Distribution of normalized water‐leaving radiances at UV and visible wave bands in relation with chlorophyll <i>a</i> 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 ( nL w ( 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 nL w ( λ ) ( μ W cm −2 sr −1 ) were recorded in the hyperoligotrophic waters of the South Pacific Gyre, with values increasing with wavelength from 305 ( nL w = 0.64) to 380 nm ( nL w = 3.18) in the UV range and decreasing from 412 ( nL w = 4.46) to 565 nm ( nL w = 0.23) in the visible region. The intense nL w ( 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 nL w ( λ ) and surface total chlorophyll a (TChl a ) concentration and CDM absorption ( a CDM ( λ )). For TChl a , the best correlation was found with the blue/green ratio at 443 nm: TChl a (mg m −3 ) = 2.37[ nL w (443)/ nL w (565)] −1.51 ( r 2 = 0.86 and RMS error (RMSE) = 23%). By contrast, for a CDM ( λ ), the best correlation was observed when using the UV/green ratio at 325 nm: a CDM (325) (m −1 ) = 0.16[ nL w (325)/ nL w (565)] −0.69 ( r 2 = 0.82 and RMSE = 16%). These results show the potential role of nL w ( λ ) at UV wave bands for the assessment, through empirical algorithms, of colored detrital matter in the surface oceanic waters.