Lake trophic structure is a result of complex interactions among consumers and their resources. Algal productivity is channeled to higher trophic levels, which in turn structure planktonic consumer food webs and control algal biomass through cascading trophic interactions (Shapiro 1980, Carpenter et al. 1985. Top predators also affect benthic food web structure (Bronmark et al. 1992). However, cascading effects in benthic food webs are confounded by habitat complexity in the littoral zone and accumulation of organic matter in lake sediments (Crowder and Cooper 1982, Gilinsky 1984, Hershey 1985, 1992, Carpenter and Lodge 1986). Thus, benthic-pelagic coupling is not well integrated into the current understanding of lake trophic dynamics. However, because arctic lake food webs are dominated by benthic-pelagic interactions, a model that focuses on planktonic food webs does not describe them well. Lake ecosystems are embedded within the landscape but have rarely been studied in that context. One of the best contexts to examine this question is the Arctic, where geomorphic constraints on fish distribution are especially apparent because the fish community is small and geographically constrained. The benthic and pelagic invertebrate communities are also not species rich, although their trophic diversity is similar to that found in temperate lakes. The arctic landscape therefore provides a unique opportunity to resolve the relationships between geomorphic setting, fish distribution, and lake trophic structure. A unifying conceptual model of the arctic landscape is the geomorphic-trophic hypothesis (Figure 1). This model integrates lake processes into their geomorphic setting by illustrating how the stream network constrains or channels the dispersal of fish and links fish dispersal with fish control of lake food webs; because landscape criteria control the distribution of fishes, and fish control lake trophic structure, the landscape indirectly controls lake trophic structure. Similar principles have been applied to fish distributions in other systems (see Magnuson et al. 1998), and landscape factors are also known to affect hydrologically driven chemical parameters, which may in turn have implications for food web structure (Kratz et al. 1991(Kratz et al. , 1997). According to the geomorphic-trophic hypothesis for arctic lake food webs, extant and paleological landscape BECAUSE LANDSCAPE CRITERIA CONTROL THE DISTRIBUTION OF FISHES, AND FISH CONTROL LAKE TROPHIC STRUCTURE, THE LANDSCAPE INDIRECTLY CONTROLS LAKE TROPHIC STRUCTUREconstraints provide a template resulting in lakes with six different types of fish communities (Figure 1): lakes dominated by lake trout (Salvelinus namaycush) but also usually containing grayling (Thymallus arcticus), sculpin (Cottus cognatus), burbot (Lota Iota), round whitefish (Prosopium cylindraceum), and, rarely, arctic char (Salvelinus alpinus); lakes containing arctic char and sculpin; lakes with grayling and sculpin; lakes with grayling only throughout the year; lakes with grayling in the...

We examined how physical and chemical factors and fish populations affected the presence and density of zooplankton species in 104 Alaskan arctic lakes. Five fish species and nine zooplankters were sampled from the lakes. Lake depth was related to the distribution of zooplankton in that smaller zooplankton species were found in deep lakes and large species in shallow lakes. Neither ionic strength nor chlorophyll was a major factor in zooplankton presence or absence. When fish were present, few, if any, very large zooplankton occurred together. However, the relationship between the presence and density of smaller-sized zooplankton species and the presence of specific species of fish was complex and did not fit anticipated patterns. The diversity of zooplankton in these arctic lakes was not related to chemistry or fish distribution, but species richness increased with increasing lake area and depth. For a few zooplankters (notably Holopedium gibberum), the fraction of lakes containing the zooplankter varied by region.The study of freshwater zooplankton and zooplankton communities has a long and storied history (Forbes 1887).

Glacial thrust systems composed of blocks of drift and bedrock, associated with hummocky stagnation moraine along the margin of the Rainy lobe of the Laurentide Ice Sheet in Minnesota, are used in conjunction with paleoecological studies to constrain a numerical model of the ice-marginal thermal regime. Subglacial meltwater production in the thawed-bed zone was at least two orders of magnitude greater than the amount that could refreeze to the base of the glacier near the margin. The excess water recharged a thick subglacial aquifer, and thrust-system development was enhanced by the presence of a frozen toe and high pore-water pressures beneath the outer 2 km the glacier. The pore-water pressure required for thrusting is calculated from overburden pressures and basal shear stresses determined by numerical modeling. The heat generated by flow of water through the subglacial aquifer substantially affects the ice-marginal thermal regime, making a steady-state frozen toe 1 .O-2.0km in width unstable. Thrusting apparently occurred during multiple oscillations, or surges, when the ice was advancing over permafrost.Les systhmes de chevauchement glaciaire, composCs de blocs de drift et du substratum rocheux, associCs aux moraines de stagnation mamelonnCes le long du lobe Rainy de l'inlandsis laurentidien dans le Minnesota, sont utilists de pair avec les Ctudes palCoCcologiques pour imposer des bornes numCriques dans un modkle de rCgime thermique de bordure de glacier. La production d'eau de fonte sous-glaciaire dans la zone OD se trouve la couche fondante Ctait au moins de deux ordres de magnitude plus grande que la masse d'eau qui pouvait recongeler a la base du glacier prks de la marge. Le surplus d'eau rechargait un Cpais aquif&re sous-glaciaire, et le dtveloppement du systkme de chevauchement Ctait facilitt par la presence d'une langue de glace et des pressions d'eau interstitielles ClevCes sous les deux premiers kilomktres du front du glacier. La pression d'eau interstitielle nCcessaire pour crCer le chevauchement est calculte a partir des pressions gkostatiques et des forces de cisaillement basales dBterminBes par la modtlisation nurnkrique. La chaleur engendrke par 1'Ccoulement de l'eau au travers l'aquifhre sous-glaciaire affecte substantiellement le rCgime thermique de la bordure du glacier, rendant instable une langue de glace rCguliere sur une largeur de 1,O-2,O km. Le chevauchement est apparu vraisemblablement durant les multiples oscillations, ou avancCes rapides, du glacier sur le pergCliso1.[Traduit par la revue]Can, J. Eanh Sci. 27, 849-862 (1990) Introduction Studies of glacial sediments and landforms provide valuable information on the physical nature and dynamics of Pleistocene ice ,sheets. The list of researchers who have used glacial geological mapping to interpret ice characteristics is long, and it is not the purpose of this paper to review the extensive literature. Within the midcontinent region of North America, the studies of Moran (1971), Moran (1974), andMathews (1974) are notable, as they est...

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