Improving the representation of internal nutrient recycling with phosphorus mass balance models: A case study in the Bay of Quinte, Ontario, Canada

We evaluate the relative importance of the causal connection between exogenous total phosphorus (TP) loading and internal nutrient recycling with the water quality conditions in the Bay of Quinte, Ontario, Canada. First, we examine the temporal trends of all the major point and non-point loading sources over the last four decades. We then enhance the mechanistic foundation of an existing simple mass-balance total phosphorus (TP) model, originally developed to guide the eutrophication management in the system. The structural improvements include the incorporation of macrophyte dynamics, the explicit representation of the role of dreissenids in the system, and the improved portrayal of the interplay between water column and sediments. The upgraded model was in good agreement with the observed TP variability in the system during the study period (2002–2009) and successfully reproduced the TP accumulation patterns toward the end of the summer-early fall. We provide evidence that phosphorus dynamics in the upper Bay are predominantly driven by the inflows from Trent River, while the middle and lower segments likely receive substantial internal subsidies from the sediment diagenesis mechanisms and/or the activity of macrophytes and dreissenids (e.g., pseudofeces production, nutrient pump effect). We also forced the model with scenarios of reduced nutrient loading and examine the likelihood of the system to meet its water quality delisting targets, although we caution that our complex overparameterized modeling construct is primarily intended for heuristic purposes. The present study together with the companion paper by Zhang et al. (2013) illustrate how phosphorus mass balance models can offer useful tools for improving our understanding of freshwater ecosystems.