The filamentous green alga Cladophora grows to nuisance proportions in Lake Ontario. Stimulated by high phosphorus concentrations, nuisance growth results in the degradation of beaches and clogging of industrial water intakes with attendant loss of beneficial uses. We develop a multi-module bioavailable phosphorus model to examine the efficacy of phosphorus management strategies in mitigating nuisance algal growth. The model platform includes modules simulating hydrodynamics (FVCOM), phosphorus-phytoplankton dynamics (GEM) and Cladophora growth (GLCMv3). The model is applied along a 25 km stretch of the Lake Ontario nearshore, extending east from Toronto, ON and receiving effluent from three wastewater treatment plants. Simulation results identify the Duffin Creek wastewater treatment plant effluent as a driving force for nuisance conditions of Cladophora growth, as reflected in effluent bioavailable phosphorus concentrations and the dimensions of the plant’s phosphorus footprint. Simulation results demonstrate that phosphorus removal by chemically enhanced secondary treatment is insufficient to provide relief from nuisance conditions. Tertiary treatment (chemically enhanced secondary treatment with ballasted flocculation) is shown to eliminate phosphorus-saturated conditions associated with the Duffin Creek wastewater treatment plant effluent, providing local relief from nuisance conditions. Management guidance presented here has wider application at sites along the highly urbanized Canadian nearshore of Lake Ontario.

The emerging shift in Great Lakes management from offshore to nearshore waters will require attention to complexities of coastal hydrodynamics and biogeochemical transformations. Emphasizing hydrodynamics, this work resolves transport processes in quantifying discharge plume and pollutant of concern (POC) footprint dimensions, the latter being the portion of the plume where water quality standards are not met. A generic approach, isolated from pollutant-specific biokinetics, provides first-approximation estimates of the footprint area. A high-resolution, linked hydrodynamic-tracer model is applied at a site in the Greater Toronto Area on Lake Ontario. Model results agree with observed meteorological and hydrodynamic conditions and satisfactorily simulate plume dimensions. Footprints are examined in the context of guidelines for regulatory mixing zone size and attendant loss of beneficial use. We demonstrate that the ratio of the water quality standard to the POC concentration at discharge is a key determinant of footprint dimensions. Footprint size for traditional pollutants (ammonia, total phosphorus) meets regulatory guidelines; however, that for soluble reactive phosphorus, a presently unattended pollutant, is ~1–2 orders of magnitude larger. This suggests that it may be necessary to upgrade treatment technologies to maintain consistency with regulatory guidelines and mitigate manifestations of the eutrophication-related soluble reactive phosphorus POC.

The historical development of environmental stressors in the watershed of Onondaga Lake (N.Y.) and the adjoining Seneca River is outlined as a prelude to the presentation of a novel approach for accessing assimilative capacity in this water quality-limited system. Proposed efforts to reclaim lost uses in the lake through heroic treatment at a major metropolitan wastewater treatment plant with continued discharge to the lake have been called into question. One option, diversion of the treatment plant effluent to the Seneca River, is complicated by phenomena impacting receiving water oxygen resources. A dual discharge strategy, which takes advantage of seasonal variations in assimilative capacity and the differing response times of the lake–river systems, is proposed as an alternative. In this approach, the effluent is routed to the river except when oxygen standards may be compromised, in which case all or part of the effluent is routed to the lake. Mathematical models demonstrate the feasibility of this option to meet water quality goals for both the lake (phosphorus) and the river (oxygen).

Phosphorus release rates were measured on intact sediment cores collected from the major depositional basin of Onondaga Lake, a dimictic, calcareous, hypereutrophic system in Syracuse, N.Y., U.S.A. Release experiments were conducted under a matrix of redox and pH conditions to investigate the importance of Ca- and Fe-related physicochemistry on sediment cores collected seasonally, i.e., during the spring, summer, fall, and winter periods. Strong seasonal variation in P release was observed, with rates ranging from ~3 to 38 mg P·m^-2·day^-1. This variation is attributed to changes in redox status and P concentration gradients at the sediment-water interface. An oxidized microlayer at the sediment-water interface partially inhibits sediment P release under isothermal, well-mixed conditions in the spring and fall. Phosphorus trapped in the oxic microlayer (sorption) is freed when the microlayer is chemically reduced at the onset of anoxia and high P release rates are observed. The oxidized microlayer serves to regulate seasonality in rates of sediment P release but does not influence long-term sediment-water exchange. It is proposed that the long-term P release process is best represented by a time-weighted annual average rate, calculated here to be ~10 mg P·m^-2·day^-1.

Diagenesis of particulate organic matter in sediments can have important effects on the dissolved oxygen (DO) resources of lakes and delay potential benefits from reductions in phosphorus (P) loading. A sediment diagenesis model of intermediate complexity, $\mathrm{Sed}2K_s$ , is developed and tested for Onondaga Lake, New York, and linked to a validated one-dimensional mechanistic hydrodynamic/stratification model (UFILS4). Modeling was supported by detailed characterization of the pelagic sediments and long-term monitoring of the lake’s water column that included transformation from hypereutrophy to upper mesotrophy and coupled improvements in hypolimnetic DO. $\mathrm{Sed}2K_s$ performed well in representing the observed decreasing trajectory of the downward flux of particulate organic carbon and in predicting the decreases in sediment effluxes of dissolved inorganic carbon and sediment oxygen demand over a 30-year period. $\mathrm{Sed}2K_s$ and the linked overall water-quality model, $\mathrm{UFILS}4/\mathrm{Sed}2K_s$ , were applied to project the extent and timing of improvements in sediment fluxes and hypolimnetic DO resources for an array of P management alternatives. Substantial delays in response associated with the effects of diagenesis were predicted that extended $\ge 40\mathrm{years}$ to reach 95% of steady state. Long-term projections indicate that improvements from the most aggressive P control alternatives may be sufficient to maintain certain coldwater fish.