The biogeography of subsurface chlorophyll maxima in the Southern Ocean
Kimberlee Anne Baldry, University of Tasmania, Institute of Marine and Antarctic Science, Hobart, TAS, Australia, Nicole A Hill, Institute of Marine and Antarctic Studies, University of Tasmania, Hobart, TAS, Australia, Peter G Strutton, University of Tasmania, Institute for Marine and Antarctic Studies, Hobart, TAS, Australia and Philip W Boyd, University of Tasmania, Institute for Marine and Antarctic Studies, Antarctic Climate Ecosystems Cooperative Research Centre, Hobart, Australia
Abstract:
The Iron hypothesis alludes to an absence of subsurface chlorophyll maxima (SCMs) in the High-Nutrient Low-Chlorophyll Southern Ocean due to its iron-limited state. Most SCMs are sustained by nutrient-light co-limitation, which relies on subsurface inputs of a limiting nutrient into the mixed layer to create a depth of optimum growth below the surface. In the Southern Ocean, iron is typically trapped tens-of-meters below the mixed layer. Consequently, it is thought that there is rarely a large sub-surface input of iron to sustain SCMs in the region. Despite this, a review by Holm-Hansen et al. (
Mar Ecol Prog Ser 2005) showed a geographically-wide presence of SCMs in the Southern Ocean, suggesting SCM formation through other mechanisms.
Here we investigate the prevalence of SCMs in the Southern Ocean using a synthesis of ship-based chlorophyll measurements and a review of Southern Ocean literature. We find that Southern Ocean SCMs are indeed a common occurrence and that mechanisms of SCM formation are variable, often dependant on biogeography. Moreover, some of these SCMs are ecologically important, contributing to water column primary production, community shifts and carbon export. To integrate subsurface phytoplankton variability captured by this study into basin-wide carbon budgets, our understanding needs to extend beyond the spatiotemporal limitations of ship-based data. This can be achieved by exploiting the capabilities of the Biogeochemical Argo program to study different types of SCMs.
