In a climate change perspective, increased air temperatures are already a reality and are expected to increase even more in the future, especially in areas at high latitudes. The present thesis therefore addresses the influence of climate change on the physical properties and the phytoplankton communities of typical small and oligotrophic lakes in northern Sweden (62-64˚N). In the first part of the study, we found a significant trend (10 lakes from 1916 to 2010) of ice break-ups occurring increasingly earlier. The timing of ice break-up was strongly influenced by the April air temperature indicating that expected increases in air temperature in the future will also result in an earlier ice break-up. We also used concentrations of chlorophyll a (chl a) as estimations of phytoplankton biomass and discovered a positive relationship between surface water temperature and concentrations of chl a in Lake Remmaren (from 1991 to 2008). The second part of the thesis focuses on climatic conditions and cyanobacteria abundance in three small, oligotrophic lakes in northern Sweden; Lake Remmaren, Lake S. Bergsjön and Lake Gransjön. The concentration and relative abundance of cyanobacteria differ between 2011 and 2012, with different climatic conditions. The "warm" year of 2011 had higher concentrations and relative abundance of cyanobacteria than the "cold" year of 2012. Trends in increasing surface water temperatures as well as increasing abundance of cyanobacteria in August were found in Lake Remmaren (from 1988 to 2011). The direct or indirect effects of warming had a positive effect on the cyanobacteria abundance, since nutrients (Tot N and Tot P) did not display an increasing trend in Lake Remmaren. An analysis on the composition of phytoplankton species in Lake Remmaren, Lake S. Bergsjön and Lake Gransjön revealed that the cyanobacteria Merismopedia sp. was more common in 2011 than 2012. If different cyanobacteria become more common in oligotrophic lakes in the future, the functioning of lake ecosystems may be impacted. Small zooplankton eats small phytoplankton and if smaller phytoplankton species, e.g. cyanobacteria, increase at the expense of other phytoplankton groups, an extra step in the food chain might be added. Less energy might be transferred to the upper levels because many cyanobacteria contain toxic compounds and are less edible than other phytoplankton groups. An increase of toxic containing cyanobacteria in lakes can also make lakes less attractive for recreational purposes in the future.

The frequency, magnitude, and atmospheric drivers of winter hydroclimatic conditions conducive to snowmelt in western Canada were evaluated. These hydroclimatic variables were linked to the mid-winter break-up of river ice that included the creation of a comprehensive database including 46 mid-winter river ice break-up events in western Canada (1950-2008) and six events in Alaska (1950-2014). Widespread increases in above-freezing temperatures and spatially diverse increases in rainfall were detected over the study period (1946-2012), particularly during January and March. Critical elevation zones representing the greatest rate of change were identified for major river basins. Specifically, low-elevation (500-1000 m) temperature changes dominated the Stikine, Nass, Skeena, and Fraser river basins and low to mid-elevation changes (700-1500 m) dominated the Peace, Athabasca, Saskatchewan, and Columbia river basins. The greatest increases in rainfall were seen below 700 m and between 1200-1900 m in the Fraser and at mid- to high-elevations (1500-2200 m) in the Peace, Athabasca, and Saskatchewan river basins. Daily synoptic-scale atmospheric circulation patterns were classified using Self-Organizing Maps (SOM) and corresponding hydroclimatic variables were evaluated. Frequency, persistence, and preferred shifts of identified synoptic types provided additional insight into characteristics of dominant atmospheric circulation patterns. Trend analyses revealed significant (p < 0.05) decreases in two dominant synoptic types: a ridge of high pressure over the Pacific Ocean and adjacent trough of low pressure over western Canada, which directs the movement of cold, dry air over the study region, and zonal flow with westerly flow from the Pacific Ocean over the study region. Conversely, trend analyses revealed an increase in the frequency and persistence of a ridge of high pressure over western Canada over the study period. However, step-change analysis revealed a decrease in zonal flows and an increase in the occurrence of high-pressure ridges over western Canada in 1977, coinciding with a shift to a positive Pacific Decadal Oscillation regime. A ridge of high pressure over western Canada was associated with a high frequency and magnitude of above-freezing temperatures and rainfall in the study region. This pattern is highly persistent and elicits a strong surface climate response. A ridge of high pressure and associated above-freezing temperatures and rainfall was also found to be the primary driver of mid-winter river ice break-up with rainfall being a stronger driver west of the Rocky Mountains and temperature to the east. These results improve our understanding of the drivers of threats to snowpack integrity and the generation of extreme hydrologic events. / Graduate

Recently, Hudson Bay experienced unidirectional trends in temperature, sea-ice extent, time of break-up, and length of the open-water season. Predicted impacts on population dynamics of ice-associated species include habitat loss and shift in prey availability. The ringed seal (Phoca hispida) depends on a stable ice platform with sufficient snow depth and a productive open-water season for reproduction and survival. Evidence of ringed seal sensitivity to environmental variations has been reported, but mechanisms involved were poorly understood. In western Hudson Bay, density, life-history traits, and diet of ringed seals were monitored over two decades, providing an opportunity to understand the effects of climatic variations on the population dynamics of this long-lived carnivore. Ringed seal density was estimated through strip-transect analyses after aerial surveys were flown in western Hudson Bay in late spring during the annual moult in the 1990s and 2000s. During these periods, ringed seals were also sampled from Inuit subsistence fall harvests In Arviat, NU, and ages, reproductive status, percentage of pups in the harvest, body condition, and diet were assessed. Strong inter-annual variations in these parameters were observed, and a decadal cycle was suggested and related to variations in the sea-ice regime. The cold and heavy ice conditions that prevailed in western Hudson Bay in 1991-92 likely induced a decrease in pelagic productivity, reducing the availability to ringed seals of sand lances (Ammodytes sp.), their major prey. The nutritional stress endured, combined with a strong predation pressure, led to a decrease in ringed seal reproductive performances, pup survival, and density during the 1990s. The recovery of ringed seal demographic parameters and number in the 2000s was associated with the immigration of pups, juveniles, and young adults into western Hudson Bay. Impact of current climatic trends on ringed seal population dynamics was not apparent, but considering the limited range of environmental variations tolerated by ringed seals, the response of this species to climate warming might be of a catastrophic type. Ringed seals were found to be good indicators of ecosystem changes, and long-term monitoring of the species in Hudson Bay should be a priority.

Recently, Hudson Bay experienced unidirectional trends in temperature, sea-ice extent, time of break-up, and length of the open-water season. Predicted impacts on population dynamics of ice-associated species include habitat loss and shift in prey availability. The ringed seal (Phoca hispida) depends on a stable ice platform with sufficient snow depth and a productive open-water season for reproduction and survival. Evidence of ringed seal sensitivity to environmental variations has been reported, but mechanisms involved were poorly understood. In western Hudson Bay, density, life-history traits, and diet of ringed seals were monitored over two decades, providing an opportunity to understand the effects of climatic variations on the population dynamics of this long-lived carnivore. Ringed seal density was estimated through strip-transect analyses after aerial surveys were flown in western Hudson Bay in late spring during the annual moult in the 1990s and 2000s. During these periods, ringed seals were also sampled from Inuit subsistence fall harvests In Arviat, NU, and ages, reproductive status, percentage of pups in the harvest, body condition, and diet were assessed. Strong inter-annual variations in these parameters were observed, and a decadal cycle was suggested and related to variations in the sea-ice regime. The cold and heavy ice conditions that prevailed in western Hudson Bay in 1991-92 likely induced a decrease in pelagic productivity, reducing the availability to ringed seals of sand lances (Ammodytes sp.), their major prey. The nutritional stress endured, combined with a strong predation pressure, led to a decrease in ringed seal reproductive performances, pup survival, and density during the 1990s. The recovery of ringed seal demographic parameters and number in the 2000s was associated with the immigration of pups, juveniles, and young adults into western Hudson Bay. Impact of current climatic trends on ringed seal population dynamics was not apparent, but considering the limited range of environmental variations tolerated by ringed seals, the response of this species to climate warming might be of a catastrophic type. Ringed seals were found to be good indicators of ecosystem changes, and long-term monitoring of the species in Hudson Bay should be a priority.

A return-period analysis of annual peak spring break-up and open-water levels for 136 Water Survey of Canada hydrometric stations was used to classify rivers across Canada and to assess the physical controls on peak break-up water-levels. According to the peak water-level river-regime classification and subsequent analysis, 32% of rivers were classified as spring break-up dominated, characterized by low elevations and slopes and large basin sizes while 45% were open-water dominated and associated with alpine environments of high elevations and channel slopes, and smaller basin sizes. The remaining 23% of rivers were classified as a mixed regime. A spatial and temporal analysis (1969-2006) of the river ice break-up season using hydrometric variables of timing and water levels, never before assessed at the northern Canada-wide scale, revealed significant declines in break-up water levels and significant trends towards earlier and prolonged break-up in western and central Canada. The spatial and temporal influence of air temperature on break-up timing was assessed using the spring 0°C isotherm, which revealed a significant positive relationship but no spatial patterns. In the case of major ocean/atmosphere oscillations, significant negative (positive) correlations indicate that break-up occurs earlier (later) during the positive phases of the Pacific North American Pattern (El Niño Southern Oscillation) over most of western Canada. Fewer significant positive correlations show that break-up occurs later during the positive phases of the Arctic Oscillation and North Atlantic Oscillation in eastern Canada. / Graduate

River-ice break-up in high-latitude regions, despite its brevity, is a fundamental process, representing the most dynamic and complex period of fluvial processes. Moreover, ice break-up has significant cascading ecological effects, with a different severity for mechanical vs. thermal break-up, and thus, motivates the importance of monitoring efforts. Classical research methods, such as fieldwork or analysis of photographs and aerial imagery, offer a general perspective on the timing of ice break-up but have safety and logistic issues caused by the dangers of unstable ice cover, the lag times between event occurrence and observation, and the frequent low visibilities. The emerging field of environmental seismology, which studies surface processes through seismic signals, provides an alternative solution to these shortcomings by continuously recording high temporal resolution data. Seismic sensing can potentially record any event within a set distance if the produced signal is powerful enough. Three geophones had monitored the subarctic Sävar River reach for 185 days to test the efficiency of seismic methods to capture ice-cracking events, and based on their characteristics, to identify thermal vs. mechanical ice break-up. With visual and multivariate analysis, seismic methods provided a conservative set of 2 228 events, detected at milliseconds precision, described, and located. Besides, both trigger lag times and principal component analysis depicted correlations between environmental drivers and ice-cracking events. The automatic picker based on duration and trigger thresholds required manual supervision because of the initial numerous false signals that accounted for 96% of total initial events. Ice-cracking signals as short as 0.2s and frequencies of 8-40 Hz with an average power of -117 dB were statistically defined, classified, and described by case events as two types, associated, based on their spectral and temporal patterns, with the two ice break-up modes. With an estimated Rayleigh wave velocity of 680 m/s, all ice-cracking signals' locations were within the instrumented area. Trigger lag times analysis improved detection and showed a strong link between ice-cracking events and drivers of lag times less than three hours, including near-immediate responses (&lt; 2s). With multivariate analysis, the lag times showed a mainly climatic control for thermal melting and a primarily fluvial control in mechanical ice break-up. The combination of statistical and seismic analysis provides, despite the considerable manual screening, a valid and potentially site-transferable method to extract and describe ice-cracking signals and thus identify ice break-up modes in northern rivers.

Concern has been expressed regarding the impacts of climate change on the hydroecology of the Mackenzie Delta, thus identifying a need for better understanding of the ice break-up regime. Archived records at hydrometric stations in the delta for the period 1974 to 2006, supplemented with observations and remotely sensed imagery, are used to assemble a break-up chronology and examine spatial and temporal patterns of break-up flooding. Hydroclimatic controls on break-up are assessed by statistical, qualitative, and trend analysis of upstream discharge and downstream ice characteristics. For the most severe break-up flooding, two event types are identified: ice-driven events, with high backwater and high peak levels in the southern, eastern and western delta, and discharge-driven events, with high levels in the mid and outer delta and along Middle Channel. Break-up initiation during ice (discharge) events occurs earlier (later) than the delta average. Severity of break-up water levels is most influenced by upstream discharge, while timing is related to ice conditions and spring hydrograph rise. Rapid upstream melt and lower intensity melt in the delta prior to break-up characterize the most severe events. Trend analysis reveals a tendency toward earlier break-up, a longer prebreak-up melt interval, and a lower magnitude of hydroclimatic controls.

Concern has been expressed regarding the impacts of climate change on the hydroecology of the Mackenzie Delta, thus identifying a need for better understanding of the ice break-up regime. Archived records at hydrometric stations in the delta for the period 1974 to 2006, supplemented with observations and remotely sensed imagery, are used to assemble a break-up chronology and examine spatial and temporal patterns of break-up flooding. Hydroclimatic controls on break-up are assessed by statistical, qualitative, and trend analysis of upstream discharge and downstream ice characteristics. For the most severe break-up flooding, two event types are identified: ice-driven events, with high backwater and high peak levels in the southern, eastern and western delta, and discharge-driven events, with high levels in the mid and outer delta and along Middle Channel. Break-up initiation during ice (discharge) events occurs earlier (later) than the delta average. Severity of break-up water levels is most influenced by upstream discharge, while timing is related to ice conditions and spring hydrograph rise. Rapid upstream melt and lower intensity melt in the delta prior to break-up characterize the most severe events. Trend analysis reveals a tendency toward earlier break-up, a longer prebreak-up melt interval, and a lower magnitude of hydroclimatic controls.