Differential effects of the urban heat island on thermal responses of freshwater fishes from unmanaged and managed systems.

A lack of understanding exists regarding how freshwater species will respond to increases in temperature associated with ongoing changes in climate. Non-urban to urban thermal gradients generated by urban heat islands can serve as models to characterize the effects of relatively consistent increases in temperature on freshwater ecosystems over several decades. This study investigates the apparent responses of two freshwater fish species, Campostoma anomalum (Central Stoneroller) and Lepomis macrochirus (Bluegill), to directional changes in temperature over the past century across the non-urban to urban gradient in the Saint Louis, Missouri region in the central United States. Differences in air temperature across this gradient have increased by approximately 3 °C since 1920. Critical thermal maximum (CTMax) assays were conducted on individuals from fish populations across this gradient from either streams (C. anomalum) or ponds (L. macrochirus) to assess whether thermal tolerance is associated with water temperature among sites. According to expectations based on the effect of an urban heat island, maximum water temperature at stream sites was positively correlated with percent urban landcover around the sites. Moreover, CTMax among populations of C. anomalum was positively correlated with maximum water temperature at each site, suggesting that this species has likely responded to increases in temperature over the past several decades. There was no relationship between percent urban landcover and maximum water temperature in the pond systems. There was also no relationship between CTMax and maximum water temperature among L. macrochirus populations. The pond systems and populations of L. macrochirus are highly managed, which may limit local physical and biological responses to increases in air temperature. Results suggest that freshwater habitats in urban environments and the species inhabiting these areas are responding differently to recent increases in air temperature, highlighting the complexity of the physical and biological components of these systems.