Distribution of normalized water‐leaving radiances at UV and visible wave bands in relation with chlorophyll a and colored detrital matter content in the southeast Pacific

Abstract

In‐water radiometric measurements were performed in the southeast Pacific (8°S–35°S, 141°W–72°W) from October to December 2004 during the Biogeochemistry and Optics South Pacific Experiment cruise. Normalized water‐leaving radiances (nLw(λ)) were determined at eight wave bands within the ultraviolet (UV) (305, 325, 340, and 380 nm) and visible (412, 443, 490, and 565 nm) spectral domains. The highest nLw(λ) (μW cm−2 sr−1) were recorded in the hyperoligotrophic waters of the South Pacific Gyre, with values increasing with wavelength from 305 (nLw = 0.64) to 380 nm (nLw = 3.18) in the UV range and decreasing from 412 (nLw = 4.46) to 565 nm (nLw = 0.23) in the visible region. The intense nLw(λ) observed in the violet‐blue domains were attributed to very low absorptions of colored detrital matter (CDM), likely related to a strong photobleaching of colored dissolved organic matter in the surface waters. We evaluated the relationships between the UV, violet, or blue/green wave band ratios of nLw(λ) and surface total chlorophyll a (TChl a) concentration and CDM absorption (aCDM(λ)). For TChl a, the best correlation was found with the blue/green ratio at 443 nm: TChl a (mg m−3) = 2.37[nLw(443)/nLw(565)]−1.51 (r2 = 0.86 and RMS error (RMSE) = 23%). By contrast, for aCDM(λ), the best correlation was observed when using the UV/green ratio at 325 nm: aCDM(325) (m−1) = 0.16[nLw(325)/nLw(565)]−0.69 (r2 = 0.82 and RMSE = 16%). These results show the potential role of nLw(λ) at UV wave bands for the assessment, through empirical algorithms, of colored detrital matter in the surface oceanic waters.