Long-term epilimnetic temperature trends in Lake Mendota and Trout Lake, Wisconsin

Warming of lake surface waters has become a concern to limnologists and water managers because air temperatures, which directly affect near-surface water temperatures, are projected to increase in Wisconsin (WICCI 2011) as well as globally (IPCC 2018). This projected increase is in addition to the changes in air temperatures that have already occurred in recent decades(WICCI 2011, NOAA 2017).The deleterious effects of increased temperatures in lake surface waters have been extensively reviewed (e.g., Blenckner 2005, Keller 2007, Adrian et al. 2009, George 2010). Briefly, the exceedance of thermal preferences or tolerances of aquatic biota can cause altered food webs and loss of biodiversity in lakes (DeStasio et al. 1996, Chu et al. 2005, Graham and Harrod 2009, Woodward et al. 2010, Comte et al. 2013). Warmer surface water temperature scan result in stronger and longer thermal stratification in deep lakes(Robertson and Ragotzkie1990, Hondzo and Stefan 1993, Livingstone 2003, Butcher et al. 2015). This process in turn can cause the duration and extent of hypolimnetic anoxia to increase, thus reducing hypolimnetic refugia needed for cold-and cool-water fish (De Stasio et al. 1996, Magnuson et al. 1997, Jeppesen et al. 2012, Missaghi et al. 2017). Longer duration of hypolimnetic anoxia can enhance eutrophication because of more internal loading of phosphorus from bottom sediments (Blenckner et al. 2002, North et al. 2014). Of particular concern, warmer water temperatures favor the growth of toxic cyanobacteria in eutrophic systems (Paerl and Huisman 2008, Wagner and Adrian 2009, Kosten et al. 2012).Another effect of warmer lake surface temperatures is increased evaporation that can result in lower water levels(Spence et al. 2013, Gronewold and Stow 2014).