Land Use Controls on Stream and Lake Dissolved Silica Concentrations: A Case Study from the Finger Lakes, Central New York State, USA.

Wednesday, 17 December 2014
John D Halfman, Hobart and William Smith Colleges, Dept of Geoscience, Geneva, NY, United States
Bedrock geology, climate and time are important controls on chemical weathering and release of dissolved silica. Forested land vs. other land uses was recently hypothesized as another control. The Finger Lakes region is an ideal natural laboratory to test this hypothesis as local watersheds vary in area, bedrock and agricultural to forested land cover in this rural setting.

Annual mean dissolved silica data from 11 watersheds in our ongoing monitoring program ranged from 100 to 4,000 μg/L Si, analyzing filtered (0.45 μm) samples by spectrophotometer (molybdate indicator with metol/oxalic acid reagents). Like earlier work, only forested land use (12 to 73%) correlated to the mean silica concentrations (r2 = 0.3), which improves (r2 = 0.6) when a small, primarily (24%) developed watershed is excluded from the correlation. Bedrock (Devonian carbonates, 0 to 8% and clastics, 0 to 99% covered by till) and basin area (10 to 500 km2) did not correlate (r2 <= 0.1). Event and base flow samples of an agricultural (64%) watershed revealed peak to base flow fluctuations in silica concentrations that more closely mimic nitrates and other groundwater solutes than suspended particles, phosphates and other runoff signature parameters.

Annual mean epilimnion and hypolimnion dissolved silica data from the 8 easternmost Finger Lakes in our ongoing monitoring program ranged from 250 to 1,500 μg/L Si. Forested cover (30 to 75%) positively correlated to epilimnion silica concentrations (r2 = 0.6). Lake water residence time (1 to 17 yr) negatively correlated to hypolimnion silica concentrations (r2 = 0.5). Agricultural land use, bedrock, and productivity indicators (chlorophyll-a, total phosphate, and secchi disk depth) lacked correlation (r2 <= 0.1). It suggested that land use impacts stream and, surprisingly, lake dissolved silica chemistry. Biogeochemical processes in the lakes like diatom uptake appears to increasingly decrease silica concentrations in lakes with longer residence times.