Export of plankton functional types from Austral island blooms naturally fertilized by Iron

Large areas of the Southern Ocean are characterised by low phytoplankton biomass, in spite of adequate concentrations of macronutrients, so-called High-Nutrient Low Chlorophyll (HNLC) areas. It has been demonstrated previously from mesoscale enrichment experiments that phytoplankton blooms in these HNLC areas are limited by iron availability. Furthermore, it has been hypothesised that glacial-interglacial cycles in atmospheric CO2 concentrations are regulated in part by natural iron supplies to the Southern Ocean which control the growth and export of plankton functional groups. However, understanding the impact of iron on the selective export of plankton functional groups and the resulting impact on biogeochemical cycles has remained a challenge. Island systems in the Southern Ocean can serve as a source of iron to HNLC areas and act as natural laboratories for studying marine biogeochemistry. In this project the objectives were to study natural island systems in the austral Southern Ocean and (i)Characterise diatom export assemblages and estimate their contribution towards Si and C export from Austral Island blooms, (ii) characterise the contribution of coccoliths, foraminifers and pteropods to carbonate flux south of the polar front and their importance for CO2 sequestration, (iii) measure the degradation state of particulate organic carbon exported from naturally iron fertilized blooms in the Southern Ocean.

There are some areas of the global ocean that have high nutrient concentrations but low production. Iron, an important micronutrient, has been shown to limit phytoplankton growth in these areas. The growth of these phytoplankton is important for sequestering atmospheric carbon dioxide and providing a food source to deep benthic communities. In this project we aim to identify which species of phytoplankton grow and are exported to the deep-ocean in iron-fertilised areas and examine the potential impact on energy supply to deep ocean ecosystems.

As part of this project we demonstrated the the export of phytoplankton from naturally iron fertilised areas in the Southern Ocean is typically dominated by resting spores of species such as Chaetoceros. These are responsible for sequestering atmospheric CO2 upon iron fertilisation and provide an important food source to benthic ecosystems through the delivery of fresh and labile fatty acids. We also quantified the proportion of caclium carbonate exported through different plankton functional groups such as coccolithophores, foraminifera and pteropods and estimated there is a reduction in carbon sequestration efficiency that results from the carbonate counter pump.