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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...

A linkage between Cladophora mats and exceedances of recreational water quality criteria has been suggested, but not directly studied. This study investigates the spatial and temporal association between Escherichia coli concentrations within and near Cladophora mats at two northwestern Lake Michigan beaches in Door County, Wisconsin. Escherichia coli concentrations in water underlying mats were significantly greater than surrounding water (p < 0.001). Below mat E. coli increased as the stranded mats persisted at the beach swash zone. Water adjacent to Cladophora mats had lower E. coli concentrations, but surpassed EPA swimming criteria the majority of sampling days. A significant positive association was found between E. coli concentrations attached to Cladophora and in underlying water (p < 0.001). The attached E. coli likely acted as a reservoir for populating water underlying the mat. Fecal bacterial pathogens, however, could not be detected by microbiological culture methods either attached to mat biomass or in underlying water. Removal of Cladophora mats from beach areas may improve aesthetic and microbial water quality at affected beaches. These associations and potential natural growth of E. coli in bathing waters call into question the efficacy of using E. coli as a recreational water quality indicator of fecal contaminations.

We conducted a series of nutrient manipulation experiments over the first 5 y of Dreissena colonization in Saginaw Bay, Lake Huron, to evaluate benthic algal nutrient limitation and community composition. We placed nutrient-diffusing substrata in the littoral zone of the Bay during 1991 (early Dreissena colonization) and from 1992 to 1995 (post-Dreissena colonization). The treatments consisted of P, N, and PϩN additions, and a control. Chlorophyll a decreased through time from 1992 to 1995. Phosphorus limited biovolume only in 1994. Treatments with P additions had significantly more chlorophyll a than the controls each year after 1992. This result was consistent with an observed decrease in dissolved P throughout the study. Nitrogen additions had no significant effect throughout the 5-y period. Major shifts in species composition did not result from nutrient additions but rather seemed to be consistent with changes in light penetration and Dreissena herbivory. Our data demonstrated that the pre-Dreissena benthic algal community was dominated by tychoplanktonic diatoms (i.e., Aulacoseira granulata and Tabellaria fenestrata), which would be susceptible to filter-feeding Dreissena. Early postinvasion conditions were marked by an increase in light penetration, and benthic algae were dominated by filamentous green algae (mostly Spirogyra sp.). Late post-invasion conditions were marked by a reduction of light caused by planktonic blooms of Microcystis sp., which were resistant to zebra mussel herbivory. The benthic algal dominance shifted to periphytic diatoms (i.e., Gomphonema clevei), which were also resistant to zebra mussel filter-feeding. A new equilibrium may be developing where Dreissena herbivory limits tychoplanktonic diatoms, which promotes Microcystis bloom, which in turn limits Dreissena filtering rates.

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