Responses of Fish and Zooplankton to Climate Variation on the Prairies, and Their Sensitivity to Climate Change

Abstract

Climate change is anticipated to significantly increase temperatures and alter current rainfall patterns, which will have important ramifications for aquatic habitats and their biological communities. Current observations indicate that climate effects will vary depending on region and lake type, and some lacustrine areas, such as the Great Plains, are particularly sensitive to these effects. Variations in local climate and lake morphometry create different habitats, which each have unique environmental controls. The anticipated impacts of climate change on aquatic biota can be difficult to evaluate because of potentially contrasting effects of temperature and hydrology on lake ecosystems, particularly in closed-basin lakes within semi-arid regions. To address these challenges, I quantified decade-scale changes in chemical and biological properties of 20 endorheic lakes in central North America in response to a pronounced transition from a drought to a pluvial period during the early 21st century. Lakes exhibited marked changes in chemical characteristics and formed two discrete clusters corresponding to periods of substantially differing effective moisture (as per Palmer Drought Severity Index, PDSI). Discriminant function analysis (DFA) explained 90% of variability in fish assemblage composition and showed that fish communities were predicted best by environmental conditions during the arid interval (PDSI < -2). DFA also predicted that lakes could support more fish species during pluvial periods, but their realized occurrences may be limited by periodic stress due to recurrent droughts and physical barriers to colonization. Zooplankton taxonomic compositions in fishless lakes were resilient to short-term changes in meteorological conditions, and did not vary between drought and deluge periods. Conversely, zooplankton taxa that were exposed to fish decreased substantially in biomass during the wet interval, likely due to increased zooplanktivory by fish. Based on my results, climate change is expected to alter fish species distributions, but it is less clear to what extent non-lethal environmental effects will influence physical health of populations in fish-habitable lakes. To address this question, I investigated the environmental controls of body condition and parasite load in walleye (Sander vitreus), northern pike (Esox lucius) and yellow perch (Perca flavescens) in seven lakes from the prior study. Over a two-year observation period (2009 vs. 2010), I observed large differences in the number of days within the favorable temperature range for ambient fish species. Surprisingly, environmental variables such as lake morphometry and nutrient levels had little relevance, despite their importance in previous studies conducted in boreal lakes. Instead, temperature and salinity were important correlates of fish health. In regard to species-specific effects, walleye was most sensitive to interannual temperature differences, as well as salinity, while yellow perch and northern pike exhibited temperature sensitivity to a lesser degree. Apparently, temperature increases are of particular concern in prairie lakes, as their polymictic nature deprives fishes of a hypolimnetic thermal refuge. Together these findings suggest that semi-arid lakes provide a useful model system for anticipating the effects of global climate change on aquatic communities in closed-basin lakes of semi-arid regions. The particular importance of temperature and salinity indicates that the interaction of global climate change and local hydrology may have particularly detrimental effects not only on the health but also the survival of established fish populations of the Great Plains.