234Th-based export fluxes during a natural iron fertilization experiment in the Southern Ocean (KEOPS)
Savoye, N and Trull, T and Jacquet, SHM and Navez, J and Dehairs, F, 234Th-based export fluxes during a natural iron fertilization experiment in the Southern Ocean (KEOPS), Deep Sea Research Part II: Topical Studies in Oceanography, 55, (5-7) pp. 841-855. ISSN 0967-0645 (2008) [Refereed Article]
Five iron-fertilization experiments in the Southern Ocean have clearly demonstrated that adding iron increases primary production, but the
implications for carbon export to the ocean interior have been less clear. This reflects both observational limitations of short-term
experiments and their uncertain relevance to quantifying ecosystem level processes that are likely to be structured differently under conditions
of punctual versus persistent stimulation. To avoid these biases, KEOPS (KErguelen Ocean and Plateau compared Study) investigated the
naturally iron-fertilized Kerguelen Plateau region in the Indian Sector of the Southern Ocean that exhibits an annual phytoplankton bloom.
Here, we report particulate organic carbon (POC) and nitrogen export from this system based on the 234Th approach.
Results indicate that the export fluxes were variable both on and off the Kerguelen Plateau (9.0–38.4 mmolCm 2 d 1 and
1.6–4.8 mmolNm 2 d 1) and were in the range of values reported for natural Southern Ocean ecosystems. Export fluxes were compared
at two reference stations, one above and one outside the Plateau. The station above the plateau was characterized by higher iron supply
and export fluxes compared to the station outside the plateau. The difference in the export flux between these two reference stations
defines the export excess induced by iron fertilization. It was 10.874.9 mmolCm 2 d 1 and 0.970.7 mmolNm 2 d 1 at 100 m, and
14.277.7 mmolCm 2 d 1 and 2.071.3 mmolNm 2 d 1 at 200 m. This POC export excess was similar to those found during other
studies of artificial (SOFeX) and natural (CROZEX) iron fertilization in the Southern Ocean.
The examination of the export efficiency (defined as the ratio of export to primary production) revealed significant variability over the
plateau related to the temporal decoupling of production and export during the demise of the bloom. On average, the export efficiency
was lower over the plateau than in surrounding waters, suggesting that increased iron supply may increase total export but lower export
efficiency. Our findings are very important for evaluating present and past carbon cycling in the Southern and global oceans and for
assessing predictive scenarios of carbon cycling and budget.