Water Quality 2013

It’s been three years now since the last in-depth analysis of the Bridge Lake water quality in 2010 (see below). This time I went out with Marge Sidney, a fishery biologist who works for the Ministry of Environment out of Kamloops. A beautiful calm and sunny day found us on the lake by 10 o’clock and we puttered in my small boat out to the testing site. For the next two hours we anchored just east of Rainbow Island in about 40m (120 feet) of water.

Marge had brought a top-of-the-line water tester the price of a small car, so in one go we could measure the water temperature, specific conductivity, dissolved oxygen levels, pH and the turbidity of the water. Attached to a long data cable, this tester was lowered into the lake one meter at a time and we recorded the readings. (You can download an Excel spreadsheet with all the measurements here).

The first surprise was a negative reading for the turbidity, i.e. the cloudiness of the water. Normally a sample of clear water would result in a reading of zero, with increasingly higher values for water with greater amounts of suspended solids (sediments, algae, plankton). This shows that Bridge Lake has very clear water, with little variation from the surface all the way to the lake floor.

The next surprise was the thickness of the thermocline. The temperature differences for the first 5 meters were only minimal, followed by a rapid cooling of the water during the next 5–6 meters (the thermocline), again followed by a slow, but steady cooling down to the bottom of the lake. These three distinct temperature layers lead to a phenomenon called stratification. The thermocline acts as a barrier between warm and cold water, and very little mixing occurs. A result of this stability is that as the summer wears on, there is less and less oxygen below the thermocline as this water never circulates to the surface and organisms in the water deplete the available oxygen.

Our measurements confirmed this, however with 4–6 mg/L dissolved oxygen in the lower layer there is still plenty for the fish in our lake to “breathe” happily. Eventually in fall (and in spring after ice-off) the surface water will be cool enough to break down the thermocline and the lake will “turn over” (a complete lake mixing will occur).

As with Chris in 2010, I learned a lot from Marge about the many factors that contribute to the quality of our water and the well-being of the fish living in it. Bridge Lake is still in excellent condition: Let’s keep it that way!

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Water Quality 2010     

On July 14th, 2010 Ministry of Environment Impact Assessment Biologist Chris Swan, her nine year old son Justin (getting a science lesson during the summer break) and our 12 year water testing veteran Karl Schmitz went out on the lake to do some in-depth evaluation of Bridge Lake's water quality. They anchored their boat for several hours at one of the lake's deepest spots, right in the middle between Grassy Island and Long Island.

      Temperature and Oxygen Levels

Chris Swan checks the water temperature and oxygen
levels at 2 meter intervals from the lake surface down
to the lake bottom in over 30 meter depth

On a long cable a probe was slowly lowered down to the bottom of the lake, recording at every meter various water quality data like pH, temperature, electrical conductivity, oxygen level and turbidity. A provisional look at the data suggested an excellent water quality: the oxygen levels stayed in the ‘green zone’ right down to the bottom of the lake. “The fish must be really happy in this lake”, remarked Chris.

While Karl recorded the data, Chris programmed, calibrated and tested a smaller device that ‘only’ measures temperature and oxygen levels (right). This device will remain here with Karl and he will go out every week for the rest of the season and repeat these tests, collecting valuable long term data for Bridge Lake. The results of Karl's weekly measurements can be found here.

Another contributing factor to the well-being of our stocked fish is the pH of the water. Both kokanee and rainbow trout prefer slightly alkaline water, so the measured value of about 8.5 is just what they like!

      Kokanee food

Justin holds the collector jar with the 50 micron screen, retaining all the zooplankton collected in the net

In the mean time, Justin had readied the funnel like net to collect zooplankton samples. Lowered to the lake bottom and slowly retrieved, the usually not noticeable zooplankton (tiny aquatic animals the size of a pinprick) remained highly concentrated in the collector jar attached to the bottom of the net. This sample (left) shows mainly red copepods and green-brown transparent daphnia (water fleas), both species a major part of the kokanee diet.

Once the final data of this outing are available, we will of course report them right here.

A great day on the lake, Thank you Chris (and Justin)!