Optical properties of the waters of the Strait of Georgia, BC, Canada

Abstract

Ocean optical studies have been conducted extensively in open ocean waters but less so in coastal waters where the influence of human population is increasing dramatically. The waters of the Strait of Georgia, British Columbia, Canada, are very important to the rearing of young salmon and herring, and to the fishing industry of British Columbia overall. The oceanography and plankton communities of the Strait have been researched extensively, however the forces behind the frequent occurrence of phytoplankton blooms in these waters still causes debate among researchers. In order to shed some light onto this topic and increase our knowledge of the characteristics of the waters of the Strait of Georgia, optical and bio-physical data were measured in the euphotic waters of the Strait in late spring and early summer of 2006. Hyperspectral optical data were measured for the first time in these waters using in situ optical profilers to collect inherent optical properties and radiometric quantities that were later used to derive apparent optical properties. The inherent optical properties included absorption coefficient, spectral beam attenuation coefficient, chromophoric dissolved organic matter absorption coefficient, particulate absorption coefficient, and particulate scattering coefficient. In situ irradiances and radiances were used to derive various diffuse attenuation coefficients. Water masses in the euphotic zone of the Strait of Georgia were then classified into three optical water masses according to their inherent optical properties using a clustering algorithm. OM1 waters were characterized by high and spectrally-invariant particulate scattering due to inorganic particles carried by the Fraser River plume. Absorption and scattering showed some spectral dependence in OM2 waters, with particles and chromophoric dissolved organic matter contributing equally to light absorption. The deepest waters, OM3, were the least influenced by the Fraser River, and the contribution of chromophoric dissolved organic matter to absorption was greater than in OM1 and OM2. A radiative transfer model, Hydrolight, was used to model some of the optical properties that were not collected in situ and then used to assess the magnitude of light available to phytoplankton in the Strait. Based on the minimum light requirements for photosynthesis of two of the main phytoplankton species in the Strait, the analysis presented here showed that there was enough light available for photosynthesis in the photosynthetically-available radiation range for the two phytoplankton species in all three optical water masses.