A new assessment in North Atlantic waters of the relationship between DMS concentration and the upper mixed layer solar radiation dose

The results of the POMME experiment, conducted in the northeast Atlantic Ocean in 2001, were used to explore whether dimethylsulfide (DMS) concentrations are linked to epipelagic ecosystem exposure to solar radiation as proposed by Vallina and Simo (2007). According to the seasonal variations in the DMS-to-dimethylsulfoniopropionate (DMSP) ratio, we found that the summer surface water concentration of DMS was, on average, threefold higher than expected from the abundance of DMSP. This is in agreement with previous observations and confirms that seasonal changes in the trophic regime, from mesotrophy in winter and spring to oligotrophy in summer, are accompanied by a relative enhancement of DMS over DMSP. However, contrary to the observations carried out at Hydrostation S in the northwest Atlantic Ocean, no strong relationship between DMS and the solar radiation dose (SRD) exists in the northeast Atlantic Ocean. From a series of sensitivity tests, where different combinations of the three parameters that drive the SRD were investigated (i.e., the solar irradiance, the law of its attenuation in the sea, and the mixed layer depth), we found that the SRD accounted for only 19% to 24% of the variance associated with monthly surface DMS concentrations. Additionally, the slope of the relationship between DMS and SRD was particularly sensitive to the choice of the irradiance attenuation law. Overall, we find that the DMS versus SRD relationship is much less significant in the northeast Atlantic Ocean than in the Sargasso Sea. In addition, we suggest a large impact of algal community structure on summer DMS concentrations in the mesotrophic coastal waters of the Mediterranean Sea. Therefore, we question the consistency between DMS versus SRD relationships at local, basin, and global scales and propose that empirical relationships relating DMS to SRD be applied with caution.