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Absorption properties of shoal-dominated waters in the Atchafalaya Shelf, Louisiana, USA
Abstract : Spectral absorption coefficients of coloured dissolved organic matter (a CDOM(λ)) and particulate matter (a p(λ)) (phytoplankton (a PHY(λ)) plus non-algal particles (a NAP(λ)), measured on the shoal-dominated region off the Atchafalaya River (AR) Shelf, Louisiana, USA, are analysed, and their effect on chlorophyll-a retrievals from ocean-colour sensors examined. Compared to a CDOM(λ) and a NAP(λ), a PHY(λ) is relatively constant, with a CDOM(λ) and a NAP(λ) varying by approximately 1.2 and 1.8 times as much as a PHY(λ) at 443 nm, respectively. The specific a PHY(λ) (a*PHY(λ)) ranges from 0.006 to 0.0612 m−2(mg chla)−1 at 443 nm, which indicates a pigment-packaging effect or a variation in pigment composition. The a NAP(λ) accounts for approximately 3–93% of a p(λ) at 443 nm, with a higher contribution to a p(λ) during an October 2007 cruise (62–93%) as compared to an August 2007 cruise (31–89%). Our results indicate that a CDOM(λ) and a NAP(λ) collectively dominate light absorption, even at higher wavelengths where their effect is expected to be minimal. In situ and satellite data match-up of chlorophyll-a yield root-mean square errors of 2.17 and 2.62 for the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) and the Medium Resolution Imaging Spectrometer (MERIS), respectively. The non-covarying a CDOM(λ) and a NAP(λ), along with variable a*PHY(λ), greatly influenced the remote retrieval of biogeochemical variables using satellite ocean-colour algorithms in this region.