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Lake Superior is a forcing factor for local weather systems, causing substantial amounts of lake effect snow in the winter (particularly on the south shore). This study assesses decreasing ice cover of Lake Superior and its effects upon synoptic weather factors. Data were collected from eleven National Weather Service (NWS) stations located on the south shore of the lake. Rainfall and snowfall amounts from December to May were regressed on percent ice coverage and average monthly temperatures from 1972-2002. Ice coverage and average monthly temperature had a negative relationship with snowfall and rainfall.

Over the past few decades, there has been increased research focus on carbon cycling within aquatic systems, especially with the changing global climate. Inland waters play a major role in the global carbon cycle, but the fundamental features remain poorly understood, particularly the large lakes of the world. Our experimental approach assessing the carbon budget of Lake Superior, the largest freshwater lake by area, provides spatial and temporal variability that has been previously overlooked but may be critical to our understanding on the biogeochemical processes controlling the lake. Multiple stations were chosen across the lake, both nearshore and offshore, to evaluate the variability in physical mixing regimes and biogeochemical processing. Short and long-term carbon consumption measurements were coupled to assess the heterotrophic activity relative to the lability of dissolved organic carbon. Partial pressure of carbon dioxide (pCO2) was directly measured to determine the metabolic nature of the lake and the amount of carbon dioxide (CO2) that fluxes across the air-water interface. The pCO2 results were further coupled with an isotopic approach measuring oxygen-18 (δ18O) to evaluate how the metabolism of Lake Superior has changed over a decadal scale. A range of environmental factors, including temperature, photodegradation and source/quality of organic carbon, influenced short and long-term carbon consumption. In-situ pCO2 observations supported a temporal switch in metabolism from the lake being a source of CO2 in the spring to being a sink in the summer driven by biological components of the system. When the pCO2 results were coupled with the isotopic measurements over the past decade (1999-2011), Lake Superior was dominated by respiration during isothermal conditions and production during stratification. In the past decade, Lake Superior has experienced increased surface water temperatures, shifting the metabolic state to a shorter net heterotrophic period in the spring and a longer net autotrophic period in the summer. This research highlights fundamental aspects of Lake Superior’s metabolism that have been previously understudied, as well as providing key information about processes controlling its carbon budget, and giving a better understanding of how climate change will continue to impact Lake Superior.

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The propensity for animals to migrate can vary markedly among species, populations within species, and individuals within populations. Many taxa displaying variation in migratory behaviour and life history are attracting conservation concern as migratory populations decline in abundance. I developed a conceptual framework for delineating the nature of the broad phenotypic variation observed among and within migratory populations and apply this to Lake Superior brook trout (Salvelinus fontinalis). My literature review of variation in the migratory behaviour of fishes revealed that subsets of the predictions outlined in my framework have been tested for most study systems, but few systems had tested all five predictions. My field studies of Lake Superior brook trout (i) compared isotopic signatures of brook trout sampled from Lake Superior and tributary streams to demonstrate that the fish sampled from the lake and stream differ consistently in habitat use and trophic ecology, (ii) applied age and growth analyses to demonstrate that fish sampled from the lake live longer and grow faster than fish sampled from the streams, with the differences in growth apparent by the end of the first year of life, (iii) used histological measures to demonstrate that fish residing in streams, and exhibiting a stream growth history, reach sexual maturity, and (iv) provided no evidence based on growth histories to suggest that individuals change migratory behaviours later in life. Together, these field studies suggest that partial migration is the best hypothesis to describe the variation observed in the migratory behaviour of brook trout in the Nipigon Bay area of Lake Superior. Although the possibility remains that some individuals could complete their life cycle in the lake. My review and field studies demonstrate that the conceptual framework can be used to help delineate variation in the migratory behaviour exhibited by fishes whose lifetime movements can be difficult to track. / Funding was provided by the Ontario Ministry of Natural Resources through the Canada-Ontario Agreement Respecting the Great Lakes Basin Ecosystem, Great Lakes Fishery Commission Fishery Research Program, a Natural Science and Engineering Research Council (NSERC) Postgraduate Scholarship, and a NSERC Discovery Grant.

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