Nutrients

The total amounts of major nutrients (N and P) in lakes are usually set by the surrounding geology, rates of weathering, and geochemical reactions, but the observed concentrations of inorganic nutrients dissolved in lakes tend to be strongly controlled by biological processes. Arctic systems follow this generality as well, but what is special in these lakes is that the supply rates, geochemistry and biological controls are often at the extremes of typical behavior. Because of the isolation of mineral soils by permafrost there are low amounts of P weathered from rocks, and because of low temperatures and shallow soil depths the generation of N and P through decomposition of organic matter on land is also low. Add to this the fact that tundra plants are critically limited by both N and P, and the result is a low release rate of nutrients from land to surface waters. Consider that an average boreal forest may retain and recycle 60-70% of the N needed to support annual primary production, while in low-arctic tundra this retention is likely much greater than 90%. Within at least some arctic lakes, a further governor is the geochemistry of sediments exceptionally rich in iron and manganese, which operates to bind P and both scavenge it from the overlying water and prevent its return. The net consequences of these extremes are that inorganic nutrient concentrations are extremely low in arctic (and alpine tundra) lakes, and one implication is that arctic lake primary production is more often co-limited by both N and P than is found in other regions.

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