Seasonal and regional differentiation of bio‐optical properties within the north polar Atlantic

Abstract

Using field data from the north polar Atlantic, we examined seasonal variability of the spectral absorption, a(λ), and backscattering, bb(λ), coefficients of surface waters in relation to phytoplankton pigments. For a given chlorophyll a concentration, the concentrations of accessory pigments were lower in spring than in summer. This effect contributed to lower chlorophyll‐specific absorption of phytoplankton and total particulate matter in spring. The spring values of the green‐to‐blue band ratio of a(λ) were higher than the summer ratios. The blue‐to‐green ratios of bb(λ) were also higher in spring. The higher bb values and lower blue‐to‐green bb ratios in summer were likely associated with higher concentrations of detrital particles in summer compared to spring. Because the product of these band ratios of a and bb is a proxy for the blue‐to‐green ratio of remote‐sensing reflectance, the performance of ocean color band‐ratio algorithms for estimating pigments is significantly affected by seasonal shifts in the relationships between absorption, backscattering, and chlorophyll a. Our results suggest that the algorithm for the spring season would predict chlorophyll a that is higher by as much as a factor of 4–6 compared to that predicted from the summer algorithm. This indicates a need for a seasonal approach in the north polar Atlantic. However, we also found that a fairly good estimate of the particulate beam attenuation coefficient at 660 nm (a proxy for total particulate matter or particulate organic carbon concentration) can be obtained by applying a single blue‐to‐green band‐ratio algorithm regardless of the season.