ALBERT

Description: Optical properties are used to characterize the biogeochemistry of cyclonic eddy Opal in the lee of Hawaii. The eddy featured an intense diatom bloom. Our results show that the ratio of chlorophyll concentration to particulate beam attenuation coefficient, [chl]/cp, is not a good indicator of the changes in particle composition through the water column. The ratio is controlled primarily by the variation in chlorophyll concentration per cell with depth (photoadaptation), so that its values increase throughout the Deep Chlorophyll Maximum Layer (DCML). Below the DCML, high values of [chl]/cp suggest that remineralization might be another important controlling factor. On the other hand, the backscattering ratio (particle backscattering to particle scattering ratio, b~bp) clearly indicates a shift from a small phytoplankton to a diatom dominated community. Below an upper layer characterized by constant values, the b~bp ratio showed a rapid decrease to a broad minimum within the DCML. The higher values below the DCML are consistent with enhanced remineralization below the eddy-induced bloom. The DCML was characterized by a layer of "healthy" diatoms underlying a layer of "senescent" diatoms. These two layers are characterized by similar optical properties, indicating some possible limitations in using optical measurements to fully characterize the composition of suspended material in the water column. An inverse relationship between b~bp and [chl]/cp, also reported by others, is observed as deep as the DCML. There, [chl]/cp increases whereas b~bp remains similar to values found in the empty frustule layer. This is a further indication that [chl]/cp might not be a good alternative to the backscattering ratio for investigating changes in particle composition with depth in Case I waters.

Description: Optical properties were collected along a transect across cyclonic eddy Opal in the lee of Hawaii during the E-Flux III field experiment (10–27 March 2005). The eddy was characterized by an intense doming of isopycnal surfaces, and by an enhanced Deep Chlorophyll Maximum Layer (DCML) within its core. The phytoplankton bloom was diatom dominated, evidencing an eddy-induced shift in ecological community. Four distinct regions were identified throughout the water column at Opal's core: a surface 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 deep layer characterized by decreasing phytoplankton activity. We focused on two parameters, the ratio of chlorophyll concentration to particulate beam attenuation coefficient, [chl]/cp, and the backscattering ratio (the particle backscattering to particle scattering ratio), b~bp, and tested their sensitivity to the changes in particle composition observed through the water column at the eddy center. Our results show that [chl]/cp is not a good indicator. Despite the shift in ecological community, the ratio remains controlled primarily by the variation in chlorophyll concentration per cell with depth (photoadaptation), so that its values increase throughout the DCML. Steeper increase of [chl]/cp below the DCML suggest that remineralization might be another important controlling factor. On the other hand, b~bp clearly indicates a shift from a small phytoplankton to a diatom dominated community. Below an upper layer characterized by constant values, the b~bp showed a rapid decrease to a broad minimum within the DCML. The higher values below the DCML are consistent with enhanced remineralization below the eddy-induced bloom. Both the "senescent" and the "healthy" diatom layers are characterized by similar optical properties, indicating some possible limitations in using optical measurements to fully characterize the composition of suspended material in the water column. The inverse relationship between b~bp, reported by others for Case II waters, is observed neither for the background conditions, nor in the presence of the eddy-induced diatom bloom. Between the two parameters, only the backscattering ratio showed the potential to be a successful indicator for changes in particle composition in Case I waters.