Lakes_on_the_shield

Jeff Green | Nov 24, 2005

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Did you know?Lakes on the shield receive less phosphorus

Commentary by Gray Merriam

Lakes on the limestone areas in the south of our region are allowed to send in samples to the Lake Partners Program every summer month for phosphorus analysis. Lakes on the Precambrian Shield get only a single sample analyzed each summer. This difference is because the granitic bedrock of the Shield releases much less phosphorus than the limestone to the south. So, if all else is equal, the seasonal development of eutrophication (or over-feeding causing excess algal growth) of Shield lakes is likely to be less severe.

But all else may not be equal. The watersheds, or drainage basins, of lakes on the Shield are more likely to be fully covered in forest, even second growth, and runoff from forest does not carry much phosphorus into the lakes. Cleared or agricultural land, more common in the watersheds of lakes on the limestone, produces runoff into the lakes that contains much more phosphorus.

On the other hand, the septic systems of dwellings and cottages around the lakes can cancel out any advantages from forest cover. Phosphorus input into lakes from septic systems, especially old or poorly maintained systems, is usually the single largest source of phosphorus flowing into lakes.

Phosphorus also enters in the rainfall, and some lakes with a small watershed area but a big lake surface area may get their second largest contribution of phosphorus from the rain.

Because the largest source of phosphorus getting into our lakes is usually flowing in our plumbing at some point, it makes sense that we try to control phosphorus before worrying about other nutrients such as nitrogen.