The metabolic balance at two contrasting sites in the Southern Ocean: The iron-fertilized Kerguelen area and HNLC waters

We investigated the balance between autotrophic and heterotrophic processes during the third month of an intense phytoplankton bloom induced by natural iron enrichment above the Kerguelen plateau (Southern Ocean). Gross (GCP)- and net community production (NCP) and dark community respiration (DCR) were determined using concurrent measurements of dissolved oxygen (O2) and total carbon dioxide (TCO2) evolution in light- and dark-bottle incubations. Experiments were carried out during four visits to one station located in the core of the phytoplankton bloom and at a high-nutrient low-chlorophyll (HNLC) site. Within the bloom, euphotic zone (around 45 m) integrated fluxes of NCP were high during the first three visits, varying between 64 and 92 mmol O2 m−2 d−1 and between −43 and −105 mmol TCO2 m−2 d−1. On the last visit, however, fluxes of NCP indicated a shift from autotrophic to heterotrophic plankton metabolism, with a net consumption of O2 (−72 mmol O2 m−2 d−1) and a net production of TCO2 (64 mmol TCO2 m−2 d−1). This shift was accompanied by a 5-fold increase in DCR, while integrated fluxes of GCP remained similar throughout the sampling period (110±21 mmol O2 m−2 d−1 and −100±27 mmol TCO2 m−2 d−1). Fluxes of NCP integrated to the euphotic zone (around 100 m) were low at a typical HLNC site, ranging from 0 to 5 mmol O2 m−2 d−1 during three visits. These data suggest that above the Kerguelen plateau organic matter production largely exceeded its immediate respiration, while at the HLNC site autotrophic and heterotrophic processes are close to balance. During the spring phytoplankton bloom above the Kerguelen plateau, a substantial part of primary production is therefore potentially available for mesozooplankton grazing and export, or respiration at a different time period.