Shaping Water Quality

What shapes the water quality in Western Durham region?

A combination of climatic and physical characteristics within the lake affect the nutrient patterns observed in the nearshore. For example:

  • Precipitation and river or creek discharge affects the nutrients transported to the lake, in addition to point sources
  • Solar intensity affects biological growth and uptake of nutrients within the lake
  • Wind affects the direction and speed of currents in the lake
  • Currents affect which way the nutrients travel
  • Mixing affects which layer of water the nutrients are contained within

We therefore need to consider:

  1. Nutrients entering the system
  2. Climatic variables
  3. Physical dynamics

1. Nutrients Entering The System

Concentrations are important as they are used to calculate the mass or quantity of nutrients that are entering the system (load).

Sometimes we look at the nutrient concentrations in something called high conductivity quartiles (e.g. the top 25% of samples with the highest conductivity). Conductivity is a measure of the ability of water to pass an electrical current. It is affected by temperature, inorganic dissolved solids (e.g. chloride, nitrate, sulphate), anions (e.g. phosphate) and cations (e.g. sodium, calcium, iron). Conductivity is a good indicator of land-based influences.

Nutrients are often elevated in areas where there is high conductivity (e.g. red, yellow, and green on the maps below). These include rivers and creeks, storm drains, and outfalls to treatment plants.

Spatial conductivity maps from data collected in 2008 by the Ontario Ministry of the Environment Conservation and Parks
Figure 1: Spatial conductivity maps from data that the Ontario Ministry of the Environment, Conservation and Parks collected in 2008. Data used to create krigged images courtesy of the Environmental Monitoring and Reporting Branch at the Ontario Ministry of the Environment, Conservation and Parks.

We can use therefore use conductivity as a land-based and anthropogenic tracer to figure out where nutrients might be elevated. Conductivity is high in waters originating from the land and low in the open lake. There are no natural conductivity sources in the open-lake.

If we look at the concentrations of nutrients in sampling locations where there is high conductivity, we can try to figure out what influences the nutrient patterns we see by the shoreline.

2. Climatic Variables

Rain and snow melt affect discharge from rivers and storm drains into the lake. The red line is the 30 year precipitation average from Environment and Climate Change Canada, while the remaining bars are the years of the monitoring program. We have wet and dry years in comparison to the average (above and below the red line respectively).

Lake Ontario Waterfront nearshore monitoring water quality

High discharge events (blue line) in the image below occur with high precipitation events (black bars). This affects nutrient delivery to the lake. Conductivity from the Ontario Ministry of the Environment, Conservation and Parks is plotted in red, while the green vertical lines are the Toronto and Region Conservation Authority (TRCA) sampling dates in 2009. From this image, we see that some of the large discharge events are in line with the large precipitation events.

Lake Ontario Waterfront nearshore monitoring water quality

The sun helps things grow in the lake, and heats the lake creating different thermal layers of water. The daily solar average peaks in the summertime.

Lake Ontario Waterfront nearshore monitoring water quality

Winds affect currents and mixing in the lake. Below is the wind stress separated into alongshore (blue) and on-shore/off-shore components (red).

Lake Ontario Waterfront nearshore monitoring water quality

3. Physical Dynamics

Current velocities and directions help move nutrients in the lake. Alongshore transport is faster than on-shore/off-shore transport.

Lake Ontario Waterfront nearshore monitoring water quality

Stratification (different water layers) has the potential to hold nutrients in different layers of water. Data used to create the images below were collected by the Ontario Ministry of the Environment, Conservation and Parks and processed by TRCA.

Lake Ontario Waterfront nearshore monitoring water quality

However, natural mixing events (upwelling and downwelling) will help move nutrients from the different stratified layers to the nearshore. We cannot control these mixing events.

figure 9 - total phosphorus overlain on top of current modeled data

Currents can help transport nutrients around the nearshore (images of current directions and velocities modeled by Environment and Climate Change Canada).

Lake Ontario Waterfront nearshore monitoring water quality

Although based on our monitoring program we cannot verify the Environment and Climate Change Canada modeled current data above, water quality results match patterns of circulation supporting the precision of the modeling.

Bringing Everything Together

Redundancy Analysis is a multivariate statistical technique.

Lake Ontario Waterfront nearshore monitoring water quality

Using this technique on the water quality from the high conductivity quartile (e.g. samples with a high land-based influence) shows that more than 75% of the variability we see in water quality in the high conductivity quartile (e.g. in areas with high land-based or anthropogenic influences) can be explained by climatic variables and physical characteristics measured within the lake. This also works using average water quality concentrations from the shoreline to 1 km from shore.

If we use a different multivariate statistics technique (multiple regression) to look at specific nutrients one by one, we see that approximately 50-60% of the differences can be explained by environmental characteristics that have occurred up to 3 days in advance of the sampling event.

Take Home Messages

  1. Environmental factors affect nutrient differences we see in the nearshore environment.