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The influences of physico-chemical and biological variables on the concentrations of dimethyl sulphide (DMS) and its precursor β-dimethylsulphoniopropionate (DMSP) were investigated through an annual cycle in the temperate shelf seas of the western English Channel. Total DMSP to chlorophyll a ratios (DMSPt/chl a) varied seasonally by 40-fold, and DMS and DMSP concentrations became temporally uncoupled, with elevated relative DMS concentrations during spring and midsummer. Taxonomic succession of high DMSP-producing phytoplankton, including Phaeocystis pouchetii, Scrippsiella trochoidea and Prorocentrum minimum, is apparent in the seasonal pattern of DMSPt concentrations. Peridinin and DMSPt concentrations showed similar seasonal trends (p < 0.0001), illustrating the substantial contribution by dinoflagellate taxa to DMSP production. Summertime stratification of the water column coincided with increased mixed layer doses of photosynthetically active radiation (PAR), increased surface ultraviolet-B (UVB) irradiance relative to PAR and a decrease in nitrate and phosphate availability. PAR dose explained 68% of the variability in DMSP/chl a during the seasonal study; whilst nitrate concentrations were inversely related to DMSP/chl a and explained 64% of the variability in log-transformed DMSP/chl a. PAR dose explained only 25% of the variation in DMS concentration, whilst nitrate concentration was inversely related to DMS and explained 49% of the variation in log-transformed DMS concentration. The highly significant relationship between DMSP/chl a and PAR dose was similar to those observed for the chlorophyll-specific accumulation of the photoprotective xanthophyll compounds diadinoxanthin and diatoxanthin and the chlorophyll-specific concentrations of UV-absorbing mycosporine-like amino acids. These results lend further, indirect evidence for a photoprotective role of DMSP, possibly associated with physiological stress caused by high PAR and UV radiation and intensified by nutrient limitation. KEY WORDS: DMSP · DMS · Seasonal cycle · Xanthophyll pigments · MAAs · NutrientsResale or republication not permitted without written consent of the publisher Mar Ecol Prog Ser 394: 111-124, 2009 vary by > 3 orders of magnitude (Keller et al. 1989), and the ability to convert DMSP into DMS through DMSPlyase activity also varies markedly between phytoplankton taxa (Niki et al. 2000). DMSP appears to play a range of roles in phytoplankton, in addition to acting as an osmolyte or compatible solute (Stefels 2000). Which physiological function(s) drive(s) the taxonomic variation in DMSP intracellular content remains unclear and is one of the hurdles to the development of mechanistic models that are capable of predicting DMS emission both seasonally and regionally.Studies of seasonal variations in DMS and DMSP concentrations have played an important part in helping to elucidate some of the important factors controlling DMS emissions. Leck et al. (1990) demonstrated the lack of a relationship between DMS concentrations ...

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