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

M Grenier,T W Trull,A Della Penna

doi:10.5194/bg-12-2707-2015

Abstract

Abstract. Natural iron fertilisation from Southern Ocean islands results in high primary production and phytoplankton biomass accumulations readily visible in satellite ocean colour observations. These images reveal great spatial complexity with highly varying concentrations of chlorophyll, presumably reflecting both variations in iron supply and conditions favouring phytoplankton accumulation. To examine the second aspect, in particular the influences of variations in temperature and mixed layer depth, we deployed four autonomous profiling floats in the Antarctic Circumpolar Current near the 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 particulate backscattering (bbp) in the top 300 m of the water column, sampling up to 5 profiles per day along meandering trajectories extending up to 1000 km. Comparison of surface Chl a estimates (analogous to values from satellite images) with total water column inventories revealed largely linear relationships, suggesting that these images provide credible information on total and not just surface biomass spatial distributions. However, they also showed that physical mixed layer depths are often not a reliable guide to biomass distributions. Regions of very high Chl a accumulation (1.5–10 μg L−1) were associated predominantly with a narrow T–S class of surface waters. In contrast, waters with only moderate Chl a enrichments (0.5–1.5 μg L−1) displayed no clear correlation with specific water properties, including no dependence on mixed layer depth or the intensity of stratification. Geostrophic trajectory analysis suggests that both these observations can be explained if the main determinant of biomass in a given water parcel is the time since leaving the Kerguelen Plateau. One float became trapped in a cyclonic eddy, allowing temporal evaluation of the water column in early autumn. During this period, decreasing surface Chl a inventories corresponded with decreases 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 the ocean interior. These results are encouraging for the expanded use of autonomous observing platforms to study biogeochemical, carbon cycle, and ecological problems, although the complex blend of Lagrangian and Eulerian sampling achieved by the floats suggests that arrays rather than single floats will often be required, and that frequent profiling offers important benefits in terms of resolving the role of mesoscale structures on biomass accumulation.

Highlights

The productivity of the Southern Ocean is important for many reasons
The drifts of the bio-profilers provided coverage of a large portion of the elevated biomass plume (Fig. 1), from near the Kerguelen Plateau to more than 700 nautical miles downstream (71 to 95◦ E) and nearly 400 nautical miles from north to south (47.5 to 54◦ S), thereby spanning waters of the Polar Frontal and Antarctic zones (Orsi et al, 1995; Park et al, 2008b; Sokolov and Rintoul, 2009). This breadth of spatial coverage of the plume did not extend to full temporal seasonal coverage, and this is important to keep in mind given the strong seasonal cycle of biomass accumulation (Trull et al, 2015; Blain et al, 2007; Mongin et al, 2008)
As shown in these images, the 2011 bio-profiler covered the period of highest biomass accumulation, while the 2014 deployments occurred after this seasonal peak and sampled the system during its senescence

Summary

Introduction

The productivity of the Southern Ocean is important for many reasons It supports fisheries and high-conservationvalue marine mammal and bird populations (Constable et al, 2003; Nicol et al, 2000), influences the carbon dioxide content of the atmosphere (Sarmiento and Le Quéré, 1996; Sigman and Boyle, 2000; Watson et al, 2000), and affects the magnitude of nutrient supply to large portions of the global surface ocean (Sarmiento et al, 2004). This diversity derives from interactions between the supply and bioavailability of iron with other drivers of productivity such as temperature, water column stratification and stability, light levels, and the possibility of co-limitation by other nutrients (Assmy et al, 2013; Boyd et al, 1999, 2001; Queguiner, 2013)

Methods

Results

Discussion

Conclusion