Understanding century to millennial-scale variation in snowfall and extreme hydrologic events in the Sierra Nevada of California is hampered by short instrumental records (<120 years) and uncertainty caused by extrapolating paleoclimate data from lower elevation to the alpine, snow deposition zone. We investigated Holocene hydrologic variability and lake productivity in the alpine Sierra Nevada using a ca. 12,000 year record of sediments collected from Pear Lake (36°35'49"N, 118°40'29"W, 2904 m asl, maximum depth 24 m). High-resolution scanning revealed alternating, fine grained, light-dark bands (1 mm to 5 mm thick) for most of the core length. This pattern was interrupted at intervals by visually homogenous layers (up to 75 mm thick) containing coarse grain particles up to 10 mm in diameter. Bulk elemental and stable isotope measurements were used to investigate sources of organic matter in lake sediments and used as a proxy for aquatic productivity. Laser diffraction particle analysis was used to obtain particle size distributions of the core at 1 cm increments to quantify the frequency of periods of relative wetness (rain-on-snow flooding, large avalanches, atmospheric rivers) and dryness (decadal to centennial-scale droughts).

At Pear Lake, the frequency of pluvial episodes declined during the Holocene from 37 in the early Holocene (11250 to 7250 cal yr BP) to 22 episodes during the middle Holocene (7250 to 3000 cal yr BP) to 15 episodes during the late Holocene (3000 cal yr BP to present). High percent silt and silt unimodal distributions, which indicate drier conditions, were more frequent in the middle and late Holocene relative to the early Holocene. The highest concentration of dry periods occurred within the last 1500 years. The δ13C and δ15N of terrestrial and aquatic organic matter overlaps substantially in the Sierra Nevada, thus they do not indicate organic matter sources in Pear Lake sediments. However, correlations between elemental and stable isotope data in the middle and late Holocene may indicate the influence of climate change on lake productivity. Overall, the data suggest a wetter, more dynamic, early Holocene followed by a drier, more drought-proned middle Holocene, and a variable early Holocene with intense climate extremes.

Stocking of salmonid fish in naturally fishless Sierra Nevada lakes over the last century has had a profound impact on aquatic foodwebs. Current research has focused on removal of fish from several lakes to observe recovery of zooplankton, benthic invertebrates and, in particular, the endangered southern mountain yellow-legged frog (Rana muscosa). Biological samples from two currently fish containing lakes and two lakes where fish have been removed were collected over the last 15 years to determine recovery rates of species found in the lakes. Due to the remote location of the lakes, the samples were preserved with ethanol (EtOH) for later analysis. However, long-term preservation methods can have significant impacts on the δ13C values of stable isotopes by resulting in enrichment in isotope values. I developed a rinse method to remove EtOH from the archived samples and to reduce the impact preservatives on stable isotope

analysis. After removing the preservative, samples were rinsed for. The rinsing method resulted in enrichment of 13C suggesting a much shorter rinsing time (< 1 day) as well as species-specific corrections for δ13C content were required to improve the accuracy of stable isotope analysis of preserved samples.

I examined the trophic structure of lakes containing and lacking fish using carbon and nitrogen stable isotopes. δ15N values revealed distinct trophic fractionation values for each type of lake. Trophic fractionation for fish-containing and fish-removed lakes was 2.03 / ± 0.517 (n=116) and1.76 / ± 0.384 (n=186) respectively. Nitrogen values in fish-containing lakes species tended to be enriched relative to fish-removed lakes, suggesting increased P availability. Time-series of δ15N for multiple species indicated small variations in nitrogen suggesting changes in diet from year to year. However significant depletion of 4.42 / was noted for the copepod Leptodiaptomus signicauda and correlated with the reemergence of Daphnia melanica post fish removal. Computations of trophic position indicated that frogs and fish occupy the same trophic level and likely rely on the same food sources. This finding suggests that fish cause extirpation of frogs both through direct predation and competition for food resources.

Human activities have more than tripled the amount of N and P available to ecosystems, resulting in complex effects on the environment. The objectives of this study are to evaluate the N saturation status of chaparral ecosystems exposed to elevated atmospheric N deposition and identify mechanisms for the destabilization of P in soils and lacustrine sediments that may be contributing to the eutrophication of high-elevation Sierra Nevada lakes. The N saturation status of a chaparral catchment was assessed through an N budget including atmospheric N inputs, hydrologic and gaseous N outputs, belowground N dynamics, and isotopic separation of streamwater nitrate sources. Phosphorus pools in soil and lake sediment were identified and monitored seasonally to understand how changes in physicochemical conditions and hydrology affect the transport of P to Sierran lakes. In chaparral, seasonal transitions, microbial C limitation, and the asynchrony between N availability and plant N demand modulated the kinetic N saturation of the catchment. The transition from dry to wet soil conditions induced rapid nitrification of ammonium leading to large hydrologic and gaseous N losses. However, during the growing season, N losses were minimal highlighting the limitations of applying standard N-saturation theory to xeric landscapes. In high elevation catchments, P is tightly bound by Al and atmospheric P inputs and rock weathering during the Holocene are the primary sources of present day soil and sediment P. Lake sediments represent a strong sink for P in high elevation lakes even during periods of anoxia. During snowmelt, 27% of the soil P cycles between the inorganic and organic pools raising the possibility that changes in snowpack dynamics could potentially explain long-term trends in P supply to lakes. Although intrinsic differences exist between chaparral and subalpine catchments, they respond similarly to the Mediterranean climate of California. The loss of tight biological control of N and P during seasonal transitions in the Sierra Nevada is similar to the response of ecosystems to periodic disturbances like fire and logging. Thus, nutrient limitation of these terrestrial and aquatic ecosystems and their sensitivity to anthropogenic pollution is tightly linked to interannual and long-term changes in climate.

Increased inputs of nutrients and acid anions to oligotrophic mountain lakes are contributing to ANC depression, elevated nitrate concentrations, shifts in nutrient limitation, and changes in the productivity and structure of aquatic communities. A need for stricter standards based on measurable ecological effects has been identified as an important step toward the long-term protection of mountain lakes. The objectives of this research were to link atmospheric deposition with acidification and eutrophication effects, develop critical loads and nutrient criteria, and assess status and trends of Sierra Nevada lakes. Investigation of multiple proxies of deposition, climate, acidification, and eutrophication indicated that early 20th century ANC decline in a Sierra Nevada lake is attributed to atmospheric deposition and the subsequent recovery in the late 20th century is attributed to the success of the Clean Air Act. Correlation analysis indicated ANC was correlated with atmospheric deposition indicators, but was not correlated with climate measures or productivity proxies. However, analyses looking more broadly across the landscape found a correlation with present day indicators of atmospheric deposition (SCPs) and ANC. These results indicate that not all lakes have fully recovered from acid deposition and stricter regulatory standards are needed. I aimed to link atmospheric deposition indicators with effects of eutrophication and acidification at a landscape scale and found that atmospheric deposition indicators were correlated with acidification, but not with eutrophication. Quantifying the relationship between nitrogen deposition and eutrophication across complex mountain landscapes is presently challenging, leading to the conclusion that critical loads based on acidification are a more robust approach. An acidification critical load was calculated based on 20th century ANC and acid deposition patterns and is 73.9 eq ha^-1 yr^-1 for acid anions, which translates to 0.68 kg-N ha^-1 yr^-1 and 1.2 kg-SO4 ha^-1 yr-1. Nitrogen criteria were calculated and ranged from were 0.33 - 0.89 µM (10% ED), 1.0 - 4.0 µM (50% ED), and 3.1 - 18 µM (90% ED). Application of criteria to Sierra Nevada lakes indicated the 10% effective dose was exceeded by 28-37 %, the 50% effective dose was exceeded by 18-29%, and the 90% effective dose was exceeded by 0.0-21%.

The Integrated Total Nitrogen Input (ITNI) method is a technique for evaluating nitrogen deposition by utilizing plants as collection interfaces. The ITNI method employs a plant-liquid-sand system (PLS system) in which a plant is hydroponically grown in silica sand and labeled with 15N tracer while growing in a greenhouse. After plants are labeled, they are deployed into the environment where the 15N tracer in the plant tissues is diluted as a result of atmospheric nitrogen deposition input via gaseous, leaf and root uptake. At the end of the sampling period, all components of the plant and system are harvested and analyzed on a mass spectrometer to determine the degree of dilution of the tracer. The 15N values obtained were incorporated into a mass balance equation that accounts for the total deposition occurring on the PLS system surfaces and yields the total nitrogen uptake from the atmosphere. In this study, we will employed coastal sage scrub (CSS), a declining native California plant assemblage, and invasive Mediterranean annual plants to determine total nitrogen deposition occurring in the Inland Empire of Southern California. Traditional nitrogen deposition collection devices such as throughfall and bulk ion exchange resins, inferential method collectors, and a rain collector was co-located with the ITNI PLS systems to compare and assess the accuracy of such traditional collectors. We found that the ITNI method is applicable to non-agricultural systems such as coastal sage scrub habitat, where the method was formerly utilized. Our findings suggest that ITNI methods compare to that of inferential techniques in estimating total nitrogen deposition to Southern California coastal sage scrub habitats. Additionally, it was found that the ITNI method is the most cost effective total nitrogen deposition measurement method when compared to the inferential method. Lastly, it was found that the ITNI system was sensitive enough to delineate between study sites on a nitrogen deposition gradient in the California Inland Empire, ranging from a "high" deposition urban site to a "moderate" deposition rural site.

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