Annual contribution of different plankton size classes to particulate dimethylsulfoniopropionate in a marine perturbed ecosystem

Dimethylsulfoniopropionate (DMSP) concentrations were observed from October 1999 to September 2000 in a Mediterranean ecosystem (Little Bay of Toulon) submitted to eutrophication. DMSP was measured in the particulate material (DMSPp), and more specifically in the >90, 5–90 and 0.2–5 μm fractions. DMSP was first converted into dimethylsulfide (DMS) by a cold alkali treatment. DMS was then analysed by gas chromatography equipped with a flame photometric detector (FPD). DMSPp concentrations were relatively high, showing a strong temporal variability with maxima in February–March (58.8 nM). The most elevated values were recorded in the 5–90 μm size class, and represented between 60% and 100% of the total DMSPp. This fraction was mostly composed of Dinoflagellates whose biomass was significantly correlated with DMSPp concentrations. These concentrations showed better correlations with Ceratium furca, Dinophysis acuminata, Prorocentrum arcuatum and also Alexandrium minutum. The intracellular contents of DMSP were much higher in Dinoflagellates (124.9±5.7 mM) than in Diatoms (25.1±1.1 mM). A. minutum produced the strongest intracellular concentrations (3387.6±121.9 mM). High DMSP contents were also found in D. acuminata (477.4±64.3 mM) and P. arcuatum (442.2±22.9 mM). The >90 μm size class had a minor importance in DMSP production, generally below 20% of the total DMSPp. However, DMSP in the >90 μm fraction was well correlated with cladoceran abundance. Plankton cells between 0.2 and 5 μm contained a low part of the DMSPp pool, lower than 20%. Besides, no correlation was found between their algal abundances and DMSP in the 0.2–5 μm size class. Temperature and photoperiod could have influenced the Dinoflagellate development, and consequently, the DMSPp concentrations. The strong autumnal rains affected the composition of the phytoplankton community and the production of sulfur compounds. These particular climatic conditions have induced an increase in nutrient concentrations and a drop in salinity, which may explain the low autumnal DMSPp concentrations.