BIOGENIC (DIATOM) SILICA PRODUCTION IN MANZANITA LAKE, LASSEN VOLCANIC NATIONAL PARK, CA

Manzanita lake (1790 masl) is a small (0.2 km²), sub-alpine lake located on the northwest flank of Lassen Peak, an active Cascade Range volcano. The Manzanita lake watershed (~ 30 km²) is dominated by extrusive volcanic rocks, principally rhyodacite. In 2014, observed surface water inflow concentrations of dissolved silica to the lake from Upper Manzanita Creek ranged from 16-18 mg/ L. Si isotope values, Chlorophyll a measurements, and diatom biovolume from 2014 water samples suggest ongoing production of diatom biogenic silica (bSi, an Opal-A phase) in May, August, and September in the epilimnion, and a greater influence of ground water or surface water inputs on dissolved silica (dSi) pools in the metalimnion and hypolimnion. Inflow surface water δ³⁰Si values were the same (~1.0‰) in May and September 2014. δ³⁰Si values for dSi in the epilimnion in 2014 were 2.1‰ in May, 1.7‰ in August, and 2.8‰ in September. Diatom (plankton) δ³⁰Si values from the epilimnion were lowest in May 2014 (-1.3‰) and highest in September 2014 (-1.1‰) showing a larger fractionation than previously assumed for lakes. δ³⁰Si values of discrete water samples from the metalimnion and hypolimnion ranged from 1.3-1.6‰ in 2014. Outflow dSi δ³⁰Si values for May and September 2014 were 1.9‰ and 1.8‰ respectively, and are lower than lake dSi δ³⁰Si values observed in the epilimnion. Differences in outflow and lake dSi δ³⁰Si values may be the result of mixing occurring between the dSi pools of the epilimnion, metalimnion, and hypolimnion or mixing with groundwater before outflow. A Si budget constructed for the lake indicates that inflowing surface water dSi is being retained as biogenic silica in lake sediments and represents ~5% (0.008 g/m²/yr) of the average total sedimentary Si mass accumulation rate of ~0.023 g/m²/yr. Biogenic silica in Manzanita lake sediments ranges from 6%-30% wt% SiO₂, and down-core shifts in bulk sediment bSi and Si isotope values may point toward a trend of increasing biogenic silica production over the last ~70 years. Concurrent shifts in organic N and C isotope signatures in Manzanita lake sediments may reflect perturbations in nitrogen biogeochemistry, specifically increased atmospheric deposition of nitrogen or watershed level human impacts, resulting in increased diatom biogenic silica production.