We tested the hypotheses that benthic macroinvertebrates, especially chironomid larvae in the Tribe Chironomini, from small oligotrophic arctic lakes are only weakly linked to pelagic food sources, and that they are trophically linked to biogenic methane. All offshore benthic macroinvertebrates sampled from 20 lakes were depleted in 13 C compared with nearshore benthic macroinvertebrates. Nearshore macroinvertebrates had ␦ 13 C consistent with feeding on periphyton and/or terrestrial detritus. However, ␦ 13 C of offshore Chironomini, the dominant macroinvertebrate group, and oligochaetes, was more depleted than that of other offshore macroinvertebrates to a degree that could only be explained by a diet that included carbon derived from biogenic methane. Seston ␦ 13 C showed a small shift toward greater depletion in deeper lakes, but ␦ 13 C of Chironomini, oligochaetes, and predatory chironomids varied as a function of dissolved oxygen concentration (DO) at the sediment-water interface, which was generally lower in shallow lakes that were deep enough to stratify. In lakes with lower DO in bottom waters, the importance of methane-based carbon to Chironomini was greater. Experimental 15 N enrichment of phytoplankton in small-and mid-sized arctic lakes resulted in limited 15 N enrichment of most benthic macroinvertebrates, indicating that their productivity was not closely tied to pelagic production. It is likely that aerobic benthic metabolism of dissolved organic carbon (DOC) also is an important energy source for benthic macroinvertebrates in sediments because the link between benthic macroinvertebrate production and contemporary pelagic primary production was weak regardless of dependence on biogenic methane.

1. The distribution of species is affected by many factors operating at a variety of temporal and spatial scales in a heterogeneous landscape. In lakes, fish communities are dynamic, influenced by landscape-level factors that control colonisation and extinction. 2. We used classification and regression tree (CART) analyses to quantify the importance of landscape-level factors in determining the distribution of fish species in 168 arctic Alaskan lakes. Factors including lake size, depth, outflow gradient, distance to other lakes, lake order, altitude, river drainage and age of glacial surface were analysed. These factors could affect either access of fish to a lake (colonisation variables), or their survival in a lake that already had been colonised (extinction variables). 3. The presence of a species was predicted accurately in 78.4% ± 10.5% (mean ± SD) of cases, and absence in 75.0% ± 6.1% of cases. The relative importance of extinction versus colonisation variables varied with species. Extinction variables were most important for lake trout (Salvelinus namaycush) and slimy sculpin (Cottus cognatus), a mixture of extinction and colonisation variables was important for arctic char (Salvelinus alpinus), and colonisation variables were most important for arctic grayling (Thymallus arcticus) and round whitefish (Prosopium cylindraceum). 4. Ecological differences among species account for much of the difference in relative importance of colonisation versus extinction variables. In addition, stream piracy events have occurred over geologic time scales, which have resulted in lakes that are currently inaccessible but support relict fish populations. 5. Climate warming, currently occurring in the arctic, is likely to alter further the stream network, which could have dramatic effects on fish distributions by affecting access to isolated lakes or isolating lakes that are currently accessible.

No abstract

We examined the effect of large (potentially piscivorous) lake trout (Salvelinus namaycush) on the dietary habits of small lake trout in an arctic lake. We hypothesized that large lake trout constrain the foraging of small lake trout, thus, in the absence of large lake trout, small lake trout will shift their diet from littoral to more abundant prey offshore. We tested this hypothesis using samples from a removal experiment where all lake trout large enough to be susceptible to gill nets were removed from a small arctic Alaskan lake during 1988-1989. We examined size at age and conducted stomach content and stable isotope analyses of lake trout collected during removal (1988), early recovery (1990), and late recovery (1999) portions of the study. Lake trout grew more quickly following removal. All lake trout fed on a variety of prey, but stomach analyses provided little information on segregation of diet between size classes. d 15 N and d 13 C analyses showed that small lake trout shifted their diet after large lake trout were removed, apparently toward more reliance on offshore zooplankton, which also implies a habitat shift to open water. Thus, we conclude that large lake trout are restricting the dietary habits of small lake trout, a restriction that was removed in an exploited population.

The movement of sediment between the lake bottom and water column of shallow lakes can be sizeable due to the large potential for resuspension in these systems. Resuspended sediments have been shown to alter phytoplankton community composition and elevate water column production and nutrient concentrations. We measured the summer sedimentation rates of two lakes in 2003 and six lakes in 2004. All lakes were shallow and located in the Alaskan Arctic. In 2004, turbidity, light attenuation, total sediment:chlorophyll a mass in the sediment traps, and thermal stratification were also measured in each of the lakes. The sediment:chlorophyll a mass was much greater than if the sediment was derived from phytoplankton production in all of the lakes, indicating that the source of the sedimenting material was resuspension and allochthonous inputs. Consistent with these findings, the temporal variation in sedimentation rate was synchronous between most lakes, and sedimentation rate was positively related to wind speed and rainfall suggesting that sedimentation rate was strongly influenced by landscape-scale factors (e.g., wind and rain events). Two of the lakes are located on deposits of loess that accumulated during past glacial periods. These two lakes had sedimentation rates that were significantly greater and more variable than any of the other lakes in the study, as well as high turbidity and light attenuation. Our results indicate that sedimentation in these shallow arctic lakes is supported primarily by allochthonous inputs and resuspension and that landscape-scale factors (e.g., weather and geology) impact on the transport of materials between the lake bottom and water column.