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As a result of increased harmful algal blooms and hypoxia in Lake Erie, the US and Canada revised their phosphorus loading targets under the 2012 Great Lakes Water Quality Agreement. The focus of this paper is the Detroit River and its watershed, a source of 25% of the total phosphorus (TP) load to Lake Erie. Its load declined 37% since 1998, due chiefly to improvements at the regional Great Lakes Water Authority Water Resource Recovery Facility (WRRF) in Detroit and phosphorus sequestered by zebra and quagga mussels in Lake Huron. In addition to the 54% of the load from Lake Huron, nonpoint sources contribute 57% of the TP load and 50% of the dissolved reactive phosphorus load, with the remaining balance from point sources. After Lake Huron, the largest source is the WRRF, which has already reduced its load by over 40%. Currently, loads from Lake Huron and further reductions from the WRRF are not part of the reduction strategy, therefore remaining watershed sources will need to decline by 72% to meet the Water Quality Agreement target - a daunting challenge. Because other urban sources are very small, most of the reduction would have to come from agriculturally-dominated lands. The most effective way to reduce those loads is to apply combinations of practices like cover crops, buffer strips, wetlands, and applying fertilizer below the soil surface on the lands with the highest phosphorus losses. However, our simulations suggest even extensive conservation on those lands may not be enough.

To support the 2012 Great Lakes Water Quality Agreement on reducing Lake Erie's phosphorus inputs, we integrated US and Canadian data to update and extend total phosphorus (TP) loads into and out of the St. Clair-Detroit River System for 1998–2016. The most significant changes were decreased loads from Lake Huron caused by mussel-induced oligotrophication of the lake, and decreased loads from upgraded Great Lakes Water Authority sewage treatment facilities in Detroit. By comparing Lake St. Clair inputs and outputs, we demonstrated that on average the lake retains 20% of its TP inputs. We also identified for the first time that loads from resuspended Lake Huron sediment were likely not always detected in US and Canadian monitoring programs due to mismatches in sampling and resuspension event frequencies, substantially underestimating the load. This additional load increased over time due to climate-induced decreases in Lake Huron ice cover and increases in winter storm frequencies. Given this more complete load inventory, we estimated that to reach a 40% reduction in the Detroit River TP load to Lake Erie, accounting for the missed load, point and non-point sources other than that coming from Lake Huron and the atmosphere would have to be reduced by at least 50%. We also discuss the implications of discontinuous monitoring efforts.

Hypoxia, or low dissolved oxygen (DO) concentrations, in lakes is commonly linked to eutrophication caused by excessive nutrient loadings. While nutrient-driven eutrophication creates a potential for hypoxia, the full realization of this potential, as well as its location, ultimate size, and duration, is to a large degree dependent on the lake's physics. Herein, we employed a three-dimensional coupled hydrodynamic and ecological model of Lake Erie to explore the potential for spatial and temporal developments of hypoxia and its response to nutrient load reductions. Reducing loads by 40 to 50% relative to the 2008 load will result in significant reductions (~ 50%) of hypoxia in terms of its maximum and mean areal extents and duration, as well as providing significant improvements in mean hypolimnetic DO. We also explored the impact of different DO threshold concentrations to characterize hypoxia, and found that responses at lower thresholds are most sensitive to variation in nutrient loads.

The ratio of gross photosynthesis to community respiration ( P : R ) is a vital characteristic of ecosystem function but is poorly defined in large lakes because traditional methods are impractical for measuring P and R on appropriate time and space scales. The new steady state in situ 18 O method has a great power to assess metabolic patterns in large lakes. However it could be confounded by seasonal and/or episodic temperature changes, which can violate the steady-state assumptions by altering the saturation level of dissolved O 2 without actual gain or loss of O 2 when air–water gas exchange is too slow to compensate for the altered solubility. We estimated P , R and P : R at three coastal sites of varied depth, productivity and physical exposure in the Laurentian Great Lakes by both the 18 O method and experimental incubations. At two shallow sites with low and moderate exposure to open lake forces, the 18 O and incubation methods returned similar estimates for P , R and P : R . The deepest site experienced large and frequent upwelling episodes and the 18 O method was unreliable, with P and R estimates usually negative. We define an O 2 saturation ratio ( γ ) that determines the applicability of the 18 O method to any given system. Estimates obtained using observed O 2 saturation above this threshold are invalid. The γ ratio increases with lake productivity. Higher productivity and/or shallower lakes appear good candidates for use of the 18 O method, but applications in deeper, physically-driven lakes will likely need explicit attention to temperature-driven non-steady state effects.

A three dimensional hydrodynamic model, the Estuary and Lake Computer Model (ELCOM), validated by field data collected in 2008, is used to investigate the thermal structure response in Lake Erie to changes in air temperature and wind speed. We define spatially and temporally varying regions for the epilimnion, thermocline, and hypolimnion. Increasing the air temperature warms up the epilimnion but has little effect on the hypolimnion. The stratification forms earlier and breaks down later. The thermocline is raised modestly in the warmer air temperature scenario. Stronger winds cool the epilimnion slightly, but warm up the hypolimnion with much larger temperature changes. The stratification duration is shortened, and the thermocline depth is noticeably deepened. Due to the large differences in depths and layer thicknesses of the three basins, the responses to changes in meteorological forcing vary among the basin. Exploiting the power of the three dimensional model to provide a more authentic characterization of thermal stratification in large lakes, it is shown that patterns inferred from simple isotherm dynamics when studying the stratification period as typically done with one dimensional models are not always accurate. The present results for Lake Erie show the potential for complicated and interactive effects of climate forcing on important biogeochemical processes, especially hypolimnetic oxygen depletion.

A 3-dimensional numerical model of Lake Erie was set up for a winter season using a coupled hydrodynamic and water quality model (ELCOM-CAEDYM) and validated against observations. The model was successful in predicting average lake surface temperature (root mean square deviation, RMSD 3 km 2 , ~ 10% of the lake surface, and thickness RMSD = 2.1 cm). This study illustrates the importance of the inclusion of ice cover when simulating hydrodynamics and lake water quality. For example, the water level oscillations were significantly reduced under ice cover conditions. Although the model is not calibrated for water quality during winter conditions, the predicted variations in DO and Chl-a are qualitatively in agreement for all three basins of Lake Erie. Similar to recent field observations, our model results also suggest that despite low temperatures and low under-ice light availability, winter conditions can support high phytoplankton biomass in the central basin which is at least comparable to that typically observed in the summer. Our results also indicate that simulations without ice-cover during the winter results in higher phytoplankton biomass in the central basin compared to that in the scenario with ice. This suggests that changes in the extent of ice cover, its thickness and duration will influence winter productivity with the consequences for hypoxia to develop later in the season.

In large lakes, temporal variability is compounded by strong spatial variability associated with mesoscale physical processes such as upwelling and basin-scale circulation. Here we explore the ability of a three dimensional model (ELCOM–CAEDYM) to capture temporal and spatial variability of phytoplankton and nutrients in Lake Erie. We emphasized the east basin of the lake, where an invasion by dreissenid mussels has given special importance to the question of spatial (particularly nearshore–offshore) variability and many comparative observations were available. We found that the model, which did not include any simulation of the mussels or of smaller diffuse nutrient sources, could capture the major features of the temperature, nutrient and phytoplankton variations. Within basin variability was large compared to among-basin variability, especially but not exclusively in the western regions. Consistent with observations in years prior to, but not after, the mussel invasion the model predicted generally higher phytoplankton concentrations in the nearshore than the offshore zones. The results suggest that the elevated phytoplankton abundance commonly observed in the nearshore of large lakes in the absence of dreissenid mussels does not have to depend on localized nutrient inputs but can be explained by the favourable light, temperature and nutrient environment in the shallower and energetic nearshore zone. The model is currently being extended to allow simulation of the effects of dreissenid mussels.