Spatial Patterns

What does nearshore monitoring in the Ajax and Pickering region tell us about water quality patterns?

If we look at the water quality from the monitoring program for Ajax and Pickering nearshore water quality, four different spatial patterns emerge:

1. Shoreside to open water
2. Surface versus bottom waters
3. Shoreside versus 100 metres from shore
4. West versus east

Lake Ontario Waterfront nearshore monitoring water quality patterns
Figure 1: A map illustrating the four spatial patterns to be explored. “Map Source: Data provided by Ontario Ministry of Natural Resources”.

1. Shoreside to Open Waters

Most of the changes we see in water quality concentrations in Ajax and Pickering occur within the first kilometre of the shoreline. Following the definitions by the Ontario Ministry of the Environment and Climate Change, the shoreside is the water closest to the shore that is “wadeable” (depths < 1.2 m). In all years, we see a general trend of higher nutrient concentrations closer to shore and decreasing concentrations farther away from shore.

Lake concentrations from sites closest to the shoreline are the highest of all sampling locations in the lake. Lake concentrations towards the open waters are the lowest concentrations of all the sampling locations. This is similar to other sites in Lake Ontario. Ontario Ministry of the Environment and Climate Change scientists have seen a similar pattern across different locations in Lake Ontario. This spatial trend, in part, is due to the importance of alongshore transport in the region which is outlined in the alongshore transport section.

E. coli

Lake Ontario Waterfront nearshore monitoring water quality patterns
Figure 2: Lake surface E. coli concentrations from 2007.

E. coli concentrations are greatest by the shoreline, and decrease as we move away from the shore. At a distance of 1 km from the shore, concentrations are below the Provincial Water Quality Objective for recreational waters set by the Ontario Ministry of the Environment and Climate Change.

Total Phosphorus

Lake Ontario Waterfront nearshore monitoring water quality patterns
Figure 3: Lake surface total phosphorus concentrations from 2012.

Total Phosphorus concentrations are highest at the shoreline and decrease as we move away from the shore towards open waters. At distances greater than 1 km from the shore in 2012, the concentrations fell below the International Joint Commission’s water quality objective for open waters.

Nitrate + Nitrite

Lake Ontario Waterfront nearshore monitoring water quality patterns
Figure 4: Lake surface nitrate+nitrite concentrations from 2011.

Beyond a distance of 1 km, nitrate + nitrite concentrations decrease. The maximum acceptable limit for nitrate + nitrite in drinking water is 10 mg/L (or 10,000 µg/L). There is no Provincial Water Quality Objective for nitrate in nearshore waters.

2. Surface Waters versus Bottom Waters

Richardson numbers tell us about mixing happening in the lake (see Upwelling and Downwelling for more detail). Richardson numbers in our results show that surface waters and bottom waters are not always mixed, which means that the nutrient concentrations in these different water layers can be different. If we take the transect west of Duffins Creek as an example, we can see that the surface waters are higher in some concentrations, or water quality measures. For example:

(i) Surface waters tend to be higher in: pH, and turbidity.

Lake Ontario Waterfront nearshore monitoring water quality patterns
Figure 5: An example of surface and bottom water pH values from summer 2008, 2011, and 2012.
Lake Ontario Waterfront nearshore monitoring water quality patterns
Figure 6: An example of surface and bottom water nitrate+nitrite concentrations from summer 2008, 2011, and 2012.

(ii) With other nutrients, like phosphorus, the patterns are not as obvious.

The plots below show that sometimes the surface waters are higher in total phosphorus (August 2012), sometimes the bottom waters are higher in total phosphorus (August 2008), and sometimes the surface and bottom waters are nearly equal in total phosphorus (August 2011). This is because there are many different processes at work and the balance of these processes may change over the nearshore.

Some examples of different processes affecting phosphorus concentrations in the lake are: dilution, biological uptake from different species, sediment release, deposition, and release during decay of dying algae and aquatic species.

Lake Ontario Waterfront nearshore monitoring water quality patterns
Figure 7: An example of surface and bottom water total phosphorus concentrations from summer 2008, 2011, and 2012.

3. Shoreside versus 100 Metres from Shore

If we look at sites along the shore from Duffins Creek to Carruthers Creek, we notice that the shoreside concentrations are high compared to the rest of the monitoring area. We also notice that the concentrations are highest near the creek mouths, and also near storm drains emptying to the lake. Depending on the direction in which the alongshore currents travel, we may see the nutrient concentrations decrease in the direction of the current.

Lake Ontario Waterfront nearshore monitoring water quality patterns

Lake Ontario Waterfront nearshore monitoring water quality patterns
Figure 8: An example of the a) E.coli, and b) total phosphorus concentration differences along the shore and 100 m from the shore. “Map Source: Data provided by Ontario Ministry of Natural Resources © Copyright: 2005 First Base Solutions Inc. All Rights Reserved”.

West versus East

If we compare the nutrient concentrations from the transects located by the mouths of Duffins Creek and Carruthers Creek, we notice some similarities and some differences.

(i) Near the Creek mouths, concentrations in the west by Duffins Creek are generally higher than in the east by Carruthers Creek for both phosphorus and nitrogen concentrations.

Example: Total Phosphorus

Lake Ontario Waterfront nearshore monitoring water quality patterns
Figure 9: Examples of total phosphorus concentrations from the Transects by Duffins and Carruthers Creeks in spring 2007, 2008, and 2010.

*Red arrow denotes increasing concentration.

*There are a few exceptions to this general observation, as seen in 2007 – where Carruthers Creek is higher than Duffins Creek.

(ii) Concentrations in the west and east are similar at distances greater than 1 km from the shoreline (with some exceptions).

Example: Nitrate + Nitrite

Lake Ontario Waterfront nearshore monitoring water quality patterns
Figure 10: Examples of nitrate+nitrite concentrations from the Transects by Duffins and Carruthers Creeks in spring 2007, 2008, and 2010.

*Red arrow denotes increasing concentration

*Exceptions to this trend exist, as shown in spring 2009.

It is expected that the concentrations in the west will be higher than in the east because there are many local inputs, including Duffins Creek, Duffins Pollution Control Plant outfall, and a large storm sewer outfall. Concentrations in the western region are generally higher, but from a water quality perspective the use of term “higher” is in reference to levels observed in the east. From a classical water quality standpoint – the majority of the reported concentrations (and median values) are at levels referred to as oligotrophic.

Take Home Messages

  1. Concentrations in lake water decrease as you move from areas close to land (creeks, drains, etc.) towards open water.
  2. Top layers of water are different than bottom layers as you move away from the shore.
  3. Shoreside concentrations are elevated by storm drains and creek mouths.
  4. Generally, the west (near Duffins Creek) has higher shoreline concentrations than the east (near Carruthers Creek.)