Beneath the Surface: Spring and the aquatic food web

The shift on the lake’s surface from ice to water happened so quickly that I had to blink to make sure I wasn’t imagining it.

Lake Windermere’s ice melted on April 13th this year.  Nine days later, when a Lake Windermere Ambassadors volunteer and I headed out to test the water, the lake was a balmy 8 C.  The shift on the lake’s surface from ice to water happened so quickly that I had to blink to make sure I wasn’t imagining it.

Change is happening beneath the lake surface right now as quickly as it is happening above.

Here in our lake, a process known as the “spring bloom” is underway.  Spring bloom is when the microorganisms at the bottom of the food chain, known as phytoplankton, start converting sun energy into the building blocks for life through the process of photosynthesis.

Why do these microscopic things in our lake matter? Consider this: photosynthesis is required to produce life, and phytoplankton account for half of all photosynthetic activity on Earth.

With the right combination of three things — light, temperature and nutrients — phytoplankton will begin to reproduce rapidly.  Below is a summary of how these conditions are optimal at this time of year.

Light: Given how cold the air is this time of year, many of us forget that we have the same day lengths on April 21st as we do on August 21st.  Although we might feel chilly, all of that sunlight is bringing a lot of energy to the earth’s surface. The long days, combined with the clear water at this time of year, means the sun is providing a burst of energy to the lake. Later in the spring, the amount of sun that can penetrate into the water is reduced because of increased plant growth and murkier waters caused by snowmelt runoff. This means that now is the time of year with the maximum amount of light penetrating into the water to support aquatic growth; and that’s quite a lot of light. Our lake is quite shallow so, in most parts of the lake, light penetrates all the way to the bottom.

Temperature: As I mentioned earlier, the temperature on the lake’s surface warmed up 8 C in a little over a week. The greater sun intensity had a lot to do with that, as did warmer air temperatures. The warmer conditions allow phytoplankton to reproduce faster.

Nutrients: Phytoplankton growth is also supported by the nutrients available in the water. In freshwater, the nutrient phosphorus is especially important for photosynthesis. It is usually only available in limited quantities, so is always in demand by aquatic primary producers. If the other nutrients like nitrogen and silica are abundant as more phosphorus is added to water, more primary production will occur.

The Lake Windermere Ambassadors sample Lake Windermere’s water to test the amount of phosphorus, as well as the temperature and clarity, in order to get a better understanding of the primary production processes in the lake, the overall condition of the lake, and to understand how these might be changing over time.

Kirsten Harma is the program co-ordinator for the Lake Windermere Ambassadors. She can be reached by phone at 250- 341-6898 or by email at info@lakeambassadors.ca.

Invermere Valley Echo

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