In the desert Southwest, predation and monsoon thunderstorms are determinants of aquatic invertebrate communities. Recent research has focused on the potential of non-native species, like bullfrogs (Rana catesbeiana), to change communities into which they are introduced. Bullfrogs were introduced into Arizona in the mid-1900s and have established widespread populations that predate on and compete with native fauna. In an effort to control bullfrog populations, state and federal agencies have focused on the eradication of bullfrogs in Southeast Arizona – a biodiversity hotspot with many endemic species. Although the effects of bullfrogs on vertebrate species are documented, how and whether bullfrogs alter aquatic invertebrate communities remains uncertain. We examined how the effects of bullfrogs compare to the seasonal effects of the monsoon in influencing invertebrate communities. The objectives of this study were to 1) examine and quantify the effects of bullfrogs on aquatic invertebrate communities by comparing ponds with and without bullfrogs; 2) determine the effect of monsoonal seasonality by sampling invertebrate communities during the early- and late-monsoon, and 3) observe the interaction of bullfrogs and monsoonal seasonality to explore the relative importance of these two factors in shaping aquatic invertebrate communities. We hypothesized a lower invertebrate abundance and richness in ponds with bullfrogs. We expected to see taxonomic differences in community composition based on body size, dispersal ability, habitat use, feeding group, and defense. We anticipated smaller invertebrates and a higher number of defended invertebrates in ponds with bullfrogs. Finally, we expected that the relative abundance of high dispersing insects would increase in late monsoon samples. Preliminary analyses suggest that invertebrate abundance and richness is negatively related to the number of bullfrogs present at a pond. By studying the effects of bullfrogs on aquatic invertebrate communities, we can begin to understand and quantify the impact of this heavily managed non-native predator.

This project seeks to better understand climate change corollary to the migration of the Intertropical Convergence Zone (ITCZ) in the tropical Pacific from the Little Ice Age (~AD 1400-1850) to the present using hydrogen isotopes as a proxy for climate change. During this time, the ITCZ is hypothesized to have reached its most southern location, the consequences of which were evident on a global scale. However, more research is needed to further investigate this hypothesis. Specifically, this study examines hydrogen isotope ratios of the source specific lipids Taraxerol and Dinosterol from Lake Escondido, Isabela Island, Galapagos. Lipids were solvent-extracted from sections of the cores, separated through silica gel columns, and the ratio of Deuterium and Hydrogen (δD) of specific lipids was quantified using a Gas Chromatograph-Isotope Ratio Mass Spectrometer. The combined fluxes of both terrestrially and aquatically sourced δD ratios over time provide a proxy for climate change through the present. Due to the widespread effects of the ITCZ on global hydrology, understanding its movement, and what triggers migration, can provide insight into future climate scenarios.

Over the past decade, measurements of hydrogen isotopes of lipid biomarkers, in particular leaf waxes, have been used to reconstruct past climate change. This method is appealing because lipids are well preserved in sediment and their hydrogen isotope ratio is primarily controlled by the hydrogen isotope ratio of their source water, which itself is controlled by factors such as precipitation amount and temperature. However, it has been shown that the isotopic difference between source water and lipids is distinctive among different plant species. In particular, it has been shown that salinity strongly affects the hydrogen isotope ratio of lipids produced by mangroves, but the mechanism that controls this relationship is unknown. Plants, including mangroves, take in water mainly from surrounding source water, but the deuterium to hydrogen ratio within the plant may be impacted by differences in transpiration rate, photosynthetic rate and respiratory water production. The photosynthetic rate (mol CO2 m-2s-1), internal carbon, (μmol CO2/ mol air) and conductance (mol H2O m-2s-1) of three species of mangrove, Rhizophora apiculata, Avicennia germinans, and Xylocarpus granatum, were measured at six different salinities from 5 to 30 ppt. The date, time, and leaf number were also recorded as additional measurements. Determining whether salinity, time of day and/or differences among species affects photosynthetic rate, internal carbon, and conductance is important in comparing the biosynthetic pathways of the three species of mangroves. It will allow us to investigate the cause of increased deuterium discrimination in mangroves at high salinity.

The fact that hydrogen isotope deuterium/protium (D/H) ratios in algal lipids co-vary with their surrounding water D/H ratios is well established. Paleoclimatologists have taken advantage of this relationship in their compound-specific D/H analyses, but little is known about the individual environmental parameters, such as temperature, salinity, light levels, and nutrient concentrations that influence D/H fractionation, or the state of unequal isotope composition. Laboratory experiments conducted with batch cultured green algae, Eudorina unicocca and Volvox aureus, grown at temperatures of 15°C and 25°C suggest that there is increased fractionation with increased temperature. The work presented here aims to elucidate the magnitude of D/H fractionation in a natural population of algae where temperature is the primary environmental variable. To test this relationship, surface water was sampled from Lake Washington once a month for one year, with a surface temperature range from ~8°C in February to ~24°C in August. Phytoplankton were filtered from the water and lipids are extracted and analyzed by gas-chromatography isotope ratio mass spectrometry (GC-IRMS) to determine the extent of D/H fractionation. We found that there was a positive correlation between temperature and D/H fractionation, in which fractionation increased with increasing temperature. The results from this research, as well as the results of similar experiments with other environmental parameters, will give paleoclimatologists accurate modern-day calibrations to apply to the analysis of lipids extracted from sediment cores in order to reveal climatic variations for the past.