Non-native consumers can significantly alter processes at the population, community, and ecosystem level, and they are a major concern in many aquatic systems. Although the community-level effects of non-native anuran tadpoles are well understood, their ecosystem-level effects have been less studied. Here, I tested the hypothesis that natural densities of non-native bullfrog tadpoles (Lithobates catesbeianus) and native Woodhouse's toad tadpoles (Anaxyrus woodhousii) have dissimilar effects on aquatic ecosystem processes because of differences in grazing and nutrient recycling (excretion and egestion). I measured bullfrog and Woodhouse's carbon, nitrogen, and phosphorus nutrient recycling rates. Then, I determined the impact of tadpole grazing on periphyton biomass (chlorophyll a) during a 39-day mesocosm experiment. Using the same experiment, I also quantified the effect of tadpole grazing and nutrient excretion on periphyton net primary production (NPP). Lastly I measured how dissolved and particulate nutrient concentrations and respiration rates changed in the presence of the two tadpole species. Per unit biomass, I found that bullfrog and Woodhouse's tadpoles excreted nitrogen and phosphorus at similar rates, though Woodhouse's tadpoles egested more carbon, nitrogen, and phosphorus. However, bullfrogs recycled nutrients at higher N:C and N:P ratios. Tadpole excretion did not cause a detectable change in dissolved nutrient concentrations. However, the percent phosphorus in mesocosm detritus was significantly higher in both tadpole treatments, compared to a tadpole-free control. Neither tadpole species decreased periphyton biomass through grazing, although bullfrog nutrient excretion increased areal NPP. This result was due to higher biomass, not higher biomass-specific productivity. Woodhouse's tadpoles significantly decreased respiration in the mesocosm detritus, while bullfrog tadpoles had no effect. This research highlights functional differences between species by showing non-native bullfrog tadpoles and native Woodhouse's tadpoles may have different effects on arid, aquatic ecosystems. Specifically, it indicates bullfrog introductions may alter primary productivity and particulate nutrient dynamics.
Humans have dramatically increased phosphorus (P) availability in terrestrial and aquatic ecosystems. As P is often a limiting nutrient of primary production, changes in its availability can have dramatic effects on ecosystem processes. I examined the effects of calcium carbonate (CaCO3) deposition, which can lower P concentrations via coprecipitation of phosphate, on P availability in two systems: streams in the Huachuca Mountains, Arizona, and a stream, Río Mesquites, in Cuatro Ciénegas, México. Calcium carbonate forms as travertine in the former and within the microbialites of the latter. Despite these differences, CaCO3 deposition led to lowered P availability in both systems. By analyzing a three-year dataset of water chemistry from the Huachuca Mountain streams, I determined that P concentrations were negatively related to CaCO3 deposition rates. I also discovered that CaCO3 was positively correlated with nitrogen concentrations, suggesting that the stoichiometric effect of CaCO3 deposition on nutrient availability is due not only to coprecipitation of phosphate, but also to P-related constraints on biotic nitrogen uptake. Building from these observations, bioassays of nutrient limitation of periphyton growth suggest that P limitation is more prevalent in streams with active CaCO3 deposition than those without. Furthermore, when I experimentally reduced rates of CaCO3 deposition within one of the streams by partial light-exclusion, areal P uptake lengths decreased, periphyton P content and growth increased, and periphyton nutrient limitation by P decreased. In Río Mesquites, CaCO3 deposition was also associated with P limitation of microbial growth. There, I investigated the consequences of reductions in CaCO3 deposition with several methods. Calcium removal led to increased concentrations of P in the microbial biomass while light reductions decreased microbial biomass and chemical inhibition had no effect. These results suggest that CaCO3 deposition in microbialites does limit biological uptake of P, that photoautotrophs play an important role in nutrient acquisition, and, combined with other experimental observations, that sulfate reduction may support CaCO3 deposition in the microbialite communities of Río Mesquites. Overall, my results suggest that the effects of CaCO3 deposition on P availability are general and this process should be considered when managing nutrient flows across aquatic ecosystems.
Permanently shadowed regions (PSRs) at the lunar poles may contain substantial water ice, but radar signatures at PSRs could indicate water ice or large block populations. Mini-RF radar and Lunar Reconnaissance Orbiter Camera Narrow Angle Camera (LROC NAC) products were used to assess block abundances where radar signatures indicated potential ice deposits. While the majority of PSRs in this study indicated large block populations and a low likelihood of water ice, one crater – Rozhdestvenskiy N – showed indirect indications of water ice in its interior.
Chloride deposits indicate regions where the last substantial liquid water existed on Mars. Major ion abundances and expected precipitation sequences of terrestrial chloride brines could provide context for assessing the provenance of martian chloride deposits. Chloride minerals are most readily distinguished in the far-infrared (45+ μm), where their fundamental absorption features are strongest. Multiple chloride compositions and textures were characterized in far-infrared emission for the first time. Systematic variations in the spectra were observed; these variations will allow chloride mineralogy to be determined and large variations in texture to be constrained.
In the present day, recurring slope lineae (RSL) may indicate water flow, but fresh water is not stable on Mars. However, dissolved chloride could allow liquid water to flow transiently. Using Thermal Emission Imaging System (THEMIS) data, I determined that RSL are most likely not fed by chloride-rich brines on Mars. Substantial amounts of salt would be consumed to produce a surface water flow; therefore, these features are therefore thought to instead be surface darkening due to capillary wicking.
The discovery of cryovolcanic features on Charon and the presence of ammonia hydrates on the surfaces of other medium-sized Kuiper Belt Objects suggests that cryovolcanism may be important to their evolution. A two-dimensional, center-point finite difference, thermal hydraulic model was developed to explore the behavior of cryovolcanic conduits on midsized KBOs. Conduits on a Charon-surrogate were shown to maintain flow through over 200 km of crust and mantle down to radii of R = 0.20 m. Radii higher than this became turbulent due to high viscous dissipation and low thermal conductivity. This model was adapted to explore the emplacement of Kubrik Mons. Steady state flow was achieved with a conduit of radius R = 0.02 m for a source chamber at 2.3 km depth. Effusion rates computed from this estimated a 122 - 163 Myr upper limit formation timescale.