Winter dynamics in mountain lakes and impacts of an introduced species to the endangered Devils Hole pupfish

Abstract

Having intrinsic knowledge of our aquatic ecosystems lets us predict how they will respond to change and allows us to make better management decisions. Changes in climate and introduced species are two major threats to aquatic ecosystems. This research explores inherent characteristics of two aquatic ecosystems facing different threats; mountains lakes facing the threat of climate change and an endangered pupfish species in a limnocrene spring, facing the threat of a new colonizing species. Climate change is causing ice-periods on lakes to shorten globally and elevation-dependent warming means mountain lakes may be at a higher risk of warming and ice loss. Winter, under-ice studies of mountain lakes are limited and in order to understand the potential impacts of shorter ice periods, we must first understand the differences in basic ecological attributes of ice-cover and ice-free seasons in mountain lakes. Chapter 1 analyzed primary productivity rate, phytoplankton biovolume and composition, zooplankton biomass and composition, nutrient concentrations, and hypolimnetic dissolved oxygen of 10 global mountain lakes in ice-covered (winter) and ice-free (summer) seasons. Productivity was 44% lower in winter compared to summer but varied in magnitude by lake. Winter phytoplankton biovolume and zooplankton abundance were 35% (p=0.01) and 25% (p= 0.01) of the summer values, respectively. Phytoplankton and zooplankton compositions were not significantly different between seasons at the class level of taxonomic analysis. Total dissolved nitrogen was 78% higher in winter compared to summer (p= 0.001) indicating nitrogen build-up during the winter months. Hypolimnetic dissolved oxygen concentrations were variable across lakes, indicating that lake thermal stratification may play a role in governing concentrations. This study shows winter is not the dormant season of which it has been historically characterized and incites the need for additional winter limnological sampling to better understand and predict how changes in climate will impact mountain lake ecosystems.Colonization by new species which become a nuisance is another major threat to aquatic ecosystems. The endangered Devils Hole pupfish (Cyprinodon diabolis) population is declining for unknown reasons and changes to algae and macroinvertebrate communities may be attributed to the recent decline. Chapter 2 examines the direct and indirect impacts of a newly established predaceous diving beetle (Neoclypeodytes cinctellus) on the Devils Hole pupfish through predator-preference experiments and diet reconstruction over time of the Devils Hole pupfish. Results from this study indicate the diving beetle is likely preying on C. diabolis eggs and early life stages, and on ostracods that may be important to the pupfish diet. The trophic position of fishes indicates direct herbivory of fishes on primary food sources (algae and plants), and invertebrates that are grazing on primary food sources. Compound specific, stable isotope amino acids analysis suggests a much more complex contribution of carbon sources to C. diabolis within a sample and across years, including contributions from autochthonous (algae and cyanobacteria, and fungus), and allochthonous (C3 and C4 plants). Decreases in invertebrates and increases in inorganic carbon in gut contents indicate higher foraging and potential food limitations. This study suggests the changing Devils Hole community may be contributing to the decline in C. diabolis populations.