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.