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Neo and paleo-limnological response to nutrient, climate, and volcanic stressors over the past 100 years at Butte, Manzanita, and Widow lakes in Lassen Volcanic National Park (CA, USA)
AuthorHoward, Kerry Louise
Geological Sciences and Engineering
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Modern limnological data, catchment characteristics, and paleolimnological data for Butte, Manzanita, and Widow lakes in Lassen Volcanic National Park (LAVO) (CA, USA) were analyzed to investigate lake response to nutrient, climate, and volcanic stressors over the past century. In addition, the quantitative use of silicon isotope ratios (expressed as δ30Si) as a diatom paleoproductivity proxy in Butte, Manzanita, and Widow lakes was also studied in order to better interpret modern and sediment core δ30Si observations from these systems. Butte Lake exhibits significant algal and water quality response to precipitation variability over sub-decadal to inter-annual time scales which is an important consideration for the paleolimnological part of this work. Extreme hydrological deficit (observed 2012-2014) resulted in changing the lake hydrology from an open, drainage-type hydrological system (surface outflow) in 2012 to a prolonged period of being a closed, seepage-type hydrological system (no surface outflow) in 2013 and 2014. Hydraulic retention times (HRTs) in Butte Lake vary according to hydrological mode of this system, where HRT may be on the order of three times longer during a closed (seepage-type) mode. Declining water quality in Butte Lake as a result of precipitation deficit in 2012-2014 corresponded with increasing lake trophic status from oligo-mesotrophic to borderline eutrophic over the same period. Shifts in water quality and lake trophic status were also reflected in reorganization of the algal community composition, where epilimnetic phytoplankton biovolume shifted from being diatom-dominated to being-cyanobacteria dominated. In contrast, Manzanita Lake appears to consistently operate in an open (drainage type) hydrological mode, and has a much shorter hydraulic retention time, with an average of ~4 months. Modern lake chemistry data and Chlorophyll-a data suggest that both Butte Lake and Manzanita Lake are N-limited. A synthesis of paleolimnological data for Butte and Manzanita lakes reveals that both lakes demonstrate down-core shifts that are characterized by: 1) significant, directional changes in diatom assemblages over the last ~100 years that accelerate during the last several decades, as well as 2) coherent changes in 15N signatures and other geochemical proxies. Coherent down-core changes in 15N signatures are also observed in Widow Lake over the past ~100 years and provide corroborating evidence to suggest a regionally-changing supply of anthropogenic reactive nitrogen from atmospheric sources. The timing of major diatom assemblage change in Butte Lake and Manzanita Lake is offset by ~30 years between the lakes (Butte Lake ~1960 CE (common era), Manzanita Lake ~1990 CE) and encompasses separate diatom taxa with differing ecological constraints. In Manzanita Lake, the diatom response associated with regional nutrient-climate forcing is characterized by increased abundances of planktonic taxa, specifically the nitrophilous indicator taxa Fragilaria crotonensis and Asterionella formosa. Alternatively, in Butte Lake, increased abundances of small araphid tychoplanktonic taxa and periphyton taxa, particularly Staurosira construens var. binodis and Karayevia clevei are observed. Differential responses among Butte and Manzanita lakes over the past century to regional-scale nutrient-climate forcing are attributed to landscape-level controls such as hydrogeologic setting, as well as other local-scale factors, such as degree of inherent resilience, and trophic-level interactions. The results of modern and paleolimnological studies for Butte and Manzanita lakes present unique implications for management of each of these keystone aquatic resources in Lassen Volcanic National Park, and thus it is recommended that management approaches be individualized for these lake systems. Modern and paleolimnological work in Butte, Manzanita, and Widow lakes additionally provides an opportunity to study Si concentrations (lake water) and Si isotope ratios (30Si) in lake water and diatom biogenic silica. Manzanita lake is identified as a sink of fluvial Si inputs; however, many other inputs and outputs of Si are as yet unquantified for LAVO lake systems. Results also indicate modern and sediment biogenic silica 30Si values are linked to diatom growth habitat and possibly species-dependent fractionation () factors. These data suggest that δ30Si records for biogenic silica from LAVO lakes may be best interpreted as a paleolimnological proxy for the aquatic-habitat related source of biogenic silica production and should be coupled with diatom community composition data and other geochemical proxies for paleolimnological interpretation. Finally, paleolimnological work in Widow Lake presented an opportunity to consider lacustrine response to volcanic disturbance in LAVO. Results point out that Widow Lake has been influenced by volcanic disturbance, explicitly by deposition of a volcanic tephra (prior to ~1880 CE). Down-core diatom assemblages shift from pre- to post-tephra, characterized by a prevalence of benthic taxa prior to tephra deposition and planktonic taxa post- tephra deposition (after ~1880 CE). Like Butte and Manzanita lakes, Widow Lake is also an N-limited system, and the 15N trend observed in Widow lake sediments are similar to those observed in Butte Lake and Manzanita Lake sediments over the past ~100 years, indicating that diatom and geochemical responses observed post- tephra (after ~1880 CE) might also be related to regional-scale nutrient-climate forcing, and not just recovery from volcanic disturbance. Given the current resolution of the available paleolimnological data, it is impossible to de-couple shifts in diatom community composition and lake geochemistry attributed to volcanic disturbance from that which may be attributed to regional nutrient-climate forcing occurring post- tephra, since ~1880 CE.