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Measurements of Under Ice Light Attenuation and the Potential Impact on Great Lakes Primary Productivity Estimates

Author(s): R. Shuchman; G. Fahnenstiel; G. Leshkevich; M. Whitley; M. Sayers; B. Hart

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Abstract: Primary productivity (PP) in the Laurentian Great Lakes Basin is reduced in the winter months by ice and snow cover on the Lakes. To quantify the effects of ice and snow cover on Lakes Superior and Michigan as well as a number of smaller inland freshwater lakes, a series of radiometric measurements in the 400-700 nm Photosynthetically Active Radiation (PAR) spectral range were made above the snow/ice interface, at the snow/ice interface and at the water/ice interface in the winter of 2015. Additionally, at the Lake Superior sites, primary productivity measurements were also made. The PAR irradiance measurements were made using a calibrated Analytical Spectral Device (ASD) Field Spec 3, Biospherical instruments 4πPAR sensor and a homemade 2πPAR sensor. The latter was used to test the use of an inexpensive sensor suite that could potentially be left on or under the ice to make time-series attenuation observations. The majority of the sites sampled in 2015 were a mixture of snow/ice, the exception being the Lake Superior sites in Keweenaw Bay. In general, the Lake Superior clear ice permitted nearly all light through (average transmittance~87%). The mixed snow/ice attenuation varied depending on the rheology of the ice, though consistently blocked 50-85% of the PAR. Snow cover of any depth blocked the majority of light and has a well-defined linear relationship from our limited field sampling. Preliminary estimates of the effect of ice and snow cover on annual primary productivity in the Upper Great Lakes were generated using, attenuation and primary productivity measurements along with basin wide ice cover statistics provided by the National Ice Center (NIC. Based on our experiments in Keweenaw Bay, Lake Superior ice cover reduced primary production by 2-39%.

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Year: 2016

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