Ash, Asterionella, and Anglers: A Paleolimnological Approach to Understanding Anthropogenic and Volcanogenic Disturbances in a Small Sub-Alpine Watershed

Abstract:

This poster summarizes geochemical, biological, hydrological, and watershed data that characterize Manzanita Lake, a small sub-alpine catchment in Lassen Volcanic National Park, CA. The future objective is to use characteristics of this system to interpret variations in diatom and sediment composition dating back to the 1914 Mt. Lassen eruption from a recently recovered lake core. Manzanita Lake is a small (0.18 km²) lake with a ~30 km² watershed area situated on the northwest flank of Mt. Lassen, one of the most active Cascade volcanoes, and is a valuable recreational spot for anglers and visitors. Hydraulic residence time is short; roughly 119 days, and is derived from lake volume (1.0 X 10⁶m³) and estimates of stream inflow (~6 ft³/sec) and outflow (~3 ft³/sec) that were made from May 2014 sampling data. Limnological sampling in 2012-2014 suggests that Manzanita Lake exhibits stable thermal stratification in the summer months, which is unusual given the shallow depth (~10m), but possibly supported by the morphometry of the lake basin and inputs of cold snowmelt from the flank of Lassen Peak. The lake is a moderately conductive (100-114 μS), mesotrophic system with secchi depths ranging from 8m to the bottom (~10m). Total phosphorus (TP) ranges from 15-25 ppb and dissolved inorganic nitrogen (DIN) from 2-15 ppb, with ammonium in the epilimnion being the largest contributor. A high concentration of silica (Si) in surface water inputs (34 mg/L) to Manzanita Lake likely reflects the rhyodacitic bedrock geology and large drainage ratio (164) of the watershed. Variations in Si concentration in the lake seem to be coupled with diatom production. During the sampling period Manzanita Lake is has been dominated by diatom blooms throughout the summer and fall months. There is a seasonal succession in the diatom species present, with abundant Asterionella formosa in the spring, transitioning to abundant Fragilaria crotonensis in the summer months, to a mixed dominance of Fragilaria crotonensis and Aulacoseira species in the fall. Persistence of diatom species with moderate to high Si requirements throughout the summer is atypical and may be related to high Si (11-29 mg/L in lake), TP, and DIN. These data provide a modern baseline with which to interpret down-core changes in diatom composition and diversity over the last century.