Using Optical Sensors on Gliders to Estimate Phytoplankton Carbon Concentrations and Chlorophyll-to-Carbon Ratios in the Southern Ocean

Abstract

One approach to deriving phytoplankton carbon biomass estimates (C phyto ) at appropriate scales is through optical products. This study uses a high-resolution glider data set in the Sub-Antarctic Zone (SAZ) of the Southern Ocean to compare four different methods of deriving C phyto from particulate backscattering and fluorescence-derived chlorophyll (chl-a). A comparison of the methods showed that at low (<0.5 mg m −3 ) chlorophyll concentrations (e.g., early spring and at depth), all four methods produced similar estimates of C phyto , whereas when chlorophyll concentrations were elevated one method derived higher concentrations of C phyto than the others. The use of methods derived from particulate backscattering rather than fluorescence can account for cellular adjustments in chl-a:C phyto that are not driven by biomass alone. A comparison of the glider chl-a:C phyto ratios from the different optical methods with ratios from laboratory cultures and cruise data found that some optical methods of deriving C phyto performed better in the SAZ than others and that regionally derived methods may be unsuitable for application to the Southern Ocean. A comparison of the glider chl-a:C phyto ratios with output from a complex biogeochemical model shows that although a ratio of 0.02 mg chl-a mg C −1 is an acceptable mean for SAZ phytoplankton (in spring-summer), the model misrepresents the seasonal cycle (with decreasing ratios from spring to summer and low sub-seasonal variability). As such, it is recommended that models expand their allowance for variable chl-a:C phyto ratios that not only account for phytoplankton acclimation to low light conditions in spring but also to higher optimal chl-a:C phyto ratios with increasing growth rates in summer.

Temporal and Vertical Variations of Particulate and Dissolved Optical Properties in the South China Sea

“…On the other hand, although b bp is often used as a proxy for particulate organic carbon or even phytoplankton carbon (C phy ; Behrenfeld et al, ; Boss & Behrenfeld, ; Graff et al, ; Mignot et al, ; Thomalla et al, ), its variability also depends on other aspects (e.g., particle size distribution, shape, structure, and refractive index; e.g., Stramski et al, ), thus sensitive to picophytoplankton (Cetinić et al, ) or highly refractive algae (e.g., cocolithophore, Balch et al, ), as well as to nonalgal particles (e.g., detritus, bacteria, and viruses, Ahn et al, , Loisel et al, ). Cetinić et al () proposed using the ratio of [Chla] and b bp as a particle size index.…”

Revisiting Ocean Color algorithms for chlorophyll a and particulate organic carbon in the Southern Ocean using biogeochemical floats

“…[], and broader than the range (20–100 g C g chl a −1 ) of Thomalla et al . [] for the SO (Figure ). Consistent with Reynolds et al .…”

Validation and Intercomparison of Ocean Color Algorithms for Estimating Particulate Organic Carbon in the Oceans

“…Databases collated included PANGAEA (https://www.pangaea.de/) and SeaBASS (Werdell and Bailey, 2005), and those compiled by Martiny et al (2014) and the Biological and Chemical Oceanography Data Management Office (BCO-DMO, USA). Further data were included from the Atlantic Meridional Transect (AMT) (including data derived from both CTD and the ship's clean water supply) and other cruises in the Southern Ocean (see a description of the Good Hope line and associated data collection in Thomalla et al, 2017). Operationally, POC is defined as all the organic carbon that is retained on GF/F filters (nominal pore size of 0.7 µm).…”