• grahamdraby

The features of the lake that you can't see.

By Dr. Nolan Pearce, post-doctoral researcher in the Trent Aquatic Research Program (Xenopolous/Frost Lab)

The cove, the rock, the hole, the point; all popular names for fishing spots I have heard over the years. But what about the features of the lake you can't see?

This past summer, we spent a day mapping the water quality of Stoney Lake to better understand the 3D distribution of water quality variables across the lake. Unlike landscapes that can be imaged from above, mapping underwater features is inherently difficult. To map features like water quality that differ across the lake as well as with depth, we needed to rely on a geostatistical method called spatial interpolation. Spatial interpolation uses mathematical relationships and discrete measurements of the variable you would like to map to estimate its value at locations where we haven't actually sampled the water.

(Above) Dissolved oxygen (mg/L) from the surface (0 m) to the bottom of the lake. The animation shows that there are hypoxic zones near the bottom in the deepest parts of Lower Stoney, and in the depths of Clear Lake, but that the deep basin of Upper Stoney (~100 feet) maintains good oxygen saturation right to the bottom. The more real measurements you make the better, but there is a trade-off between sampling effort and interpolation accuracy. To map water quality, we went to 22 locations across Stoney Lake and used a vertical profiler to take measurements of chlorophyll-a (algae), dissolved oxygen, temperature, and many other variables every 5 cm from the surface of the water to the bottom of the lake. Back in the office, these data were compiled and imported into a geographic information system (GIS) to produce a three-dimensional map of water quality in Stoney Lake using spatial interpolation. As you can see by in the animations of chlorophyll-a and dissolved oxygen, there is a lot of variation in water quality across the system. We have more work to do in mapping the three-dimensional distribution of these variables out more frequently throughout the year. When complete, these maps will be used to help us understand the potential impacts of changing water quality on fish movement and behaviour.

(Above) An animation showing the chlorophyll-a concentration in Stoney Lake, from the surface to the lake bottom. Chlorophyll-a serves as a rough index of the amount of algae in the water.

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