The effects of climate variability on Pacific salmon abundance are uncertain because historical records are short and are complicated by commercial harvesting and habitat alteration. We use lake sediment records of delta15N and biological indicators to reconstruct sockeye salmon abundance in the Bristol Bay and Kodiak Island regions of Alaska over the past 300 years. Marked shifts in populations occurred over decades during this period, and some pronounced changes appear to be related to climatic change. Variations in salmon returns due to climate or harvesting can have strong impacts on sockeye nursery lake productivity in systems where adult salmon carcasses are important nutrient sources.

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Ponds are often identified by their small size and shallow depths, but the lack of a universal evidence-based definition hampers science and weakens legal protection. Here, we compile existing pond definitions, compare ecosystem metrics (e.g., metabolism, nutrient concentrations, and gas fluxes) among ponds, wetlands, and lakes, and propose an evidence-based pond definition. Compiled definitions often mentioned surface area and depth, but were largely qualitative and variable. Government legislation rarely defined ponds, despite commonly using the term. Ponds, as defined in published studies, varied in origin and hydroperiod and were often distinct from lakes and wetlands in water chemistry. We also compared how ecosystem metrics related to three variables often seen in waterbody definitions: waterbody size, maximum depth, and emergent vegetation cover. Most ecosystem metrics (e.g., water chemistry, gas fluxes, and metabolism) exhibited nonlinear relationships with these variables, with average threshold changes at 3.7 ± 1.8 ha (median: 1.5 ha) in surface area, 5.8 ± 2.5 m (median: 5.2 m) in depth, and 13.4 ± 6.3% (median: 8.2%) emergent vegetation cover. We use this evidence and prior definitions to define ponds as waterbodies that are small (< 5 ha), shallow (< 5 m), with < 30% emergent vegetation and we highlight areas for further study near these boundaries. This definition will inform the science, policy, and management of globally abundant and ecologically significant pond ecosystems.

Fossil cladoceran remains preserved in surface sediment samples from 44 oligotrophic lakes in south-central Ontario were examined to evaluate the relationships between species assemblages and measured environmental variables. Differences in cladoceran assemblages were related to physical and chemical variables using multivariate techniques. Redundancy Analysis (RDA) identified five environmental variables as significantly influencing assemblage composition: sulphate (SO 4 2-), calcium (Ca 2+ ), pH, maximum lake depth (Z max ) and dissolved organic carbon (DOC). There was a distinct separation of lakes and taxa along the ion gradient based on SO 4 , Ca and pH. Additionally, cladoceran communities in coloured, shallow lakes had relatively higher abundances of littoral chydorid species and the pelagic taxa Holopedium spp., and the Daphnia pulex complex. Deep, clear lakes had relatively higher abundances of other pelagic taxa. Predation by fish (measured as presence-absence) and Chaoborus (measured as density) were less significant than some of the physicochemical variables in influencing cladoceran assemblage structure. However, this could be due to the limited resolution of the predation data that was available at the time of this study. The distribution of cladocerans in the surface sediment, and their relation to these important environmental variables, suggests that there is considerable potential for the use of sedimentary cladoceran remains as environmental indicators in south-central Ontario lakes.

Climate variations exert rapid and strong control on the hydrology of shallow lake-rich subarctic landscapes, but knowledge of the associated effects on limnological conditions remains limited. Based on analysis of water isotope compositions and water chemistry at 56 lakes across Old Crow Flats (Yukon), a large thermokarst landscape, we assess if differences in source water inputs (snowmelt versus rainfall) affect limnological conditions during the ice-free season of 2007 and explore influences of catchment features. Results demonstrate that lakes with snowmelt-dominated source waters, situated in catchments that support tall shrub and woodland vegetation, possess significantly higher (p < 0.05) nutrient (N, P, SiO2) and dissolved organic carbon concentrations than lakes with rainfall-dominated source waters. Conversely, rainfall-dominated lakes, located in catchments dominated by dwarf shrubs and sparse vegetation, have significantly higher concentrations of major ions (Mg2+, Na+, SO42−) and pH. These limnological differences persisted throughout the ice-free season. We suggest that interaction of snowmelt with organic-rich detritus raises nutrient concentrations in snowmelt-dominated lakes and that evaporative-concentration, shoreline erosion and possibly rainfall runoff are processes that raise the ionic content of lakes with rainfall-dominated source waters. Knowledge of these relations improves the ability to anticipate limnological responses to ongoing and future climate and hydrological change in Arctic and subarctic regions.

We examine the role of local environmental and spatial factors in explaining variation in the composition of cladoceran assemblages from surface sediments within a set of 50 lakes spanning a broad southwest to northeast transect across the central Canadian Arctic treeline region from Yellowknife (Northwest Territories) to the northern boundary of the Thelon Game Sanctuary (Nunavut Territory). Within each lake, the cladoceran fauna was identified based on the subfossil exoskeletal remains preserved in recently deposited lake sediments. Physical and chemical limnological data were measured in August of 1996 and 1998. Spatial data were generated based on latitude and longitude using Principal Coordinates of Neighbors Matrices analysis (PCNM). The relationships between cladocerans and the measured environmental and spatial variables were examined using both unconstrained (Principal Components Analysis, PCA) and constrained (Redundancy Analysis, RDA) ordination techniques. Variance partitioning, based on partial RDAs, was used to identify the relative importance of significant environmental and spatial explanatory variables. Three environmental variables were identified as significantly influencing cladoceran community structure: surface water temperature, dissolved organic carbon (DOC), and total phosphorus (TP). Five PCNM-generated spatial variables were also significant in explaining cladoceran distributions. Variance partitioning attributed 14% of the variance in the distribution of Cladocera to spatial factors, an additional 10% to spatially-structured environmental variables, and 8% to environmental factors that were not spatially-structured. Within the central Canadian Arctic treeline region, spatial and other environmental processes had an important influence on the distribution of cladoceran communities. The strong influence of spatial factors was related to the large ecoclimatic gradient across treeline. The distribution patterns of cladocerans suggest that they have potential for use in paleoenvironmental assessments of northern ecosystems, a region of considerable interest for environmental change research

The remains of cladocerans were examined from the surface sediments of 51 freshwater sites along a northsouth transect spanning Alaska. We identified 27 cladoceran taxa from the sediments, consisting primarily of littoral chydorid species. Variations in cladoceran assemblages were related to measured physical and chemical variables using multivariate techniques. Redundancy analysis (RDA) indicated that lake depth, total phosphorus (TP), and altitude all had a significant influence in determining the composition of cladoceran assemblages. Cladoceran communities in tundra and forest-tundra lakes, which were relatively shallow and nutrient-poor, had relatively low abundances of pelagic Cladocera, and were primarily composed of several littoral chydorid species. Among pelagic cladoceran species, there was a distinct shift in dominance from the Bosminidae in lakes in the southern boreal forest region to Daphniidae in lakes in the northern boreal forest. Daphnia dominated lakes had significantly higher total phosphorus, specific conductivity, and calcium concentrations than lakes dominated by Eubosmina. Overall, the relative importance of physical and chemical factors in structuring cladocerans is similar to other previously studied regions, and suggests the Cladocera may be useful as ecological and paleoenvironmental indicators in this region.

Abstract:Recent studies using remote sensing analysis of lake-rich thermokarst landscapes have documented evidence of declining lake surface area in response to recent warming. However, images alone cannot identify whether these declines are due to increasing frequency of lake drainage events associated with accelerated thermokarst activity or to increasing evaporation in response to longer ice-free season duration. Here, we explore the potential of combining aerial photograph time series with paleolimnological analyses to track changes in hydrological conditions of a thermokarst lake in the Old Crow Flats (OCF), Canada, and to identify their causes. Images show that the water level in lake OCF 48 declined markedly sometime between 1972 and 2001. In a sediment core from OCF 48, complacent stratigraphic profiles of several physical, geochemical, and biological parameters from ¾1874-1967 indicate hydro-limnological conditions were relatively stable. From ¾1967-1989, declines in organic matter content, organic carbon isotope values, and pigment concentrations are interpreted to reflect an increase in supply of minerogenic sediment, and subsequent decline in aquatic productivity, caused by increased thermo-erosion of shoreline soils. Lake expansion was likely caused by increased summer rainfall, as recorded by increased cellulose-inferred lake-water oxygen isotope compositions. Stratigraphic trends defining the lake expansion phase terminated at ¾1989, which likely marks the year when the lake drained. Above-average precipitation during the previous year probably raised the lake level and promoted further thermo-erosion of the shoreline soils that caused the lake to drain. These are meteorological conditions that have led to other recent lake-drainage events in the OCF. Thus, the decline in lake level, evident in the aerial photograph from 2001, is unlikely to have been caused by evaporation, but rather is a remnant of a drainage event that took place more than a decade earlier. After drainage, the lake began to refill, and most paleolimnological parameters approach levels that are similar to those during the stable phase. These findings indicate that combined use of aerial images and paleolimnological methods offers much promise for identifying the hydrological consequences of recent climatic variations on thermokarst lakes.