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Green Bay, Lake Michigan, USA, is the largest freshwater estuary in the Laurentian Great Lakes and receives disproportional terrestrial inputs as a result of a high watershed to bay surface area ratio. While seasonal hypoxia and the formation of "dead zones" in Green Bay have received increasing attention, there are no systematic studies on the dynamics of dissolved organic matter (DOM) and its linkage to the development of hypoxia. During summer 2014, bulk dissolved organic carbon (DOC) analysis, UV-vis spectroscopy, and fluorescence excitation-emission matrices (EEMs) coupled with PARAFAC analysis were used to quantify the abundance, composition and source of DOM and their spatiotemporal variations in Green Bay, Lake Michigan. Concentrations of DOC ranged from 202 to 571μM-C (average=361±73μM-C) in June and from 279 to 610μM-C (average=349±64μM-C) in August. In both months, absorption coefficient at 254nm (a254) was strongly correlated to bulk DOC and was most abundant in the Fox River, attesting a dominant terrestrial input. Non-chromophoric DOC comprised, on average, ~32% of bulk DOC in June with higher terrestrial DOM and ~47% in August with higher aquagenic DOM, indicating that autochthonous and more degraded DOM is of lower optical activity. PARAFAC modeling on EEM data resulted in four major fluorescent DOM components, including two terrestrial humic-like, one aquagenic humic-like, and one protein-like component. Variations in the abundance of DOM components further supported changes in DOM sources. Mixing behavior of DOM components also indicated that while bulk DOM behaved quasi-conservatively, significant compositional changes occurred during transport from the Fox River to the open bay.

Despite major investments in point source reductions, portions of the Great Lakes, like Green Bay, remain hypereutrophic and are subject to persistent seasonal hypoxia. Phosphorus (P) is generally a limiting nutrient in the Great Lakes ecosystem, but not all P species are equally bioavailable, and the dynamics of nutrients and their correlation to algal bloom remain poorly characterized, in part, due to a lack of adequate quantification of P chemical speciation. During summer 2014, water samples were collected from seasonally hypoxic Green Bay for measurements of dissolved and particulate inorganic and organic P to examine P cycling dynamics along a steep nutrient gradient ranging from Fox River inflow dominated eutrophic waters in the southern bay to mesotrophic northern waters near the bay's connection with open Lake Michigan. River-derived dissolved and particulate P was quickly removed from the water column in southern Green Bay through biological uptake and sedimentation. Concentrations of phosphate or dissolved inorganic P (DIP) dramatically decreased from 828 ± 216 nM in the Fox River, comprising 57 ± 1% of the total dissolved P, to 24 ± 9 nM in northern Green Bay where dissolved organic P (DOP) became predominant (>80%). Generally low phosphate concentrations and extremely high dissolved organic C/P ratios (2090 ± 1160 in August 2014) suggested high DOP turnover rates and active transformation between DOP and DIP through organic degradation during P-limited conditions in Green Bay. Elevated DIP levels were accompanied by low dissolved oxygen in deeper waters (10-15m) of central Green Bay where hypoxia-development occurred, suggesting the release of DIP through particle regeneration under hypoxic conditions enhanced by lateral transport and sediment resuspension. High partition coefficients (Kd) of both inorganic and organic P and their significant negative correlation with suspended particulate matter concentrations indicated the particle-reactive nature of P in freshwater environments and may imply that DOP could also be bioavailable under P-limitation.