Estimating mercury concentrations and fluxes in the water column and sediment of Lake Ontario with HERMES model.

The HERMES model-predicted Hg concentrations and fluxes in Lake Ontario were based on twelve lake and drainage basin variables (i.e., water temperature, precipitation rate, air Hg, surface area, mean depth, water volume, water inflow rate, inflow water Hg, inflow and lake suspended particulate matter, air-water and water-air mass transfer coefficients, and sedimentation rate). The HERMES model-predicted Hg water and surface sediment concentrations were found to be significantly correlated (±20%) with measured values (r(2) = 0.94, p < 0.0001, n = 13) and mechanistic model predictions (LOTOX2-Hg, r(2) = 0.95, p < 0.0001, n = 10). The predictive capacity of HERMES was previously tested on smaller (≤1 km(2)) lakes in Nova Scotia and Ontario, Canada (i.e., water and sediment Hg concentrations were ±15% of measured data). Results suggest that HERMES could be applicable to a broad range of lake sizes. Uncertainty analyses on HERMES model input variables indicated a larger atmospheric Hg contribution for Lake Ontario when compared to previous predictions for smaller lakes.

Development of resource shed delineation in aquatic ecosystems.

We apply a concept derived from food web ecology to large-scale spatial patterns of material supply within and between watersheds and coasts by generalizing the definition "resource shed" to source areas for materials supplied to a receptor (e.g., a point location) over a specified time interval. Independent hydrologic and hydrodynamic models, coupled with a particle tracking model, were used to delimit resource shed total spatial extent and relative contributory importance for selected receptors in Lake Erie (North America) over varying time intervals. One resource shed was extended into the Maumee River watershed (OH) by integrating the lake and hydrologic models. Model validation was achieved through comparison with data from the 2005 International Field Years on Lake Erie (IFYLE) study. Resource shed size, orientation, and internal structure varied with receptor location, in-lake circulation, terrestrial precipitation, time interval, and season. River plume extent and interaction were illustrated, and model integration revealed the relative contributory importance of subwatershed catchments to an off-shore receptor.

Changes in fractal dimension during aggregation.

Experiments were performed to evaluate temporal changes in the fractal dimension of aggregates formed during flocculation of an initially monodisperse suspension of latex microspheres. Particle size distributions and aggregate geometrical information at different mixing times were obtained using a non-intrusive optical sampling and digital image analysis technique, under variable conditions of mixing speed, coagulant (alum) dose and particle concentration. Pixel resolution required to determine aggregate size and geometric measures including the fractal dimension is discussed and a quantitative measure of accuracy is developed. The two-dimensional fractal dimension was found to range from 1.94 to 1.48, corresponding to aggregates that are either relatively compact or loosely structured, respectively. Changes in fractal dimension are explained using a conceptual model, which describes changes in fractal dimension associated with aggregate growth and changes in aggregate structure. For aggregation of an initially monodisperse suspension, the fractal dimension was found to decrease over time in the initial stages of floc formation.