Arctic Lakes Research Overview
Initial work at the Arctic LTER focused on Toolik Lake, however, we currently monitor 7 additional lakes for primary productivity, temperature, light penetration, water chemistry, and species composition. We describe Toolik Lake in detail because it is typical of this region. Toolik Lake (150 ha) is a deep (max = 25 m; mean = 7 m) kettle lake. It is oligotrophic (annual production of 12 g C m-2), stratifies with surface temperatures up to 18°C, and is ice-free from July through September.
The surface water temperature reaches 18°C but the volume-weighted summer temperature is 10-14°C (Miller et al. 1986); the lake is dimictic despite its northern location. Ice cover, which may reach 1.4 m thick, forms in early October. The lake first stratifies in July when the thermocline is located at 5 m. By early August, thermocline depth is at 7 m and the lake mixes in late August. The lake is fairly clear with summer Secchi disk readings around 6-7 m.
Tussock and upland heath tundra, and an occasional balsam poplar, dominate the vegetation-covered hills of the Toolik Lake watershed. The Toolik watershed covers 65 km2 and the major inlet drains 75% of the watershed including a series of 12 lakes. A secondary inlet drains another 7% of the watershed and several ephemeral streams account for the remainder of the inflow. The volume of the inflow is enough to flush the lake about once each year (Hobbie et al. 1983); much of the flow comes from snowmelt in mid- to late-May to early July. Ice-out in the lake occurs from mid to late June. The lake level fluctuates by as much as 59 cm, is generally highest at the time of major inflow, and declines throughout the summer.
The chemistry of Toolik Lake is typical of oligotrophic lakes. Oxygen is generally near saturation even during summer stratification and during the long arctic winter. The alkalinity is 0.2-0.4 meq l-1 and summer pH is around 7.6 but winter pH's may be lower (Whalen and Cornwell 1985). Plant nutrients are extremely dilute because of the low annual loadings of 290 mmol N m-2 total dissolved nitrogen and 4.64 mmol P m-2 total phosphorus. Average lake concentrations of nitrogen and phosphorus are 0.4 mM NO3-N, 0.2 mM NH4-N, and 0.2 mM total phosphorus (Whalen and Cornwell 1985).
The low levels of plant nutrients lead to low levels of phytoplankton biomass and productivity. Photic zone chlorophyll averages around 2 ug l-1 and phytoplankton primary productivity averages about 1 mol C m-2 yr-1 (Miller et al. 1986). The phytoplankton in Toolik Lake are sparse and small, around 2 x 106 cells l-1 and generally less than 6 mm in diameter. Most of the algae are flagellates belonging to the Chrysophyceae and Cryptophyceae. There are some planktonic centric diatoms (Cyclotella compta and C. bodanica) which are also quite small (Miller et al. 1986). Bacteria numbers, commonly 1.5 x 106 -1 (Hobbie et al. 1983), are close to the average for natural waters.
The zooplankton densities in Toolik Lake are also quite low (O'Brien et al. 1979). The most common species are Bosmina longirostris, Daphnia longiremis, Diaptomus pribilofensis and Cyclops scutifer. The Bosmina occasionally reach 10 per liter but are commonly 1-5 per liter as are the other three species. An invertebrate planktivore, Heterocope septentrionalis, is a very important predator on these herbivorous species but is rarely more abundant than 0.3 per liter. Surprisingly, rotifers are very rare and are of no quantitative significance in Toolik Lake.
The benthic environment consists of soft sediments (75%) and rocks (25%, around the shoreline and on the reefs). In shallower areas, the soft sediments may be covered with patchy beds of diminutive macrophytes (mosses and Nitella). The benthic invertebrate fauna is dominated by chironomids including at least 27 species spread among 21 genera, and by molluscs (Hershey 1985b). These include two clams (Pisidium and Sphaerium) and four snails (Lymnaea elodes, Valvata lewisi, Gyraulus sp., and Physa sp.). Larval caddisflies and oligochaetes also occur but are quite sparse.
Five species of fishes are present in Toolik Lake. Juvenile lake trout (Salvelinus namaycush) and round whitefish (Prosopium cylindraceum) may feed on zooplankton, but adults feed on benthos or other fish in the case of lake trout (O'Brien et al. 1979). Arctic grayling (Thymallus arcticus) are relatively common and feed on zooplankton when small but become fairly omnivorous when longer than 15 cm. The slimy sculpin (Cottus cognatus) is quite common in the rocky littoral and soft-sediment zones (McDonald et al. 1982) and feeds entirely on benthic invertebrates (Cuker 1981, Hershey 1985, Hershey and McDonald 1985). The adult burbot (Lota lota) have been observed to feed on fish, including sculpin.
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