Contemporary abundance patterns of Cyclotella sensu lato diatom taxa in Lake Superior: Assessing responses to physical and chemical gradients and potential links to climate change

Many studies attribute shifts in relative abundances of Cyclotella sensu lato diatom taxa in freshwater lakes over the past 150 years to climate change. Although abundances of several of these taxa have also changed in Lake Superior over the last century, the specific drivers are currently undear. This study used multiple linear regression to analyze long-term planktonic monitoring data to identify drivers of changing cell densities of six Cyclotella sensu lato species, four shown to be responsive to climate-driven change in smaller lakes and two species with similar morphology to two previously assessed taxa: Discostella stelligera and the morphologically similar D. pseudostelligera, Lindavia comensis and the morphologically similar L. delicatula, Lindavia ocellata, and Lindavia bodanica. Assessed variables associated with climate-driven physical change included surface water temperature and light availability, with turbidity and thermocline depth as indicators of the light environment. One to three variables explained 8-61% variation for each of 5 taxa from 2001 to 2011, significant predictive variables were not identified for D. stelligera. Multiple linear regression identified nutrients, including nitrate, phosphorus, and silica, as essential in explaining distributions of all the other assessed taxa. A physical variable associated with climate explained changes of one taxon: L. ocellata, which increased with lower water temperatures. Climate-driven species responses were not as apparent as in other systems; specifically, changes in the light environment of this clear lake did not explain abundance changes in any of the taxa. Multiple and complex environmental variables drive Cyclotella sensu lato abundance patterns in Lake Superior, possibly suggesting ecotype formation. (C) 2018 International Association for Great Lakes Research. Published by Elsevier B.V. All rights reserved.