from eutrophic lower Fox River to northern Green Bay for the measurements of bulk dissolved organic carbon (DOC), UV-vis absorbance, and fluorescence excitation-emission matrices (EEMs) in addition to hydrographic parameters. DOM abundance, composition, mixing behavior, and sources were quantified for Green Bay in June and August 2014. DOC concentrations ranged from 202 – 571 µM-C with an average of 36173 µM-C, showing a south-to-north concentration gradient, with the highest concentration, more higher-molecular-weight and aromatic DOM components in the lower Fox River. Absorption coefficient (a₂₅₄) was significantly correlated to DOC concentration and specific conductivity, showing an apparent conservative mixing behavior, especially in August. Non-chromophoric DOC comprised, on average, 33% of the bulk DOC in June and 47% in August, consistent with change in DOM sources between June and August and the lower optical active of autochthonous and more degraded DOM. Parallel factor (PARAFAC) analysis on EEMs data gave rise to two terrestrial humic-like, one aquagenic humic-like, and one protein-like DOM components. Fluorescence indices (BIX and HIX) agreed well with the relative abundance of fluorescent DOM components, with more humified DOM in June and aquagenic DOM in August. Variations in DOM abundance and composition attested the dominance of terrestrial DOM and a dynamic changes in DOM quality along the river-bay transect and between June and August.