Lakes everywhere are intimately tied to inputs of materials and nutrients from land, but this terrestrial connection is particularly strong in the Arctic for organic matter in dissolved and particulate forms. In terms of C (carbon), N, and P, the dissolved organic matter (DOM) concentrations typically represent 70-90% of the total C, N, and P in lake water or entering the lakes. This terrestrial connection is so strong mainly because the primary production of terrestrial environments is much greater than that found in aquatic habitats; as an example, Toolik Lake in northern Alaska is a typical deep-water lake (68.38° N, 1.5 km2 surface area, 25 m maximum depth) and has an annual primary productivity of —10 g C m~2, whereas in the surrounding tundra the productivity is not only 10 times that amount but the storage of organic C in soils is 1000 times that amount (—10 kg C m~2 in the upper thawed layer alone). There is, however, a distinct gradient of terrestrial influence on lakes in the Arctic, because as one travels further north or to higher elevation the climate is colder, the terrestrial vegetation becomes scarce, the organic matter in soils is much reduced, and there is less precipitation in the form of rain to flush DOM from soils into surface waters (Figure 3). The endpoint of this gradient is found in antarctic lakes, and consists of a full reversal of the pattern -there, in some areas the organic matter produced in lakes is blown onto land and may constitute the dominant C input to soils.
The strong terrestrial-aquatic linkage affecting arctic lakes is also illustrated by the case of landscapelevel carbon budgets. Lakes with ice-free periods in the Arctic, as in most other areas of the world, tend to release more CO2 to the atmosphere than they take up in photosynthesis on an annual basis (net C sources). This supersaturation with respect to atmospheric CO2 is driven by the export from land of CO2 and of organic carbon that is respired to CO2 and methane in the lakes. What is different about this landscape integration of terrestrial and aquatic ecosystems in the far north is that the surface area of water is so great that the loss of C back to the atmosphere is a significant term in the carbon balance of the entire Arctic (—20% of the net C exchange with the atmosphere; Figure 4).
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