Underwater optical environment in the coastal waters of British Columbia, Canada

Abstract

We describe the underwater light field of the Strait of Georgia in spring and summer, using apparent optical properties (reflectance, attenuation coefficient of downwelling irradiance, the average cosine of downwelling irradiance, and the attenuation of scalar irradiance). Both the attenuation and reflectance of photosynthetically available radiation (PAR; 400-700 nm) are highest in the turbid waters of the Fraser River plume, due to scattering by mainly inorganic particles and absorption by coloured dissolved organic matter, phytoplankton, and other organic particles. Light is most diffuse in the surface waters of the plume and least diffuse at depth and away from the plume. Throughout the Strait, blue and red wavelengths are attenuated most rapidly resulting in a green peak of reflectance, the portion of the electromagnetic spectrum that penetrates the most deeply. PAR is attenuated to 1% of its surface intensity within 6-22 m in the spring and 4-23 m in the summer. For red and blue light, the depth of 1% penetration is never deeper than 9 m. All of the visible radiation, with the exception of some green light, is absorbed within the outflowing layer (15-30 m) that is exported from the Strait with the estuarine circulation. The rapid extinction of light helps to explain the very shallow distribution of phytoplankton.

Validation of Non-photochemical Quenching Corrections for Chlorophyll-a Measurements Aboard Ships of Opportunity

“…We calculate the PAR 2m extinction coefficient k z , using a dataset of in situ down-welling irradiance and salinity from the study area (Loos et al, 2017). From this data, we derive a relationship between salinity and k z , which was then used to estimate k z based on salinity measurements from the ferry.…”

“…From this data, we derive a relationship between salinity and k z , which was then used to estimate k z based on salinity measurements from the ferry. PAR 2m was calculated according to Kirk (2011): In this region, light attenuation within the relatively clearer and more saline oceanic waters is primarily driven by phytoplankton, and it is generally lower than in the more turbid less saline plume waters, where attenuation is driven by CDOM and TSM (Johannessen et al, 2006;Loos and Costa, 2010;Loos et al, 2017).…”

“…As such, a relationship between salinity and k z was defined using in situ data measured by Loos et al (2017) in the SoG during the summer, matching oceanographic and irradiance conditions in this study. The authors measured PAR down-welling irradiance and salinity at different depths, allowing the calculation of k z according to Kirk (2011):…”

“…Plume and oceanic waters are distinguished based on a variable salinity threshold described in Halverson and Pawlowicz (2011) developed for the SoG. These two classes reflect differences in turbidity (Loos et al, 2017) that may in turn affect the magnitude of NPQ (Halverson and Pawlowicz, 2013). Salinity thresholds (S threshold ) were calculated for each ferry transect, and plume waters were defined where salinity was less than S threshold and oceanic waters were defined where salinity was greater than S threshold .…”

“…FIGURE 3 | Salinity and modeled kz values based on 19 CTD profiles acquired byLoos et al (2017). The solid line shows the regression fit, dashed red lines show the 95% confidence interval for the regression, and the dotted black lines show the 95% prediction interval for the data.…”