Seasonal Variability

The forcing functions of seasonal variations in lake chemistry fall into two categories. First is the concentration of salts, gases, and organic matter by evaporation in the summer or ice formation in the

Figure 3 High-arctic Lake Hazen (left, 81.8° N) and low-arctic Toolik Lake (right, 68.38° N) showing the comparison of terrestrial vegetation in the catchment, leading to reduced inputs of organic matter from land into the high-arctic versus the low-arctic lake. (Photos: left - W Vincent; right - G Kling).

Land-water connections in the arctic

1 4 Freshwater loses ~20-30g Cm-2 yr-1

1 4 Freshwater loses ~20-30g Cm-2 yr-1

Figure 4 Regional and circumpolar carbon budgets are influenced strongly by lakes acting as conduits of CO2 and CH4 to the atmosphere. On a per area basis, the annual evasion of gases to the atmosphere roughly balances the net long-term C storage on land. Shallow, organic rich soils lying above permafrost allow the products of plant and soil respiration to accumulate, which are then moved into streams and lakes and returned to the atmosphere or exported to the sea.

Figure 4 Regional and circumpolar carbon budgets are influenced strongly by lakes acting as conduits of CO2 and CH4 to the atmosphere. On a per area basis, the annual evasion of gases to the atmosphere roughly balances the net long-term C storage on land. Shallow, organic rich soils lying above permafrost allow the products of plant and soil respiration to accumulate, which are then moved into streams and lakes and returned to the atmosphere or exported to the sea.

winter; although these processes occur worldwide where lake ice forms, the generally dry conditions and development of thick ice in the Arctic accentuate the magnitude of effects. As water freezes the crystals exclude salts and dissolved organic materials, and the chemical concentrations increase in the unfrozen water just below the ice surface. This cryo-concentra-tion can affect large volumes of surface water even in deep lakes, but the most dramatic impacts are in the many shallow lakes that freeze completely or nearly to the bottom. Once the ice melts, evaporative concentration in these shallow ponds may also alter water chemistry progressively through the ice-free season. In summer, flat coastal plain areas can have high evaporation and especially transpiration of water in wetlands surrounding thermokarst ponds, which can actually reverse the hydrological gradient so that water flows out of the ponds and onto land. Although this transfer does not change water chemistry directly, it can impact the chemical loading and budgets of lakes.

The second category of processes that change lake chemistry seasonally is the external input from snow-melt and summer storms. Little happens chemically during winter once the maximum ice thickness is established, but warming temperatures in spring cause snowmelt and result in the largest, periodic mass flow in the hydrological cycle. This spring snowmelt and associated runoff can completely flush smaller lakes with relatively dilute but often nutrient-rich water. Because soils are still mostly frozen at the time of snowmelt, runoff water is short-circuited from the normal terrestrial flow path where it is exposed to soil microbes and plant roots; thus it can be rich in nutrients or labile organics leached from the surface mat of vegetation. Therefore even in larger-volume lakes the runoff water can greatly stimulate bacterial activity, and can provide the majority of inorganic nutrients used by algae in the lake throughout the summer growing season. Snow-melt typically occurs when the deeper lakes are still ice-covered, and because the inflow water is close to °C the runoff enters the lake and floats just below the lower ice surface in close proximity to surface algae and bacteria, increasing the impact of this seasonal change in water chemistry on other aspects of ecosystem function.

Large hydrological excursions during summer storm events can similarly alter the chemistry of lakes, especially through inputs of N, P, and DOM. These alterations can persist for long periods of time (many weeks) depending on the volume and character of the inflow. One important distinction from snow-melt is that summer stormwaters have variable temperature and density and can produce overflows (as occur under ice), but more often this water forms an underflow and intrudes at depth in the lake. These intrusions may remain in thin layers but can also thicken and mix nutrients into both underlying and overlying waters. The specific placement of the intrusions relative to water column structure, organism distribution, and the availability of light for photosynthesis or photochemical reactions can determine the ultimate impact of the inflow on lake chemistry and biology. In fact, determining the dynamics of these events and their impacts on arctic lakes is a critical area of current and future research.

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