A decade of dissolved sulphur compounds measurements in the southwestern Baltic Sea

Abstract. Dissolved sulphur compounds were measured at the Boknis Eck Time-Series Station (BE, Eckernförde Bay, SW Baltic Sea) during the period February 2009–December 2018. Our results show considerable interannual and seasonal variabilities in the mixed layer concentrations of dimethyl sulphide (DMS), total dimethylsulphoniopropionate (DMSP_t) and total dimethyl sulphoxide (DMSO_t). Positive correlations were found between particulate DMSP (DMSP_p) and particulate DMSO (DMSO_p) as well as DMSP_t and DMSO_t in the mixed layer, suggesting a similar source for both compounds. The decreasing long-term trends, observed for DMSP_t and DMS in the mixed layer, were linked to the concurrent trend of the sum of 19'-hexanoyloxyfucoxanthin and 19'-butanoyloxy fucoxanthin, which are the marker pigments of prymnesiophyceae and chrysophytes, respectively. Major Baltic Inflow (MBI) events influenced the distribution of sulphur compounds due to phytoplankton community changes, and sediment might be a potential source for DMS in the bottom layer during seasonal hypoxia/anoxia at BE. A modified algorithm based on the phytoplankton pigments well reproduces the DMSP_p : Chl a ratios during this study and could be used to estimate future surface (5 m) DMSP_p concentrations at BE.