COMPARING SEDIMENT RECORDS AND MODEL HINDCASTS OF RECENT CLIMATE CHANGE IN BOREAL LAKES

Lake sediment records are a powerful tool for reconstructing past changes in climate over multiple time scales, yet they lack predictive power to forecast limnological responses to the impacts of human-induced global warming. The problem lies with both the complexities of lake-climate response as well as variability among lakes in depth, size, and watershed characteristics. Climate change may disrupt aquatic ecosystems directly through changes in temperature, wind, and precipitation, indirectly through watershed effects, and in concert with man-made stressors such as nutrient pollution, invasive species, and land-use change. Recent studies have documented possible climate-induced changes in boreal-region lakes, including a longer ice-free season, stronger thermal stratification, increased inputs of dissolved organic carbon, shifts in algal communities, increased carbon burial, and an increased frequency of cyanobacterial blooms. To elucidate the mechanisms by which climate impacts lakes we used retrospective thermal modeling of temperature-depth relationships generated with MINLAKE2012 and compared model output with historical biological and physical changes in lakes as interpreted from dated lake sediment cores. Eight lakes spanning a range of surface areas and depths were studied from Voyageurs and Isle Royale National Parks (Minnesota/Michigan, USA). Models were developed for each lake spanning 1960–2011. The most common trend was increasing summer shallow-water temperatures across two time periods (1962–1986, 1987–2011), followed by increased frequency and duration of thermal gradients of 2-3° C/m for deep lakes. Sediment core data (diatoms, biogenic silica) showed changes in diatom communities between 1960 and 2010 differed among shallower and deeper lakes and park units, with shallow lake warming affecting the abundance of benthic/tychoplanktonic forms and deep lake changes impacting deep chlorophyll layer communities and species that respond to length of spring mixing. In contrast, there was little consistency among lake response in historical diatom productivity beyond temporal consistency; in those lakes where biogenic silica flux shows changes between 1960–2010, flux generally began increasing in the 1970s and 1980s with rates continuing to climb to present-day.