Historical changes in Holocene climate in northeastern Ontario were quantified using analyses of sedimentary pollen, diatoms, and pigments in a small boreal lake. Modern analog reconstructions of average temperature from Holocene pollen assemblages of Charland Lake showed temperature was ~2°C warmer than present conditions ~7800–4500 cal. yr BP, a time period consistent with the Holocene thermal maximum (HTM). Pollen data suggest a two-phase HTM: warm and dry conditions based on the presence of primarily Pinus spp., followed by warm and wet conditions based on increases in cedar. Overall, algal production was low during the HTM, as reflected by low concentrations of pigments and diatoms. In the late HTM, increases in cedar pollen and planktonic diatoms suggest sustained increases in water levels for the remainder of the Holocene. During the Post-HTM Period (~4500–2000 cal. yr BP), a period that was warmer than today but cooler than the HTM, overall pigment production was significantly higher than all other periods. However, changes in diatom species composition suggest this period was not uniform, with variation occurring between diatoms indicative of higher and lower nutrient levels. The last ~2000 cal. yr BP was less productive than the Post-HTM Period but more productive than the HTM with higher production from diatoms and cyanobacteria. This study suggests that the relationship between climate and lake water production can be quite complex, and that changes in temperature, precipitation, light, lake levels, and mixing patterns are among factors that are related to changes in subfossil phototroph assemblages.

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Abstract. While interactions between groundwater and lake-water influence water chemistry, water balance, aquatic organisms, biochemical cycles and contamination levels, they remain a poorly studied component of lake hydrology. Identifying the controls of groundwater and lake-water interactions at the landscape level and classifying lakes into categories based on their degree of interaction with the groundwater can provide insights into a lake's sensitivity and vulnerability to environmental stressors. Such information can also provide baseline conditions for comparison to future changes that are important for water management and conservation. To this end, water chemistry and water isotopic composition were investigated in a set of 50 boreal lakes located at different elevations in an esker system near Timmins, Ontario. Analyses focused on stable isotopic ratios of hydrogen and oxygen and specific conductance as indicators of the position of a lake with respect to the influence of groundwater. Both isotopic composition and specific conductance distinguished higher-elevation groundwater-recharge lakes from lower-elevation groundwater-discharge lakes. Groundwater-recharge lakes were high-elevation lakes characterized by enriched isotopic values and low values of specific conductance. In contrast, groundwater-discharge lakes were isotopically depleted and had higher values of specific conductance and occurred at lower elevations. An intermediate group of lakes was also defined (termed seepage lakes) and had intermediate isotopic and water-chemistry characteristics compared to recharge and discharge lakes. Differences in water geochemistry between field campaigns revealed that upland groundwater-recharge lakes showed evidence of evaporative drawdown, indicating sensitivity to short-term changes in climate, whereas the lower-elevation groundwater-discharge lakes showed little variation between seasonal samples and consequently would likely be affected only by hydroclimatological changes of greater duration and magnitude.

14Water chemistry and water isotopic composition were investigated in a set of 50 boreal lakes located 15 at different elevations in an esker system near Timmins, Ontario, as well as in local streams, groundwater 16 springs and information available from seasonal precipitation values. Analyses focused on stable isotopic 17 ratios of hydrogen and oxygen, as well as specific conductance as indicators of the position of a lake with 18 respect to the influence of groundwater. Both isotopic composition and specific conductance distinguished 19 higher elevation groundwater discharge lakes from lower elevation groundwater recharge lakes. 20Groundwater recharge lakes characterized by enriched isotopic values and low values of specific 21 conductance are located above the hydraulic midline elevation of the study lakes. In contrast groundwater 22 discharge lakes, were isotopically depleted and had higher values of specific conductance, and occurred 23 below the hydraulic midline of the study lakes. An intermediate group of lakes was also defined (termed 24 seepage lakes) and consisted of either recharge lakes that were alkaline, or discharge lakes that had no 25 outlet. The seepage lakes group had intermediate isotopic and water chemistry characteristics compared to 26 recharge and discharge lakes. A classification scheme for lakes was developed based on the specific 27 conductivity, water isotopic composition, the presence of an outlet, and other characteristic to define three 28 types of recharge lakes, and two types of discharge lakes. 29 30