Optical Characterization of an Eddy-induced Diatom BloomWest of the Island of Hawaii

[摘要] Optical properties were collected along a transect across cyclonic eddy Opalin the lee of Hawaii during the E-Flux III field experiment (10–27 March 2005). The eddywas characterized by an intense doming of isopycnal surfaces, and by an enhanced DeepChlorophyll Maximum Layer (DCML) within its core. The phytoplankton bloom was diatomdominated, evidencing an eddy-induced shift in ecological community. Four distinctregions were identified throughout the water columnat Opal's core: asurface mixed layer dominated by small phytoplankton; a layer dominated by "senescent"diatoms between the bottom of the upper mixed layer and the DCML; the DCML; and a deeplayer characterized by decreasing phytoplankton activity. We focused on two parameters,the ratio of chlorophyll concentration to particulate beam attenuation coefficient, [chl]/c_p,and the backscattering ratio (the particle backscattering to particle scattering ratio),b~_bp,and tested their sensitivity to the changes in particle composition observed throughthe water column at the eddy center. Our results show that [chl]/c_p is not a goodindicator. Despite the shift in ecological community, the ratio remains controlledprimarily by the variation in chlorophyll concentration per cell with depth (photoadaptation),so that its values increase throughout the DCML. Steeper increase of [chl]/c_p below theDCML suggest that remineralization might be another important controlling factor. On theother hand, b~_bp clearly indicates a shift from a small phytoplankton to adiatom dominated community. Below an upper layer characterized by constant values, theb~_bp showed a rapid decrease to a broad minimum within the DCML. The highervalues below the DCML are consistent with enhanced remineralization below the eddy-inducedbloom. Both the "senescent" and the "healthy" diatom layers are characterized bysimilar optical properties, indicating some possible limitations in using opticalmeasurements to fully characterize the composition of suspended material in thewater column. The inverse relationship between b~_bp,reported by others for Case II waters, is observed neither for the backgroundconditions, nor in the presence of the eddy-induced diatom bloom. Between thetwo parameters, only the backscattering ratio showed the potential to be asuccessful indicator for changes in particle composition in Case I waters.

[学科分类] 地球化学与岩石