Microbial absorption and backscattering coefficients from in situ and POLDER satellite data during an El Niño–Southern Oscillation cold phase in the equatorial Pacific (180°)

Abstract

Contributions of different microbial groups to absorption, backscattering, and marine reflectance (a(λ), bb(λ) and R(λ), respectively) were quantified during an El Niño‐Southern Oscillation cold phase in the equatorial Pacific during the Etude du Broutage en Zone Equatoriale cruise on board the R/V L'Atalante. In situ data were collected at every degree of latitude from 8°S to 8°N, 180° (26 October to 13 November 1996), and satellite reflectances were available from POLDER‐ADEOS for the 1–10 November 1996 decade. Bulk absorption and backscattering coefficients were estimated at 440 nm for the major microbial groups enumerated in the upper surface layer (heterotrophic bacteria, Prochlorococcus, Synechococcus, and <20‐μm eukaryotic algae). Total absorption and backscattering coefficients were retrieved from space by a new inverse method. The observed ecosystem was typical of a well‐developed equatorial upwelling, with maximal values of 0.4 mg m−3 for Tchl a, 0.026 m−1 for ap(440), 0.023 m−1 for aphy(440), and a low in situ adet (<14% of ap). Prochlorococcus and nanoeukaryotic algae (3.4‐μm mean diameter) were the dominant absorbers (97%), contributing about equally to aphy. The retrieved total absorption coefficient, asat(440), from POLDER (maximum of 0.03 m−1) was higher than ap(440), as it included absorption by CDOM (estimated to be 15% of ap(440) + aw(440), where aw = absorption by pure water). Heterotrophic bacteria were the dominant contributors (73%) to total simulated microbial backscattering, bbmic (maximum = 3.7 × 10−4 m−1), but bbmic was negligible compared to the inverted total backscattering by particles, bbp (2.7 × 10−3 m−1), indicating that unidentified small nonliving particles contributed most to the satellite signal.