Longer Ice-free Seasons Research Articles

Environmental and societal consequences of winter ice loss from lakes.

Climate change is reducing winter ice cover on lakes; yet, the full societal and environmental consequences of this ice loss are poorly understood. The socioeconomic implications of declining ice include diminished access to ice-based cultural activities, safety concerns in traversing ice, changes in fisheries, increases in shoreline erosion, and declines in water storage. Longer ice-free seasons allow more time and capacity for water to warm, threatening water quality and biodiversity. Food webs likely will reorganize, with constrained availability of ice-associated and cold-water niches, and ice loss will affect the nature, magnitude, and timing of greenhouse gas emissions. Examining these rapidly emerging changes will generate more-complete models of lake dynamics, and transdisciplinary collaborations will facilitate translation to effective management and sustainability.

Spatial and Temporal Evolution of Seasonal Sea Ice Extent of Hudson Strait, Canada, 1971–2018

The temporal and spatial variation in seasonal sea ice in Hudson Strait is examined using time series and spatial clustering analyses. For the period from 1971 to 2018, a time series of sea ice breakup and freeze-up dates and ice-free season length at twenty-four grid points were generated from sea ice charts derived from satellite and other data. These data were analyzed temporally and spatially. The temporal analyses indicated an unambiguous response to a warming climate with statistically significant earlier breakup dates, later freeze-up dates, and longer ice-free seasons, that were statistically linked to coincident regional surface air temperatures. The rate of change in freeze-up dates and ice-free season length was particularly strong in the early 2000s and less so in the 2010s. There was evidence that breakup date behaviour was not only coincident with regional temperatures but likely with temperature and ice conditions of the previous year. Later freeze-up dates were directly linked to earlier breakup dates using detrended time series. Spatial clustering analysis on the Hudson Strait gridded sea ice data revealed distinctive signatures for Ungava Bay, Frobisher Bay, and for grid points close to the shore and a clear linkage to the underlying circulation of Hudson Strait.

Climate warming accelerates somatic growth of an Arctic fish species in high-latitude lakes

High-latitude aquatic ecosystems are responding to rapid climate warming. A longer ice-free season with higher water temperatures may accelerate somatic growth in lake ectotherms, leading to widespread ecological implications. In fish, rising temperatures are expected to boost rates of food intake and conversion, and predictions based on empirical relationships between temperature and growth suggest a substantial increase in fish growth rates during the last decades. Fish abundance negatively affects growth by limiting food availability. This field study addresses the effects of climate warming on growth of a subarctic population of Arctic charr (Salvelinus alpinus (L.) over nearly 40 years. Juvenile growth of 680 individuals of Arctic charr, was reconstructed by sclerochronological analysis using sagittal otoliths sampled annually from the early 1980s to 2016. Statistical modelling revealed a positive effect of water temperature, and a negative effect of abundance on somatic growth in juvenile individuals. Temperature dependence in growth was significant for average and fast-growing individuals across all investigated age classes. These findings suggest that, as temperatures rise, somatic growth of Arctic charr will increase in high latitude lakes. Climate warming will thus influence cold water fish life history and size-structured interactions, with important consequences for their populations and ecosystems.

A MIXED METHOD TRADITIONAL KNOWLEDGE STUDY OF COASTAL FACTOR CORRELATION TO DECLINING SEA ICE IN RESOLUTE BAY, NUNAVUT

Canadian Arctic environments suffer severe consequences from global climate change. As sea surface and air temperatures rise, sea ice extent and thickness continue to decline, resulting in longer ice-free seasons. Due to the remote nature of Canadian Arctic communities, there are limited quantitative data available for calibration of predictions. However, local Inuit hunters, trappers, and elders hold a plethora of knowledge on the sea ice and coastal environment within their communities. This study seeks to answer the question, can coastal factors be correlated to declining sea ice through scientific and traditional knowledge methods. The present study examined (1) historical trends in, (2) lived experience and traditional knowledge of, and (3) correlation between, sea ice and the coastal environment in Resolute Bay, Nunavut Canada.

A 1300-year microfaunal record from the Beaufort Sea shelf indicates exceptional climate-related environmental changes over the last two centuries

The environments of Arctic Ocean nearshore areas experience high intra- and inter-annual variability, making it difficult to evaluate the impact of anthropogenic warming. However, a sediment record from the southern Canadian Beaufort Sea allowed us to reconstruct the impacts of climate and environmental changes over the last 1300 years along the northern Yukon coast, Canada. The coring site (PG2303; 69.513°N, 138.895°W; water depth 32 m) is located in the Herschel Basin, where high sedimentation rates (0.1–0.5 cm a−1) allowed analyses at sub-centennial to decadal resolutions. Benthic foraminiferal, ostracod, and tintinnid assemblages, as well as the stable isotope composition of the foraminifera Elphidium clavatum and Cassidulina reniforme were used as paleoclimatic and ecological indicators, while the age model was based on the combined radiometric data of 14C, 210Pb and 137Cs. From ca 700 to 1050 CE, our data suggest penetration of offshore shelf-break waters inferred by the dominance of C. reniforme followed by the relatively abundant Triloculina trihedra in the foraminiferal assemblages as both species are associated with stable saline conditions. Afterwards, the occurrence of ostracods Kotoracythere arctoborealis and Normanicythere leioderma suggests influx of Pacific-sourced waters until ca. 1150 CE. From ∼1150–1650 CE, persistent frigid waters, limited sediment supply, and low abundances of microfossils suggest cold conditions with pervasive annual sea-ice cover that may have restricted upwelling of oceanic waters. After ∼1800 CE, the co-occurrence of Tintinnopsis fimbriata and bacterial/complex organic carbon feeder foraminifera (Quinqueloculina stalkeri, Textularia earlandi and Stetsonia horvathi), suggest an increased influence of freshwater rich in particulate organic matter, which may be related to the spreading of the Mackenzie River plume and/or increased coastal permafrost erosion during longer ice-free seasons. Based on these proxy data, the shift at ∼1800 CE marks the onset of regional warming, which further intensified after ∼1955 CE, likely in response to the anthropogenic forcing.

Seasonal and interannual variations in the propagation of photosynthetically available radiation through the Arctic atmosphere

The Arctic atmosphere–surface system transmits visible light from the Sun to the ocean, determining the annual cycle of light available to microalgae. This light is referred to as photosynthetically available radiation (PAR). A known consequence of Arctic warming is the change at the atmosphere–ocean interface (longer ice-free season, younger ice), implying an increase in the percentage of PAR being transferred to the water. However, much less is known about the recent changes in how much PAR is being transferred by the overlaying atmosphere. We studied the transfer of PAR through the atmosphere between May 21 and July 23 at a pan-Arctic scale for the period ranging from 2000 to 2016. By combining a large data set of atmospheric and surface conditions into a radiative transfer model, we computed the percentage of PAR transferred to the surface. We found that typical Arctic atmospheres convey between 60% and 70% of the incident PAR received from the Sun, meaning the Arctic atmosphere typically transmits more light than most sea ice surfaces, with the exception of mature melt ponds. We also found that the transfer of PAR through the atmosphere decreased at a rate of 2.3% per decade over the studied period, due to the increase in cloudiness and the weaker radiative interaction between the atmosphere and the surface. Further investigation is required to address how, in the warmer Arctic climate, this negative trend would compensate for the increased surface transmittance and its consequences on marine productivity.

Examination of saffron cod Eleginus gracilis (Tilesius 1810) population genetic structure

Annual ice retreat in the northern Bering and Chukchi seas, which has occurred earlier in the past decades resulting in longer ice-free seasons and warmer ocean temperatures, is producing a cascade of ecological effects, and may open Arctic waters to commercial fishing and oil exploration. Baseline knowledge of population structure of a species is critical to developing strategies for management and conservation. There is little information for the saffron cod (Eleginus gracilis), an abundant forage fish inhabiting the coastlines of the North Pacific and Arctic oceans. We examined saffron cod population genetic structure with data from nine microsatellite loci designed to describe the genetic compositions of and variation among 40 collections of saffron cod from four regions (northwestern Alaska, Gulf of Alaska, western Pacific Ocean, and Gulf of Anadyr). The northwestern Alaska collections (Norton Sound and Chukchi Sea) exhibited little genetic divergence. The Gulf of Anadyr collection differed from other regions but was most similar to those of northwestern Alaska. The collections from the western Pacific Ocean (Sakhalin Island and Hokkaido Island) differed genetically, but not to the extent they did from other regions. Gulf of Alaska collections (Kodiak Island and Prince William Sound) comprised a lineage that was distinct from all of the other areas, including the geographically adjacent northwestern Alaska collections. The absence of genetic structure in northwestern Alaska probably reflects their recent expansion into previously unavailable habitat that became available after the Last Glacial Maximum (~ 16,000 years ago). Tracking and predicting the likely expansion of the eurythermal saffron cod may have parallels to its postglacial colonization.

Temporal variability in foraminiferal morphology and geochemistry at the West Antarctic Peninsula: a sediment trap study

Abstract. The West Antarctic Peninsula (WAP) exhibits strong spatial and temporal oceanographic variability, resulting in highly heterogeneous biological productivity. Calcifying organisms that live in the waters off the WAP respond to temporal and spatial variations in ocean temperature and chemistry. These marine calcifiers are potentially threatened by regional climate change with waters already naturally close to carbonate undersaturation. Future projections of carbonate production in the Southern Ocean are challenging due to the lack of historical data collection and complex, decadal climate variability. Here we present a 6-year-long record of the shell fluxes, morphology and stable isotope variability of the polar planktic foraminifera Neogloboquadrina pachyderma (sensu stricto) from near Palmer Station, Antarctica. This species is fundamental to Southern Ocean planktic carbonate production as it is one of the very few planktic foraminifer species adapted to the marine polar environments. We use these new data to obtain insights into its ecology and to derive a robust assessment of the response of this polar species to environmental change. Morphology and stable isotope composition reveal the presence of different growth stages within this tightly defined species. Inter- and intra-annual variability of foraminiferal flux and size is evident and driven by a combination of environmental forcing parameters, most importantly food availability, temperature and sea ice duration and extent. Foraminiferal growth occurs throughout the austral year and is influenced by environmental change, a large portion of which is driven by the Southern Annular Mode and El Niño–Southern Oscillation. A distinct seasonal production is observed, with the highest shell fluxes during the warmest and most productive months of the year. The sensitivity of calcifying foraminifera to environmental variability in this region, from weeks to decades, has implications both for their response to future climatic change and for their use as palaeoclimate indicators. A longer ice-free season could increase carbonate production in this region at least while carbonate saturation is still high enough to allow for thick tests to grow.

Interactions between climate and landscape drive Holocene ecological change in a High Arctic lake on Somerset Island, Nunavut, Canada

This study presents a diatom-based analysis of the post-glacial Holocene environmental history at Lake RS29 on Somerset Island in the Canadian High Arctic. Earliest post-glacial diatom assemblages (10 200–10 000 cal yr BP) consisted mainly of small, benthic fragilarioid taxa. Poor diatom preservation in the early Holocene (~10 000–6200 cal yr BP) is associated with warm conditions, as determined by pollen data from the same core and other paleoclimate estimates from the region. Analysis of this and other sites from across the Canadian Arctic suggest that zones of poor diatom preservation or diatom absence in lake sediment records may be associated with warm conditions. After 6200 cal yr BP, acidophilic assemblages consisting of Aulacoseira spp. and a suite of periphytic taxa indicate acidification since the mid-Holocene. During this time period, cooling causing changes in lake ice phenology was likely a major driver of the reconstructed mid-Holocene pH decline. Watershed processes, including reduced fluxes of base cations as the rate of sediment accumulation slowed, may also be contributors to long-term shifts in lake water pH and associated changes in diatom assemblages. The uppermost sediments in the Lake RS29 record were characterized by abrupt declines in Aulacoseira alpigena and increases in benthic diatom taxa Cyclotella sensu lato, suggesting an increase in lake water pH and longer ice-free seasons.

Climate change alters the reproductive phenology and investment of a lacustrine fish, the three-spine stickleback.

High-latitude lakes are particularly sensitive to the effects of global climate change, demonstrating earlier ice breakup, longer ice-free seasons, and increased water temperatures. Such physical changes have implications for diverse life-history traits in taxa across entire lake food webs. Here, we use a five-decade time series from an Alaskan lake to explore effects of climate change on growth and reproduction of a widely distributed lacustrine fish, the three-spine stickleback (Gasterosteus aculeatus). We used multivariate autoregressive state-space (MARSS) models to describe trends in the mean length for multiple size classes and to explore the influence of physical (date of ice breakup, surface water temperature) and biological (density of con- and heterospecifics) factors. As predicted, mean size of age 1 and older fish at the end of the growing season increased across years with earlier ice breakup and warmer temperatures. In contrast, mean size of age 0 fish decreased over time. Overall, lower fish density and warmer water temperatures were associated with larger size for all cohorts. Earlier ice breakup was associated with larger size for age 1 and older fish but, paradoxically, with smaller size of age 0 fish. To explore this latter result, we used mixing models on age 0 size distributions, which revealed an additional cohort in years with early ice breakup, lowering the mean size of age 0 fish. Moreover, early ice breakup was associated with earlier breeding, evidenced by earlier capture of age 0 fish. Our results suggest that early ice breakup altered both timing and frequency of breeding; three-spine stickleback spawned earlier and more often in response to earlier ice breakup date. While previous studies have shown the influence of changing conditions in northern lakes on breeding timing and growth, this is the first to document increased breeding frequency, highlighting another pathway by which climate change can alter the ecology of northern lakes.

Temperature effects on net greenhouse gas production and bacterial communities in arctic thaw ponds.

One consequence of High Arctic permafrost thawing is the formation of small ponds, which release greenhouse gases (GHG) from stored carbon through microbial activity. Under a climate with higher summer air temperatures and longer ice-free seasons, sediments of shallow ponds are likely to become warmer, which could influence enzyme kinetics or select for less cryophilic microbes. There is little data on the direct temperature effects on GHG production and consumption or on microbial communities' composition in Arctic ponds. We investigated GHG production over 16 days at 4°C and 9°C in sediments collected from four thaw ponds. Consistent with an enzymatic response, production rates of CO2 and CH4 were significantly greater at higher temperatures, with Q10 varying from 1.2 to 2.5. The bacterial community composition from one pond was followed through the incubation by targeting the V6-V8 variable regions of the 16S rRNA gene and 16S rRNA. Several rare taxa detected from rRNA accounted for significant community compositional changes. At the higher temperature, the relative community contribution from Bacteroidetes decreased by 15% with compensating increases in Betaproteobacteria, Alphaproteobacteria, Firmicutes, Acidobacteria, Verrucomicrobia and Actinobacteria. The increase in experimental GHG production accompanied by changes in community indicates an additional factor to consider in sediment environments when evaluating future climate scenarios.

Temperature effects on net greenhouse gas production and bacterial communities in arctic thaw ponds.

One consequence of High Arctic permafrost thawing is the formation of small ponds, which release greenhouse gases (GHG) from stored carbon through microbial activity. Under a climate with higher summer air temperatures and longer ice-free seasons, sediments of shallow ponds are likely to become warmer, which could influence enzyme kinetics or select for less cryophilic microbes. There is little data on the direct temperature effects on GHG production and consumption or on microbial communities' composition in Arctic ponds. We investigated GHG production over 16 days at 4°C and 9°C in sediments collected from four thaw ponds. Consistent with an enzymatic response, production rates of CO2 and CH4 were significantly greater at higher temperatures, with Q10 varying from 1.2 to 2.5. The bacterial community composition from one pond was followed through the incubation by targeting the V6-V8 variable regions of the 16S rRNA gene and 16S rRNA. Several rare taxa detected from rRNA accounted for significant community compositional changes. At the higher temperature, the relative community contribution from Bacteroidetes decreased by 15% with compensating increases in Betaproteobacteria, Alphaproteobacteria, Firmicutes, Acidobacteria, Verrucomicrobia and Actinobacteria. The increase in experimental GHG production accompanied by changes in community indicates an additional factor to consider in sediment environments when evaluating future climate scenarios.

Vertical segregation of age-0 and age-1+ polar cod (Boreogadus saida) over the annual cycle in the Canadian Beaufort Sea

The offshore marine ecosystem of the Canadian Beaufort Sea faces the double pressure of climate change and industrialization. Polar cod (Boreogadus saida) is a pivotal forage species in this ecosystem, accounting for 95 % of the pelagic fish assemblage. Its vertical distribution over the annual cycle remains poorly documented. Hydroacoustic records from 2006 to 2012 were analysed to test the hypothesis that age-0 polar cod segregate vertically from larger congeners. Trawls and ichthyoplankton nets validated the acoustic signal. Fish length, weight, and biomass were estimated from new regressions of target strength and weight on standard length. Polar cod were vertically segregated by size in all months, with small age-0 juveniles in the epipelagic (<100 m) layer and larger age-1+ deeper in the water column. From December to March, the biomass of age-1+ peaked in a mesopelagic layer between 200 and 400 m. With increasing irradiance from April to July, the mesopelagic layer deepened and extended to 600 m. Starting in July, age-0 polar cod formed an epipelagic scattering layer that persisted until November. From September onward, age-0 left the epipelagic layer to join small age-1+ in the upper mesopelagic layer. Low biomass in the mesopelagic layer from February to September likely resulted from large polar cod settling on the seafloor to avoid diving marine mammals. Longer ice-free seasons, warmer sea-surface temperatures, or an oil spill at the surface would likely impact epipelagic age-0, while mesopelagic age-1+ would be vulnerable to an eventual oil plume spreading over and above the seafloor.

Past and Future Changes in Arctic Lake and River Ice

Paleolimnological evidence from some Arctic lakes suggests that longer ice-free seasons have been experienced since the beginning of the nineteenth century. It has been inferred from some additional records that many Arctic lakes may have crossed an important ecological threshold as a result of recent warming. In the instrumental record, long-term trends exhibit increasingly later freeze-ups and earlier break-ups, closely corresponding to increasing air temperature trends, but with greater sensitivity at the more temperate latitudes. Broad spatial patterns in these trends are also related to major atmospheric circulation patterns. Future projections of lake ice indicate increasingly later freeze-ups and earlier break-ups, decreasing ice thickness, and changes in cover composition, particularly white-ice. For rivers, projected future decreases in south to north air-temperature gradients suggest that the severity of ice-jam flooding may be reduced but this could be mitigated by changes in the magnitude of spring snowmelt.

The response and role of ice cover in lake-climate interactions

This paper reviews the current state of knowledge pertaining to the interactions of lake ice and climate. Lake ice has been shown to be sensitive to climate variability through observations and modelling, and both long-term and short-term trends have been identified from ice records. Ice phenology trends have typically been associated with variations in air temperatures while ice thickness trends tend to be associated more to changes in snow cover. The role of ice cover in the regional climate is less documented and with longer ice-free seasons possible as a result of changing climate conditions, especially at higher latitudes, the effects of lakes on their surrounding climate (such as increased evaporation, lake-effect snow and thermal moderation of surrounding areas, for example) can be expected to become more prominent. The inclusion of lakes and lake ice in climate modelling is an area of increased attention in recent studies. An important step in improving predictions of ice conditions in models is the assimilation of remote sensing data in areas where in-situ data is lacking, or non-representative of the lake conditions. The ability to accurately represent ice cover on lakes will be an important step in the improvement of global circulation models, regional climate models and numerical weather forecasting.

Watershed-scale reconstruction of middle and late Holocene paleoenvironmental changes on Melville Peninsula, Nunavut, Canada

Arctic regions have heightened sensitivity to climatic changes, however regional scale differences exist in the magnitude and timing of biotic responses. Melville Peninsula in the east-central Canadian Arctic is a transitional area with respect to modern climate, with a continental climate to the west, and more maritime influences to the east. This, and the presence of Thule archaeological sites, make the Peninsula an area of high importance for paleoclimate reconstructions. Lake sediment cores from two adjacent ultra-oligotrophic and very dilute lakes (SP02 and SP04) from interior Melville Peninsula (68°33′N and 83°17′W, 220 m a.s.l.) were analyzed for diatom assemblages (microscopic algae) to track paleoclimatic changes. High magnetic susceptibility, coarse-grained sediments, low organic matter, and lack of diatoms in sediments older than 6000 years BP confirm earlier suggestions of a persistent ice cap on the Peninsula. Peak diatom concentration and biogenic silica, and maximum values for sediment organic matter, between 4400 and 2900 years BP may indicate the Holocene Thermal Maximum (HTM). The timing of the HTM is similar to that of the Baffin region to the east. Decreases in these proxies, as well as small shifts in diatom assemblages following 2900 years BP indicate the advancement of Neoglacial cooling. The changes of the pre-industrial portion of the last millennium were of smaller magnitude than those of the HTM, but subtle changes in diatom assemblages after 750 years BP may be indicative of the Little Ice Age, the timing of which agrees well with that of Boothia Peninsula to the west. The Anthropocene is well marked in these records, with increases in diatom species richness from a pre-industrial maximum of 28–51 diatom species in the modern assemblage at Lake SP02. Planktonic diatoms appear in the 20th century, signalling profound ecological changes associated with longer ice-free seasons.

Mercury in seals linked to vanishing sea ice

Longer ice-free seasons may mean that food sources for Arctic marine mammals are more contaminated.

Elevated mercury concentrations in fish in lakes in the Mackenzie River Basin: The role of physical, chemical, and biological factors

During the mid-1990s and through the early 2000s, researchers determined that elevated mercury concentrations were a common occurrence in predatory fish in many lakes in the Mackenzie River Basin (MRB), located in northern Canada. Here we present the results of studies investigating factors contributing to higher mercury concentrations in fish in many of these lakes. Twenty-two percent of lake trout, 33% of northern pike, and 50% of walleye populations had mean mercury concentrations > 0.5 μg/g, the guideline for the commercial sale of fish. Higher mercury concentrations were strongly associated with the relatively old age of MRB predatory fish; mean age ranged from 7.6 to 24.9 years for the three species. In contrast, none of the lake trout sampled in eight lakes further south in northern Saskatchewan and Alberta had mean mercury concentrations > 0.5 μg/g; fish also were younger (mean age 6 years for the 8 lakes). Mercury concentrations in MRB fish generally increased with fish length, age, and trophic feeding although the nature of these relationships varied with the lake. Mean length was a good predictor of mean mercury concentrations in walleye populations across the study lakes but not for whitefish, lake trout, and pike; age was a good predictor for lake trout and walleye. Mercury concentrations in water and invertebrates were similar to those observed in more southerly regions where fish do not have elevated mercury concentrations. Mercury concentrations tended to be higher in fish in smaller vs. larger lakes and as a probable consequence of higher summer epilimnion temperatures, which favour a higher net methylation rate, and higher mercury and methyl mercury concentrations in water which enter these lakes from the watershed. Increasing fishing pressures on MRB lakes may be a means of reducing mean fish age, improving growth rates, and decreasing mercury body burdens. Increased global warming may result in higher mercury concentrations in fish through increased water temperatures, a longer ice free season, and increased release of stored mercury from the watershed into these lakes.

Late Quaternary diatom response to vegetation and climate change in a subalpine lake in Banff National Park, Alberta

The late Quaternary diatom record from subalpine Crowfoot Lake, Banff National Park, Alberta (lat. 51° 61′N; long. 116° 31′W) has been analyzed. Results are related to independently inferred vegetation and climate changes. No diatoms were found in the basal diamict that predates 11330 14C yr BP. Very few occur until ca. 10 10014 C yr BP probably due to the short time between de-glaciation and an advance of the Crowfoot Glacier during the ‘Younger Dryas Chron’. Initial pioneering species were characteristic of alkaline water and calcareous organic sediments. They appeared as sediments became organic and laminated suggesting increasing water clarity, and as the Pinus-dominated forest expanded and the climate warmed. After ca. 9060 14C yr BP diatom numbers increased rapidly, reaching a maximum prior to the Mazama tephra; they remained high until ca. 3500 14C yr BP. The period between ca. 9060 and 3500 14C yr saw timberline elevation increase and the dominance of xerophytic taxa. These are consistent with early to mid-Holocene warmth and aridity. Diatom productivity reflects the warm climate and presumably longer ice-free season, a stable catchment and transparent water. Decreases in diatom productivity coincide with a vegetation change with reduction of xerophytic taxa and the appearance of a closed Picea-Abies forest, hence a cooler, wetter climate at ca. 4100 to 3500 14C yr BP. The diatom numbers during the Neoglacial were of the same magnitude as prior to ca. 9060 14C yr BP. Small species of Fragilaria (overwhelmingly Fragilaria construens v. venter) became extremely dominant during the period of high diatom productivity, and remained so thereafter. Recovery of the lake appears to have been rapid after deposition of the Mazama tephra. Maximum occurrence of Cyclotella radiosa occurred ca. 8000 14C yr BP during the warm early Holocene and may reflect this warmer climate, a longer ice-free season than presently, perhaps less turbid water, or it may reflect a subtly higher nutrient status of the lake water. The diatom record of Crowfoot Lake has responded with sensitivity, particularly in terms of productivity, to the Holocene vegetation and climate changes.