Optical Characterization of an Eddy-induced Diatom Bloom West of the Island of Hawaii

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]/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 through the water column at the eddy center. Our results show that [chl]/c p 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]/c p 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, thẽ 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, indicatingCorrespondence to: F. Nencioli (francesco.nencioli@opl.ucsb.edu) some possible limitations in using optical measurements to fully characterize the composition of suspended material in the water column. The inverse relationship betweenb bp and [chl]/c p , 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.

Spatial and seasonal variability of particulate matter optical and size properties in the Eastern Mediterranean Sea

“…At least two inherent optical properties are needed to determine relative composition of particles (organic versus inorganic) e.g. absorption, fluorescence and backscattering, (Holser et al, 2011), backscatter and total scatter (Loisel et al, 2007;Nencioli et al, 2010), backscatter and attenuation (Boss et al, 2009). Gardner et al (2001) demonstrated that continuous profiles of in situ beam attenuation and fluorescence, combined with discrete bottle data can provide information about particle properties, and composition.…”

A simple optical index shows spatial and temporal heterogeneity in phytoplankton community composition during the 2008 North Atlantic Bloom Experiment

“…A diversity of satellitebased algorithms for determining phytoplankton functional types from ocean color reflectance has been developed in the last decade (see review by Moisan et al, 2012), although without community consensus as to robustness. Nencioli et al (2010) implied that changes in the ratio of Chl to particulate beam attenuation coefficient (c p ) and the backscattering ratio (b bp /b p , where b bp is the total particulate scattering coefficient and b p is the backscattering coefficient) are associated with changes in phytoplankton composition and physiological (light) adaptation in eddies off Hawaii. In a mooring study of the spring bloom in the Labrador Sea, a change in phytoplankton species composition is offered as the explanation for the observed variability in Chl F /c p , although this suggestion is unconfirmed by in situ measurement of species composition (Strutton et al, 2011).…”

Evaluation of bio‐optical inversion of spectral irradiance measured from an autonomous underwater vehicle

“…The spectral downwelling attenuation in the Santa Barbara Channel showed high attenuation in the blue portion of the spectrum, consistent with a higher proportion of cDOM relative to phytoplankton (Figure 6). Even though measured attenuation by the AC‐S showed values off Hawaii an order of magnitude smaller than Santa Barbara Channel (data not shown), the values of K d ( λ ) derived from the AUV deployments in Hawaii were generally higher, with a spectral shape consistent with waters with a higher proportion of phytoplankton [ Nencioli et al , 2010]. Even though the depth of the Chl a maximum was likely deeper (∼100 to 120 m) than the diving depth of the AUV [ Sakamoto et al , 2004; Nencioli et al , 2010], the concentrations of Chl a were less than those in the Santa Barbara Channel and would not have influenced the K d ( λ ) values to the degree found in Figure 6.…”

“…Depths for Santa Barbara Channel were 1.5, 5, and 10 m. Depths for Hawaii were 5, 10, 15, 20, and 25 m for 5 September 2009 and 2, 4, 6, 12, 18 and 30 m on 6 September 2009. Derived data from the vehicle were also compared to the absorption coefficients measured with a WET Labs AC‐S following procedures outlined by Nencioli et al [2010].…”

“…Ninety-six percent of the irradiance data were collected with attitudes less than 6°. Given the limit of 10°attitude recommended by the SeaWiFS protocols [Mueller and Fargion, 2002], 3% of the data set were removed. These data occurred when the vehicle would surface for occasional GPS fixes (see below) and were not part of the underway data set used in this study.…”