Effect of natural iron fertilisation on the distribution of DMS and DMSP in the Indian sector of the Southern Ocean

Understanding the processes controlling the distribution and fate of dimethylsulphide (DMS) in marine surface waters is critical to quantifying the flux of DMS to the atmosphere. All fertilisation experiments in the Southern Ocean trying to mimic a short-term (several weeks) atmospheric, dust-related, iron supply so far have shown that DMS concentrations increase when iron is added artificially in small patches to iron-limited regions. During the Kerguelen Ocean and Plateau compared Study (KEOPS), we have investigated the impact on surface concentrations of DMS and its precursor, particulate dimethylsulphoniopropionate (DMSPp), of iron supplied by mixing from below. We found that the massive summer phytoplanktonic bloom sustained by naturally enhanced vertical mixing of deep iron-rich seawater above the Kerguelen Plateau is not a substantially larger source of DMS than the surrounding low-primary productivity waters. The sustained enrichment of iron over the Kerguelen Plateau does not translate into an increase in DMS concentration because: (1) the nano-phytoplankton, which is the main producer of DMSPp in the area, is not favoured by the continuous supply of iron in surface waters, (2) the transfer efficiency of DMSP to DMS is probably low due to the higher levels of bacterial production measured over the plateau, which, likely, also stimulate the bacterial removal of DMS, and (3) nitrates and fresh chromophoric dissolved organic matter, supplied in surface waters from enhanced mixing of deep water over the plateau, increase the photodegradation rates of DMS. Thus, artificial short-term iron supplies from above and naturally sustained ones show fundamental differences in their impacts on the distribution and net production of DMS.