Abstract
Abstract. 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.
Highlights
During the last two decades, many studies have focused on the relationships between optical properties and biogeochemical constituents in order to develop rapid, continuous, in situ techniques for retrieving information such as the amount, type and composition of suspended material in the water column
Few studies have focused on the vertical distribution of these properties to these depths in concentrations in open ocean (Case I) waters (e.g. Twardowski et al, 2007), as most open ocean studies primarily investigate the spatial and temporal evolution of optical parameters in the very upper layer of the water column (e.g. Huot et al, 2008; Behrenfeld et al, 2005; Morel et al, 2007) due to its importance for remote sensing
Vertical profiles from outside cyclone Opal are compared to the ones collected at its core to test the sensitivity of the two parameters to the changes in biogeochemical characteristics and the shift in ecological community observed within the eddy
Summary
During the last two decades, many studies have focused on the relationships between optical properties and biogeochemical constituents (e.g., phytoplankton, sediment, dissolved matter) in order to develop rapid, continuous, in situ techniques for retrieving information such as the amount, type and composition of suspended material in the water column. Chlorophyll fluorescence, beam attenuation coefficient [cp, m−1], total particulate scattering [bp, m−1], and particulate backscattering [bbp, m−1] are the optical quantities of main interest in this study. The slope of spectral cp, γ , can be used to estimate the particle size distribution (PSD) slope [ξ ] through the relationship γ ≈ ξ −3 (Boss et al, 2001b,a). Assuming a hyperbolic PSD (Junge-type model), ξ describes the shape of the distribution: higher values of ξ indicate a steep distribution, in which smaller particles dominate.
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