Evaluating Lake Response to Environmental and Climatic Change using Lake Core Records and Modeling

This dissertation evaluates how lakes respond to changes in their environmental and climatic settings. This dissertation consists of two lake modeling projects and the environmental and paleoclimatic reconstructions of four lakes in southwestern Montana. Lakes and their associated biological communities respond to environmental and climatic change on rapid timescales. Lake modeling and lake core stratigraphies are complementary tools for exploration of how lakes and their biota respond to environmental or climatic perturbations. In one modeling approach for exploring these issues, a simple hydro-climatological lake model was developed that classifies lake sensitivity to climatic perturbations based upon lake area, catchment area, precipitation, and evapotranspiration. Using simple ratios of these commonly measured parameters the model classifies lakes into three domains: ephemeral, sensitive to vegetation change, and permanent. The lakes that plot within the sensitive to vegetation change domain should show water balance fluctuations in response to environmental change, and these lakes would make good targets for paleoclimatic studies. Diatom records from four lakes, Crevice Lake, Foy Lake, Morrison Lake, and Reservoir Lake, in southwestern Montana provide late-Holocene (past 3000 years) records of environmental and climatic variability. The lakes show similarities in the timing of major changes in the fossil diatom assemblages, suggesting regional climate forcings. Spectral analysis of the lake-core records suggest periodic fluctuations at spectral frequencies that are characteristic of oceanic influence on climate, such as the Atlantic Multi-decadal Oscillation and Pacific Decadal Oscillation. The Crevice Lake core diatom record shows three distinctive diatom communities during the approximately the last 1000 years. The model DYRESM-CAEDYM was used in an inverse modeling approach to provide a means to estimate climate variables during these three stages. The model estimates of climate variables during the Medieval Period, the Little Ice Age, and the 20th century, include incoming shortwave radiation, cloud cover, vapor pressure, and wind speed. The model results suggests that changes in spring seasonality, when the climate variables differ the most, is more important in affecting diatom community composition than total deviations from modern averages

Synchronous climatic change inferred from diatom records in four western Montana lakes in the U.S. Rocky Mountains

Late-Holocene environmental and climatic conditions were reconstructed from diatom assemblages in sediment cores from four western Montana lakes: Crevice Lake, Foy Lake, Morrison Lake, and Reservoir Lake. The lakes show synchroneity in timing of shifts in diatom community structure, but the nature of these changes differs among the lakes. Two of the sites provide highly resolved records of hydrologic balance, while the other two stratigraphic sequences primarily record temperature impact on lake thermal structure. All four lakes show significant change in five discrete intervals: 2200–2100, 1700–1600, 1350–1200, 800–600, and 250 cal yr BP. The similarities in the timing of change suggest overlying regional climatic influences on lake dynamics. The 800–600 cal yr BP shift is evident in other paleorecords throughout the Great Plains and western US, associated with the transition from the Medieval Climate Anomaly to the Little Ice Age. Large-scale climatic mechanisms that influence these lake environments may result from atmospheric circulation patterns that are driven by interactions between Pacific and Atlantic sea-surface temperatures, which are then locally modified by topography

Synchronous climatic change inferred from diatom records in four western Montana lakes in the U.S. Rocky Mountains

Late-Holocene environmental and climatic conditions were reconstructed from diatom assemblages in sediment cores from four western Montana lakes: Crevice Lake, Foy Lake, Morrison Lake, and Reservoir Lake. The lakes show synchroneity in timing of shifts in diatom community structure, but the nature of these changes differs among the lakes. Two of the sites provide highly resolved records of hydrologic balance, while the other two stratigraphic sequences primarily record temperature impact on lake thermal structure. All four lakes show significant change in five discrete intervals: 2200–2100, 1700–1600, 1350–1200, 800–600, and 250 cal yr BP. The similarities in the timing of change suggest overlying regional climatic influences on lake dynamics. The 800–600 cal yr BP shift is evident in other paleorecords throughout the Great Plains and western US, associated with the transition from the Medieval Climate Anomaly to the Little Ice Age. Large-scale climatic mechanisms that influence these lake environments may result from atmospheric circulation patterns that are driven by interactions between Pacific and Atlantic sea-surface temperatures, which are then locally modified by topography

Evaluating lake response to environmental and climatic change using lake core records and modeling

This dissertation evaluates how lakes respond to changes in their environmental and climatic settings. This dissertation consists of two lake modeling projects and the environmental and paleoclimatic reconstructions of four lakes in southwestern Montana. Lakes and their associated biological communities respond to environmental and climatic change on rapid timescales. Lake modeling and lake core stratigraphies are complementary tools for exploration of how lakes and their biota respond to environmental or climatic perturbations. In one modeling approach for exploring these issues, a simple hydro-climatological lake model was developed that classifies lake sensitivity to climatic perturbations based upon lake area, catchment area, precipitation, and evapotranspiration. Using simple ratios of these commonly measured parameters the model classifies lakes into three domains: ephemeral, sensitive to vegetation change, and permanent. The lakes that plot within the sensitive to vegetation change domain should show water balance fluctuations in response to environmental change, and these lakes would make good targets for paleoclimatic studies. Diatom records from four lakes, Crevice Lake, Foy Lake, Morrison Lake, and Reservoir Lake, in southwestern Montana provide late-Holocene (past 3000 years) records of environmental and climatic variability. The lakes show similarities in the timing of major changes in the fossil diatom assemblages, suggesting regional climate forcings. Spectral analysis of the lake-core records suggest periodic fluctuations at spectral frequencies that are characteristic of oceanic influence on climate, such as the Atlantic Multi-decadal Oscillation and Pacific Decadal Oscillation. The Crevice Lake core diatom record shows three distinctive diatom communities during the approximately the last 1000 years. The model DYRESM-CAEDYM was used in an inverse modeling approach to provide a means to estimate climate variables during these three stages. The model estimates of climate variables during the Medieval Period, the Little Ice Age, and the 20th century, include incoming shortwave radiation, cloud cover, vapor pressure, and wind speed. The model results suggests that changes in spring seasonality, when the climate variables differ the most, is more important in affecting diatom community composition than total deviations from modern averages

A hydro-climatological lake classification model and its evaluation using global data

For many of the world’s lakes, particularly those in remote regions, an assessment of the basin’s sensitivity to climate change is limited by the availability of appropriate hydrologic data. A regional steady-state lake water balance model was developed that uses simple, yet easily estimated or obtained, data to generate an aridity index (potential evapotranspiration to precipitation ratio) to predict changes in lake basin area to lake surface area ratio, a non-dimensional lake-basin property that can be easily obtained from digital maps. In the model, lake water balance components include precipitation, lake evaporation, and runoff into the lake. Both basin runoff and evaporation are incorporated in the analytical model using an empirical equation (based on the Budyko hypothesis) that utilizes the aridity index and a calibration parameter to account for vegetative influence. Observed records of lake to basin area ratio, as a function of their evapotranspiration to precipitation ratio, for a range of climates and land cover conditions, were plotted and compared to a family of calculated steady state curves. Dividing the domain of calculated curves into regions of permanent, land-cover change sensitive and ephemeral lakes allows for comparison of model predicted lake classification with lake sediment records. The impact of land cover and climate change on lake persistence is also discussed. The model is most applicable for closed basin lakes in sub-humid, semi-arid, and arid environments. The model can be used: (1) as a diagnostic tool to analyze lake response to climate change; (2) to assess environmental and anthropogenic factors leading to transient lake response; and (3) as a paleoclimatic research tool, to identify lakes that can potentially provide high-resolution paleorecords of water balance

Holocene seasonal variability inferred from multiple proxy records from Crevice Lake, Yellowstone National Park, USA

A 9400-yr-old record from Crevice Lake, a semi-closed alkaline lake in northern Yellowstone National Park, was analyzed for pollen, charcoal, geochemistry, mineralogy, diatoms, and stable isotopes to develop a nuanced understanding of Holocene environmental history in a region of northern Rocky Mountains that receives both summer and winter precipitation. The limited surface area, conical bathymetry, and deep water (\u3e31 m) of Crevice Lake create oxygen-deficient conditions in the hypolimnion and preserve annually laminated sediment (varves) for much of the record. Pollen data indicate that the watershed supported a closed Pinus-dominated forest and low fire frequency prior to 8200 cal yr BP, followed by open parkland until 2600 cal yr BP, and open mixed-conifer forest thereafter. Fire activity shifted from infrequent stand replacing fires initially to frequent surface fires in the middle Holocene and stand-replacing events in recent centuries. Low values of δ18O suggest high winter precipitation in the early Holocene, followed by steadily drier conditions after 8500 cal yr BP. Carbonate-rich sediments before 5000 cal yr BP imply warmer summer conditions than after 5000 cal yr BP. High values of molybdenum (Mo), uranium (U), and sulfur (S) indicate anoxic bottom-waters before 8000 cal yr BP, between 4400 and 3900 cal yr BP, and after 2400 cal yr BP. The diatom record indicates extensive water-column mixing in spring and early summer through much of the Holocene, but a period between 2200 and 800 cal yr BP had strong summer stratification, phosphate limitation, and oxygen-deficient bottom waters. Together, the proxy data suggest wet winters, protracted springs, and warm effectively wet summers in the early Holocene and less snowpack, cool springs, warm dry summers in the middle Holocene. In the late Holocene, the region and lake experienced extreme changes in winter, spring, and summer conditions, with particularly short springs and dry summers and winters during the Roman Warm Period (~2000 cal yr BP) and Medieval Climate Anomaly (1200–800 cal yr BP). Long springs and mild summers occurred during the Little Ice Age, and these conditions persist to the present. Although the proxy data indicate effectively wet summer conditions in the early Holocene and drier conditions in the middle and late Holocene, none point specifically to changes in summer precipitation as the cause. Instead, summer conditions were governed by multi-seasonal controls on effective moisture that operated over multiple time scales