We present the first evidence of biological change in all of the pelagic Laurentian Great Lakes associated with recent climatic warming. We hypothesized that measured changes in lake temperature, and the resulting physical changes to water columns, were affecting diatom communities in the Great Lakes. A paleolimnological analysis of 10 sediment cores collected from deep locations throughout the Great Lakes basin indicates a recent (30-50 yr) reorganization of the diatom community to one characterized by elevated abundances of several species from the group Cyclotella sensu lato, coinciding with rising atmospheric and water temperatures. These Cyclotella increases are a probable mechanistic result of new physical regimes such as changing stratification depths and longer ice-free periods, and possibly water quality shifts. Efforts to understand the mechanisms of these changes are ongoing, but this compositional reorganization in primary producers could have important implications to Great Lakes food webs.As atmospheric warming continues, seasonal exposure of lake water columns to sunlight lengthens and ice periods become shorter and less intense. The frequency and duration of stratification events may be expected to increase, as has been observed in Lake Superior (Austin and Colman 2008). Changes to the biota of the Laurentian Great Lakes due to warming are likely occurring. The Great Lakes comprise almost 90% of the surface water resources of North America, so it is imperative to understand the effect of climate shifts on general biology and food webs.Studies of species shifts driven by climate have largely focused on larger flora and fauna (Smol 2012) and usually track how a species alters its phenology or shifts its range. For instance, as a region becomes warmer and wetter, some plants and animals will thrive while other species populations will fragment, shrink and be driven to extinction. Some species adjust to climate change by moving outside their historical geographic boundaries. For phytoplankton, a climate-driven shift tends to mean a change in the dominant flora and reduction or extirpation of previously dominant taxa. It has been recognized in paleorecords from freshwater aquatic systems that phytoplankton, particularly diatoms, exhibit the greatest modification due to recent climate change, followed by invertebrates such as cladocerans and chironomids (R€ uhland et al. 2014). For instance, diatoms in a dated sediment core from Lake of the Woods, northwestern Ontario, were compared to instrumental records (R€ uhland et al. 2008) and Cyclotella sensu lato taxa increased concurrently with a lengthening ice-free period (by almost 30 d in the past 40 yr) and increasing water temperature, providing evidence that warming played a role in diatom community re-organization. Also, a decline in wind speeds over the past 50 yr was significantly correlated with the increasing relative abundance of Cyclotella. Atmospheric warming and associated changes in lake thermal properties were the most plausible ex...

Lake Ontario's condition has fluctuated since European settlement, and our understanding of the linkages between observed ecosystem shifts and stressors is improving. Changes in the physical and chemical environment of the lake due to non-indigenous species, pollution, sedimentation, turbidity, and climate change altered the pelagic primary producers, so algal assessments have been valuable for tracking long-term conditions. We present a chronological account of algal assessments to summarize past and present environmental conditions in Lake Ontario. This review particularly focuses on pelagic, diatom-based assessments as their fossils in sediments have revealed the combined effects of environmental insults and recovery. This review recaps the long-term trends according to three unique regions: Hamilton Harbour, the main lake basin, and the Bay of Quinte. We summarize pre-European impact settlement; eutrophication throughout most of the 20th century; subsequent water quality changes due to nutrient reductions; and filter-feeding dreissenid colonization and contemporary pelagic, shoreline, and embayment impairments. Recent data suggest that although phytoplankton biovolume is stable, species composition has shifted to an increase in densities of spring eutrophic diatoms and summer cyanobacteria. Continued monitoring and evaluation of historical data will assist in understanding and responding to the natural and anthropogenic drivers of Lake Ontario's environmental conditions.

No abstract

The Laurentian Great Lakes are among the planet’s fastest-warming lakes. Recent paleolimnological studies have shown changes in the diatom community of the system, including shifts towards taxa characteristic of strongly stratified systems and ongoing cell-size diminution. Relationships between species’ cell size and establishment in—or extirpation from—the system have not been addressed. Examining patterns of establishment and extirpation provides insight into the effects of multiple stressors at the ecosystem scale. We evaluate the timing of the establishment or extirpation of diatom taxa from fossil records post-European settlement within the Great Lakes as a function of cell size. Relationships between establishment or extirpation date and cell size were not random, and were best expressed as cubic curves. Generally, large taxa became established early in the record, while establishments of smaller taxa continued apace until the late 20th century. Extirpations of taxa of all sizes accelerated in the late 20th and early 21st centuries, and large-celled taxa were disproportionately extirpated over the last two decades. We discuss the implications of these relationships on the overall cell-size characteristics of the community, and consider the influences of propagule pressure, nutrient status, species invasions, and climate change upon diatom establishment and extirpation.

No abstract