The region studied includes the Laurentian Great Lakes and a diversity of smaller glacial lakes, streams and wetlands south of permanent permafrost and towards the southern extent of Wisconsin glaciation. We emphasize lakes and quantitative implications. The region is warmer and wetter than it has been over most of the last 12 000 years. Since 1911 observed air temperatures have increased by about 0 . 118C per decade in spring and 0 . 068C in winter; annual precipitation has increased by about 2 . 1% per decade. Ice thaw phenologies since the 1850s indicate a late winter warming of about 2 . 58C. In future scenarios for a doubled CO 2 climate, air temperature increases in summer and winter and precipitation decreases (summer) in western Ontario but increases (winter) in western Ontario, northern Minnesota, Wisconsin and Michigan. Such changes in climate have altered and would further alter hydrological and other physical features of lakes. Warmer climates, i.e. 2 Â CO 2 climates, would lower net basin water supplies, stream¯ows and water levels owing to increased evaporation in excess of precipitation. Water levels have been responsive to drought and future scenarios for the Great Lakes simulate levels 0 . 2 to 2 . 5 m lower. Human adaptation to such changes is expensive. Warmer climates would decrease the spatial extent of ice cover on the Great Lakes; small lakes, especially to the south, would no longer freeze over every year. Temperature simulations for strati®ed lakes are 1±78C warmer for surface waters, and 68C cooler to 88C warmer for deep waters. Thermocline depth would change (4 m shallower to 3 . 5 m deeper) with warmer climates alone; deepening owing to increases in light penetration would occur with reduced input of dissolved organic carbon (DOC) from dryer catchments. Dissolved oxygen would decrease below the thermocline. These physical changes would in turn aect the phytoplankton, zooplankton, benthos and ®shes. Annual phytoplankton production may increase but many complex reactions of the phytoplankton community to altered temperatures, thermocline depths, light penetrations and nutrient inputs would be expected. Zooplankton biomass would increase, but, again, many complex interactions are expected.Generally, the thermal habitat for warm-, cool-and even cold-water ®shes would increase in size in deep strati®ed lakes, but would decrease in shallow unstrati®ed lakes and in streams. Less dissolved oxygen below the thermocline of lakes would further degrade strati®ed lakes for cold water ®shes. Growth and production would increase for ®shes that are now in thermal environments cooler than their optimum but decrease for those that are at or above their optimum, provided they cannot move to a deeper or headwater thermal refuge. The zoogeographical boundary for ®sh species could move north by 500±600 km; invasions of warmer water ®shes and extirpations of colder water ®shes should increase. Aquatic ecosystems across the region do not necessarily exhibit coherent responses to climate changes and va...

1. The relationship between surfidal sediment diatom taxa (Bacillariophyceae) and measured Hmnological variables in forty-six British Columbia lakes was explored using canonical correspondence analysis (CCA). Lake-water total phosphorus concentration (TP), maximum lake depth, conductivity, and calcium concentration each accounted for independent and statistically significant directions of variation in the distribution of diatom taxa. 2. Weighted-averaging (WA) models were developed to infer lake-water TP from the relative abundances of 131 diatom taxa in the surfidal sediments of thirty-seven lakes. WA regression and calibration with classical deshrinking provided the best model for TP reconstructions. 3. Our quantitative inference model has two major advantages over existing multiple linear-regression models: (i) inferences are based on the responses of individual taxa to TP, and do not involve grouping the taxa into a small number of ecological categories; and (ii) the model assumes that diatoms respond to TP in a unimodal, rather than a linear, fashion. 4. The WA model can now be used to infer past lake-water TP, within the range 5-28ngl"', from diatoms preserved in the sediments of British Columbia lakes. The model can provide quantitative estimates of the onset, rate,, and magnitude of lake eutrophication in response to natural processes and human disturbances.

Abstract:We used stable isotopes (υ 18 O and υ 2 H) and water chemistry to characterize the water balance and hydrolimnological relationships of 57 shallow aquatic basins in the Peace-Athabasca Delta (PAD), northern Alberta, Canada, based on sampling at the end of the 2000 thaw season. Evaporation-to-inflow ratios (E/I) were estimated using an isotope mass-balance model tailored to accommodate basin-specific input water compositions, which provided an effective, first-order, quantitative framework for identifying water balances and associated limnological characteristics spanning three main, previously identified drainage types. Open-drainage basins (E/I < 0Ð4; n D 5), characterized by low alkalinity, low concentrations of nitrogen, dissolved organic carbon (DOC) and ions, and high minerogenic turbidity, include large, shallow basins that dominate the interior of the PAD and experience frequent or continuous river channel connection. Closed-drainage basins (E/I ½ 1Ð0; n D 16), in contrast, possess high alkalinity and high concentrations of nitrogen, DOC, and ions, and low minerogenic turbidity, and are located primarily in the relict and infrequently flooded landscape of the northern Peace sector of the delta. Several basins fall into the restricted-drainage category (0Ð4 # E/I < 1Ð0; n D 26) with intermediate water chemistries and are predominant in the southern Athabasca sector, which is subject to active fluviodeltaic processes, including intermittent flooding from riverbank overflow. Integration of isotopic and limnological data also revealed evidence for a new fourth drainage type, mainly located near the large open-drainage lakes that occupy the central portion of the delta but within the Athabasca sector (n D 10). These basins were very shallow (<50 cm deep) at the time of sampling and isotopically depleted, corresponding to E/I characteristic of restricted-and open-drainage conditions. However, they are limnologically similar to closed-drainage basins except for higher conductivity and higher concentrations of Ca 2C and Na C , and lower concentrations of SiO 2 and chlorophyll c. These distinct features are due to the overriding influence of recent summer rainfall on the basin water balance and chemistry. The close relationships evident between water balances and limnological conditions suggest that past and future changes in hydrology are likely to be coupled with marked alterations in water chemistry and, hence, the ecology of aquatic environments in the PAD.

Abstract:Floods caused by ice-jams on the Peace River are considered to be important for maintaining hydro-ecological conditions of perched basins in the Peace-Athabasca Delta (PAD), Canada, a highly productive and internationally recognized northern boreal ecosystem. Concerns over the potential linkages between regulation of the Peace River in 1968 for hydroelectric production and low Peace River discharge between 1968 and 1971 during the filling of the hydroelectric reservoir, absence of a major ice-jam flood event between 1975 and 1995, and low water levels in perched basins during the 1980s and early 1990s have sparked numerous environmental studies largely aimed at restoring water levels in the PAD. Lack of sufficient long-term hydrological records, however, has limited the ability to objectively assess the importance of anthropogenic factors versus natural climatic forcing in regulating hydroecological conditions of the PAD. Here, we report results of a paleolimnological study on laminated sediments from two oxbow lakes in the PAD, which are located adjacent to major flood distributaries of the Peace River. Sediment core magnetic susceptibility measurements, supported by results from several other physical and geochemical analyses as well as stratigraphic correspondence with recorded high-water events on the Peace River, provide proxy records of flood history spanning the past ¾180 and ¾300 years in these two basins. Results indicate that inferred flood frequency has been highly variable over the past 300 years but in decline for many decades beginning as early as the late nineteenth century, well before Peace River regulation. Additionally, several multi-decadal intervals without a major flood have occurred during the past 300 years. While climate-related mechanisms responsible for this variability in flood frequency remain to be determined, as does quantifying the relative roles of river regulation and climate variability on hydro-ecological conditions in the PAD since 1968, these results suggest that ecosystem management strategies for the PAD need to explicitly account for natural variations in flood recurrence intervals.

Many northern lake‐rich regions are undergoing pronounced hydrological change, yet inadequate knowledge of the drivers of these landscape‐scale responses hampers our ability to predict future conditions. We address this challenge in the thermokarst landscape of Old Crow Flats (OCF) using a combination of remote sensing imagery and monitoring of stable isotope compositions of lake waters over three thaw seasons (2007–2009). Quantitative analysis confirmed that the hydrological behavior of lakes is strongly influenced by catchment vegetation and physiography. Catchments of snowmelt‐dominated lakes, typically located in southern peripheral areas of OCF, encompass high proportions of woodland/forest and tall shrub vegetation (mean percent land cover = ca. 60%). These land cover types effectively capture snow and generate abundant snowmelt runoff that offsets lake water evaporation. Rainfall‐dominated lakes that are not strongly influenced by evaporation are typically located in eastern and northern OCF where their catchments have higher proportions of dwarf shrub/herbaceous and sparse vegetation (ca. 45%), as well as surface water (ca. 20%). Evaporation‐dominated lakes, are located in the OCF interior where their catchments are distinguished by substantially higher lake area to catchment area ratios (LA/CA = ca. 29%) compared to low evaporation‐influenced rainfall‐dominated (ca. 10%) and snowmelt‐dominated (ca. 4%) lakes. Lakes whose catchments contain >75% combined dwarf shrub/herbaceous vegetation and surface water are most susceptible to evaporative lake‐level drawdown, especially following periods of low precipitation. Findings indicate that multiple hydrological trajectories are probable in response to climate‐driven changes in precipitation amount and seasonality, vegetation composition, and thermokarst processes. These will likely include a shift to greater snowmelt influence in catchments experiencing expansion of tall shrubs, greater influence from evaporation in catchments having higher proportions of surface water, and an increase in the rate of thermokarst lake expansion and probability of drainage. Local observations suggest that some of these changes are already underway.

Quantitative reconstructions are made of Holocene changes in climatic and environmental conditions from analyses of pollen, chironomids and diatoms in identical stratigraphic levels of a sediment core from Vuoskkujavri (68'20'43 N, 19'06'00 E, 348 m as.l.) near Abisko in northern Sweden (Lapland). Transfer functions, based on regional calibration sets, are applied to reconstruct Holocene patterns in mean July air temperature (using all three indicators), mean Januaiy air temperature (pollen), annual precipitation (pollen) and lakewater pH (diatoms).During periods with 'good' fit to the modern calibration sets all mean July air-temperature inferences based on the three proxy indicators reveal a general trend of decreasing temperature: pollen-inferred mean July air temperature shows a decrease of c. 1.10C since 7500 cal. yrs BP; the chironomids show a decrease of c. 1.20C since the early Holocene; whereas the diatoms show a decrease of C. 1.50C since 6000 cal. yrs BP. Pollen-inferred mean January air temperature indicates that winters may have been warmer by c. 3.0WC during the early Holocene, followed by a gradual cooling until 8500 cal. yrs BP (c. I .0'C warmer than today) and a subsequent warming until 7000 cal. yrs BP (c. 2.0'C warmer than today). Since 7000 cal. yrs BP, a gradual cooling towards the present-day values is inferred. According to the pollen, annual precipitation may have been considerably higher during the early Holocene than today (c. +150 mm) and increased until 7000 cal. yrs BP (c. +320 mm). Since 7000 cal. yrs BP, annual precipitation decreased continuously towards present-day values. Diatom-inferred pH trends show that natural acidification of c. 0.5 pH units followed deglaciation; present-day values were reached c. 5000 cal. yrs BP. The early Holocene is identified as a problematic time period for the application of modern calibration sets, as diatoms show 'poor' fit to the calibration set from 10 600 to 6000 cal. yrs BP, pollen from 10 600 to 7500 cal. yrs BP, and chironomids from 10 250 to 10 000 cal. yrs BP. Compared with estimates from the COHMAP GCM model, mean July air-temperature inferences based on biological proxies at Vuoskkujdvri suggest a more moderate decrease in temperature over the past 9000 years.

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