Revisiting the Ocean Color Algorithms for Particulate Organic Carbon and Chlorophyll-a Concentrations in the Ross Sea

The Ross Sea is the most productive marginal sea in the Southern Ocean and plays an important role in carbon cycling. However, limited sampling of Chlorophyll-a (Chl) and particulate organic carbon (POC) concentrations from research expeditions constrains our understanding of the biogeochemical processes there. Satellites provide a useful tool for synoptic mapping of surface water properties on regional and global scales, yet the general applicability of the published algorithms in the Ross Sea is poorly known. Based on the data collected from 18 cruises in the past 20 years, we analyzed both the NASA standard and locally developed Chl and POC algorithms applicable to the Ross Sea. Our results show that Chl and POC are markedly underestimated using the NASA standard algorithms, with root mean square difference (RMSD) of 4.72 mg m(-3) and 218.0 mg m(-3), and mean bias of -3.48 mg m(-3) and -159.1 mg m(-3), for a wide range of Chl (0.42-16.3 mg m(-3)) and POC (46.8-812 mg m(-3)). Similar poor performances were also found for other algorithms applicable in the Ross Sea. We locally tuned both Chl and POC algorithms, and found that the Rrs667-based approach showed the most robust performances in retrieving both Chl and POC, with improved RMSD of 2.86 mg m(-3) and 129.7 mg m(-3), and limited biases. Our results show that the algal bloom signals in the Ross Sea in terms of Chl and POC are significantly greater than previously determined. More field observations will further constrain the locally tuned algorithms.

Automated summary

The Ross Sea is a highly productive marginal sea in the Southern Ocean, but limited sampling has constrained understanding of its biogeochemical processes. Existing algorithms for mapping surface water properties in the Ross Sea are found to underestimate Chlorophyll-a and particulate organic carbon concentrations. Local tuning of algorithms based on data from 18 cruises in the past 20 years shows improved accuracy in estimating Chl and POC, indicating that algal bloom signals in the Ross Sea are greater than previously thought. Further field observations will be needed to refine the locally tuned algorithms.