Zahra Akbarzadeh, Sergei Bocaniov, Helen Powley, Philippe Van Cappellen (2022). Nearshore-offshore phosphorus mass balance modelling for large lakes: The Lake Erie case study. Proceedings of the GWF Annual Open Science Meeting, May 16-18, 2022.

Nearshore eutrophication is an important unsolved water quality problem in the Laurentian Great Lakes. This research focuses on Lake Erie, a large lake with unique water quality issues that are in part driven by the excess availability of phosphorus (P), including: harmful cyanobacterial blooms in the western basin, nuisance nearshore Cladophora growth in the eastern basin, and hypoxia in the central basin. We developed a simple regionalized P mass balance model for Lake Erie which explicitly accounts for the exchanges of water and P between the offshore and littoral zones. Except for the very shallow western basin, Lake Erie was divided into littoral segments and offshore areas, with the latter consisting of an epilimnion and hypolimnion. The P model includes dissolved and particulate fractions of P in?the?water column, as well as?total P (TP) in the bottom sediments. Mean annual P loadings (2003-2016 averages) from watershed flows and atmospheric deposition, along with the mean annual water exchanges between the different lake compartments estimated from 3D hydrodynamic model simulations, were imposed to the?P?model. Particular attention was given to quantifying the average littoral-offshore water exchange fluxes within the central and eastern basins lake. The resulting in-lake P exchanges thus comprise littoral-littoral (i.e., alongshore), littoral-offshore, and inter-basin fluxes. The calculations imply that P inputs from offshore waters make up to 20% of the annual TP budget of the littoral segments of the lake, while the remaining 80% is sourced from land runoff, atmospheric deposition, inflow from adjoining littoral segments, and, for the westernmost littoral segments of the central basin, the western basin. The model is applied to assess the long-term effectiveness of land-based P load management strategies in mitigating nearshore eutrophication. For instance, the model scenarios indicate that the 40% reduction in external riverine P loads to the western and central basins decided on by Canada and the US will not yield the desired nearshore P concentrations even after 10 years. Furthermore, based on our analysis of the available monitoring data collected in Lake Erie by different agencies, we recommend that increased efforts be devoted to obtaining more comprehensive P data sets along the littoral zone and filling the data gap on winter season lake biogeochemistry.

AOSM2022 First Author: Zahra Akbarzadeh (University of Waterloo) Additional Authors: Sergei Bocaniov (University of Waterloo), Helen Powley (Plymouth Marine Laboratory), Philippe Van Cappellen (University of Waterloo)