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Natural iron fertilisation from Southern Ocean islands results in highprimary production and phytoplankton biomass accumulations readily visiblein satellite ocean colour observations. These images reveal great spatialcomplexity with highly varying concentrations of chlorophyll, presumablyreflecting both variations in iron supply and conditions favouringphytoplankton accumulation. To examine the second aspect, in particular theinfluences of variations in temperature and mixed layer depth, we deployedfour autonomous profiling floats in the Antarctic Circumpolar Current nearthe Kerguelen Plateau in the Indian sector of the Southern Ocean. Each"bio-profiler" measured more than 250 profiles of temperature (T), salinity(S), dissolved oxygen, chlorophyll a (Chl a) fluorescence, and particulatebackscattering (bbp) in the top 300 m of the water column,sampling up to 5 profiles per day along meandering trajectories extending upto 1000 km. Comparison of surface Chl a estimates (analogous to values fromsatellite images) with total water column inventories revealed largelylinear relationships, suggesting that these images provide credibleinformation on total and not just surface biomass spatial distributions. However,they also showed that physical mixed layer depths are often not a reliable guide tobiomass distributions. Regions ofvery high Chl a accumulation (1.5–10 μg L−1) were associatedpredominantly with a narrow T–S class of surface waters. In contrast, waterswith only moderate Chl a enrichments (0.5–1.5 μg L−1) displayedno clear correlation with specific water properties, including no dependenceon mixed layer depth or the intensity of stratification. Geostrophictrajectory analysis suggests that both these observations can be explainedif the main determinant of biomass in a given water parcel is the time sinceleaving the Kerguelen Plateau. One float became trapped in a cyclonic eddy,allowing temporal evaluation of the water column in early autumn. Duringthis period, decreasing surface Chl a inventories corresponded withdecreases in oxygen inventories on sub-mixed-layer density surfaces,consistent with significant export of organic matter (~35%) and its respiration and storage as dissolved inorganic carbon in theocean interior. These results are encouraging for the expanded use ofautonomous observing platforms to study biogeochemical, carbon cycle, andecological problems, although the complex blend of Lagrangian and Euleriansampling achieved by the floats suggests that arrays rather than singlefloats will often be required, and that frequent profiling offers importantbenefits in terms of resolving the role of mesoscale structures on biomassaccumulation

Autonomous profiling float observations of the high-biomass plume downstream of the Kerguelen Plateau in the Southern Ocean