Decreases in diatom cell size during the 20th century in the Laurentian Great Lakes: A response to warming waters?

Several authors have postulated that lake warming favors diatom taxa characterized by smaller cell sizes and suggested that this phenomenon may affect freshwater phytoplankton communities worldwide. Here, we examined historical (∼1900-2015) trends in diatom cell size in the Laurentian Great Lakes. Cell size decreased in Lakes Superior, Erie and Ontario, while no significant trends were observed for Michigan and Huron. In Lakes Superior and Ontario, cell size within species decreased over the course of the 20th century, suggesting demographic shifts toward smaller, later-generation individuals. Contrastingly, species-specific mean cell size increased in Michigan and Erie, likely as a result of accelerated loss rates during summer stratification. Size-specific rates of relative abundance change (larger taxa decreased while smaller taxa increased), were observed in all lakes except Michigan. These shifts toward communities dominated by smaller celled taxa either reinforced (Superior, Huron, Ontario) or dampened (Michigan, Erie) the influence of demographic shifts. Notwithstanding the influences of multiple stressors on diatom cell size at the within-lake scale, we demonstrated a gradual (5.11 μm 3 /y) decline in mean diatom cell size across the basin. Historical, basin-wide decreases in cell size demonstrate the likelihood of climate change driving changes in the primary producer community of large, freshwater systems.