Some Notes on Water Color in Keweenaw Bay (Lake Superior) |
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| Affiliation: | 1. Netherlands Institute of Ecology (NIOO-KNAW), Centre for Limnology, Rijksstraatweg 6, 3631 AC Nieuwersluis, The Netherlands;2. Department of Civil and Environmental Engineering, Michigan Technological University, 1400 Townsend Drive, Houghton, Michigan 49931;1. Industrial Doctoral Centre for Offshore Renewable Energy, The University of Edinburgh, King’s Buildings, Edinburgh EH9 3JL, UK;2. E.ON Technologies Limited, Technology Centre, Ratcliffe on Soar, Nottingham NG11 0EE, UK;3. College of Engineering, Mathematics and Physical Science, Renewable Energy Research Group, University of Exeter, Penryn Campus, Penryn TR10 9EZ, UK;4. Department of Naval Architecture, Ocean and Marine Engineering, University of Strathclyde, Glasgow G4 0LZ, UK;5. Institute for Energy Systems, School of Engineering, The University of Edinburgh, King’s Buildings, Edinburgh EH9 3JL, UK;1. Unit for Marine and Coastal Systems, Deltares, P.O. Box 177, 2600 MH Delft, The Netherlands;2. Department of Hydraulic Engineering, Delft University of Technology, P.O. Box 5048, 2600 GA Delft, The Netherlands;3. Institute for Biology and Environmental Science, Carl von Ossietzky University Oldenburg, Carl-von-Ossietzky-Str. 9-11, D-26129 Oldenburg, Germany;4. Wetlands International, P.O. Box 471, 6700 AL Wageningen, The Netherlands;5. Directorate of Marine and Coasts, Ministry of Marine Affairs and Fisheries (MMAF), Jakarta, 10110, Indonesia;1. College of Resource and Environment, Hunan Agricultural University, Changsha, 410128, China;2. College of Resources and Environmental Engineering, Ludong University, Yantai, 264025, China;3. Key Laboratory for Agro-ecological Processes in Subtropical Regions, Institute of Subtropical Agriculture, Chinese Academy of Sciences, Changsha, 410125, China;4. University of Chinese Academy of Sciences, Beijing, 100049, China;5. State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, 100029, China;1. Department of Hepatology & Gastroenterology, Aarhus University Hospital, Aarhus, Denmark;2. Department of Clinical Biochemistry, Aarhus University Hospital, Aarhus, Denmark |
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| Abstract: | Spectral subsurface irradiance reflectance—intrinsic water color—was derived from above-water radiance measurements using a hand-held spectroradiometer along a transect on Keweenaw Bay, Lake Superior. The reflectance spectra were typical of oligotrophic lacustrine waters. The reflectance peak wavelength shifted from 484 nm at stations farthest offshore to 540 nm near the head of the bay. This change coincided with a decrease in Secchi-disk depth from 16 to 8 m, and an increase in concentrations of chlorophyll a and total suspended matter from about 0.45 to 0.60 mg m−3 and from 0.3 to 0.5 g m−3, respectively. The concentration of chromophoric dissolved organic matter (gilvin), expressed as the absorption of filtrate at 440 nm, varied between 0.11 and 0.2 m−1. Like almost all inland waters, Keweenaw Bay should be classified as a Case 2 water due to the concentrations of gilvin and inanimate particles relative to plankton biomass. A model for chlorophyll-a estimation from spectral reflectance that adequately predicted concentrations in mesotrophic to highly eutrophic Case 2 waters elsewhere gave negative values when applied to the Keweenaw Bay transect. Evidently, there is a need of algorithm development for oligotrophic lacustrine waters. Estimates improved using a modified blue to green band ratio algorithm previously applied for remote sensing of oceanic waters. In optimization of semi-empirical algorithms for estimation of plankton biomass in Lake Superior, absorption by gilvin as well as by inanimate particles merits special consideration. |
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