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

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:(Cphyto) 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.