The Southern Ocean (SO) is known for its atypical bio-optical regime. This complicates the interpretation of proxies measured from satellite and in situ platforms equipped with optical sensors, which occupy an important niche for monitoring the vast and remote SO. A ship-based field study in concert with time series observations from BioGeoChemical-Argo (BGC-Argo) profiling floats were used to investigate spatial and temporal variations in bio-optical relationships in the open ocean waters surrounding the Kerguelen Plateau in the Indian sector of the SO. Compared to other regions with similar chlorophyll concentrations, chlorophyll-specific phytoplankton absorption in the blue waveband presented a consistent negative anomaly. The anomaly was uniform over deep mixed layers and correlated with phytoplankton size, photoacclimation and atypically high concentrations of fucoxanthin. The BGC-Argo observation-based proxies revealed that the blue absorption anomaly increased with chlorophyll concentration both spatially and temporally and, while particularly pronounced in the naturally iron-fertilized waters, was also found in the High Nutrient Low Chlorophyll region. While phytoplankton size was an important driver of the anomaly, photoacclimation associated with self-shading of phytoplankton cells was also involved during intense booms. The backscattering coefficient exhibited negative and positive anomalies in the low and high biomass regimes, respectively. The large positive anomaly in high biomass regimes was attributed to the variable non-algal particles characteristics associated with a relatively high production of bloom by-products. With clear understanding of the bio-optical anomalies, BGC-Argo floats stand as unique tools for monitoring the bio-optical spatio-temporal complexity of the SO.

Key Points

Contribution by large diatoms and photoacclimation are major drivers of a negative anomaly in the blue chlorophyll-specific phytoplankton absorption coefficient

BioGeoChemical-Argo observations indicate a bio-optical anomaly exists over the entire year with maximal values associated with seasonal blooms

The Indian Southern Ocean shows a large seasonal and regional variability in bio-optical regimes that lie outside the global means

Plain Language Summary

The Southern Ocean (SO) plays a key role in Earth's climate. However, its remoteness and harsh climate necessitate remote sensing approaches such as optical sensors on profiling floats and satellites. These require optical measurements and shipboard samples to relate optical signals to biological properties, called optical proxies. The SO optical proxies have been found to be very different from those measured in other oceanic regions. Here the proxies in the Kerguelen region are investigated along a ship's track crossing from subtropical to subpolar latitudes. The Kerguelen Plateau impedes both wind and currents such that deep waters upwell to the surface providing nutrients to primary producers. This region in the Indian sector of the SO provides a tremendous range in ecosystem properties from nutrient-starved to nutrient-rich, making these results applicable to other SO areas. The authors find that the differences in optical proxies can be explained by the presence of different phytoplankton communities and their responses to being mixed to depths where light is limited. This explains why the proxies vary so significantly across this region and change over the seasons. This knowledge makes the observations obtained by floats and satellites invaluable for understanding the entire SO.