234Th-based export fluxes during a natural iron fertilization experiment in the Southern Ocean (KEOPS)

• Published in: Deep Sea Research Part II: Topical Studies in Oceanography
• Main Authors: Savoye, N, Trull, T, Jacquet, SHM, Navez, J, Dehairs, F
• Format: Article in Journal/Newspaper
• Language: English
• Published: Pergamon 2008
• Subjects: Earth Sciences; Oceanography; Chemical Oceanography; Southern Ocean; Kerguelen; Indian
• Online Access: http://www.sciencedirect.com; https://doi.org/10.1016/j.dsr2.2007.12.036; http://ecite.utas.edu.au/54668

Five iron-fertilization experiments in the Southern Ocean have clearly demonstrated that adding iron increases primary production, but theimplications for carbon export to the ocean interior have been less clear. This reflects both observational limitations of short-termexperiments and their uncertain relevance to quantifying ecosystem level processes that are likely to be structured differently under conditionsof punctual versus persistent stimulation. To avoid these biases, KEOPS (KErguelen Ocean and Plateau compared Study) investigated thenaturally 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 (9.038.4 mmolCm 2 d 1 and1.64.8 mmolNm 2 d 1) and were in the range of values reported for natural Southern Ocean ecosystems. Export fluxes were comparedat two reference stations, one above and one outside the Plateau. The station above the plateau was characterized by higher iron supplyand export fluxes compared to the station outside the plateau. The difference in the export flux between these two reference stationsdefines 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, and14.277.7 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 otherstudies 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 theplateau related to the temporal decoupling of production and export during the demise of the bloom. On average, the export efficiencywas lower over the plateau than in surrounding waters, suggesting that increased iron supply may increase total export but lower exportefficiency. Our findings are very important for evaluating present and past carbon cycling in the Southern and global oceans and forassessing predictive scenarios of carbon cycling and budget.