Lake and tributary monitoring continued in 2014 with an expanded scope and detective work. Diagnostic water quality data from the open water season are still being analyzed by both the ALMS LakeWatch team and by our own tributary samplers and all should be available before the end of March 2015. While we wait, four decades of data in the Alberta Environment water quality database help to indicate the state of the lake and impact of the surrounding watershed and its progressive urbanization.
Is the lake getting better or worse over time? Here’s what we know:
1. Does the lake appear to be more or less transparent than it used to be?
Simple Secchi disk observations tell us this:
The detectability of a submerged Secchi disk has varied erratically from as shallow as 2 meters (m) to 8 m even in a single year. That is a measure of the quantity of suspended light-scattering particles in the water column. More algae, or fine minerals from shoreline erosion, cause a shallow reading while fewer particles allow the disk to still be seen at greater depths.
2. Is the lake more or less green than it used to be?
Here are results from lab analysis of the green Chlorophyll-a pigment found in algae and other plant material:
The Chlorophyll-a molecule found in green plants helps to convert CO2 to O2.
Over the 30-year period shown, the chlorophyll-a data seem to have declined although in any year values can change significantly through the open-water season. Other factors affect algae growth including available plant nutrients in the water, sunlight, and water temperature.
3. Are the Secchi disk measurements affected by algae in Sylvan Lake?
Here is the graph that relates the two variables:
In general, higher water clarity (larger Secchi disk depth) is found when Chlorophyll-a is low. However, because algae tend to float near the lake surface where light can penetrate we should not expect that the two measurements will be highly correlated.
4. Do nutrient concentrations that are transported into Sylvan Lake in tributaries cause algal blooms?
The simple answer is that nitrogen (N), phosphorus (P), potassium (K) and other elements in Sylvan Lake’s water are essential for growth and are absorbed by plants. Many studies of lakes have shown that phosphorus can be a limiting growth factor. We use the standard Total Phosphorus lab analyses as one convenient indicator of the potential for algal blooms. The data collected since 1982 show that the TP concentration has varied around a mean value of 21 micrograms per litre with a standard deviation of about 5 micrograms/L. That means 95% of the data fall between about 11 and 31 micrograms/L during three decades of water quality testing. The important TP concentration (and the associated Dissolved Phosphorus concentration, the active form for plant uptake) has not changed much, likely because of a lucky balance between annual lake loading in runoff and annual loss to the lake-bottom sediments as algae, cyanobacteria and plants die.
5. Are the green Chlorophyll-a and Total Phosphorus nutrient concentrations related?
Let’s test it with a scatter plot of one variable against the other. Here is the graph of Chlorophyll-a plotted against TP:
The correlation between the two variables is poor. For example over the years of observation, at TP of 20 micrograms/L , chlorophyll-a has been found to vary from almost zero to 10 micrograms/L. And, at a reference value of 5 ug/L of chlorophyll-a, TP has varied between 15 and 30 ug/L. The statistical distributions of each variable are broad. Without measurements of all the other variables that affect chlorophyll-a we can only assume that algae growth and phosphorus nutrient concentrations should be related.
Qualitatively, the data do indicate that TP has rarely exceeded the Alberta Surface Water Quality Guideline (ASWQG) of 35 ug/L that is recommended for Sylvan Lake. The empirical evidence continues to support a lack of chronic algal/cyanobacterial blooms in Sylvan Lake. A systematic tributary monitoring program that measures nutrient transport into the lake is essential to detect any threat of increased lake loading by plant fertilizers.
6. Are there any other indicators of longer term water quality changes?
This graph shows that other common ions in Sylvan Lake have been increasing over time:
Inflowing surface and groundwater contains dissolved minerals and agricultural and urban contaminants. Continuous evaporation of lake water slowly concentrates those ions which are not lost to lake sediments by adsorption, ion exchange, or precipitation. The first graph shows that sodium, potassium, and chloride have increased since 1983. The second figure expands the concentration scale to show that chloride ion (the bottom line in the left hand graph) has increased almost three times since 1982. The most likely sources of chloride in the watershed are road salt and domestic water softener applications. Increasing chloride, while dilute, is a direct indicator of human impact on the lake.
7. How can we understand all this information? What can we do to protect Sylvan Lake?
It’s easy. You just have to know the Two Laws of the Watershed:
1. Water flows downhill.
So you can easily tell where water is coming from, and going to.
2. Stuff from the land ends up in the lake.
Sylvan Lake is like a slightly leaky bathtub. More natural and urban wastes enter the tub than leave through the drain.
Protection of Sylvan Lake requires that all contaminated surface runoff and stormwater be collected and cleaned up before it is discharged into the lake, or diverted out of the watershed in municipal stormwater sewers. New urban development that covers land with buildings and roads prevents water infiltration that presently replenishes groundwater supplies and increases the volume and rate of runoff. Diversion of snow melt and precipitation runoff that flows downhill will eventually affect the watershed’s water balance.
Understand the Two Laws, then common sense will let you create your own lake protection solutions.