A seasonal survey was undertaken, over a year, of phytoplankton from surface water in the western English Channel (Station L4) measuring mycosporine-like amino acids (MAAs), photosynthetic pigments and microscopic counts. Ground level solar radiation (ultraviolet-B, ultraviolet-A and photosynthetically active radiation; UV-B, UV-A and PAR) was measured at a nearby site. From this we estimated in situ solar irradiance received by phytoplankton using measurements of the mixed layer depth and calculations of the 50% light level cut-off. The MAAs occurred year round, with concentrations increasing rapidly during spring and summer (maximum 8·5 μg l−1) to levels exceeding those of chlorophyll-a (chl-a maximum 3·6 μg l−1). On two occasions, increases in specific MAAs coincide with algal blooms. In spring, increases in mycosporine-glycine (λmax 310 nm in the UV-B) coincide with a bloom of the prymnesiophyte Phaeocystis pouchetii and in July and August increases in an unidentified MAA (λmax 328 nm) match a bloom of the diatom, Guinardia striata (=Rhizosolenia stolterfothii). Concentration of MAAs, but not chlorophyll, correlate with in situ irradiance. The ratio of MAA to chl-a increases linearily with in situ irradiance received by phytoplankton reaching 13·9 nmol MAA (nmol chl-a)−1 at 101 W m−2. Evidence of photoinduction is observed during the P. pouchetii bloom with a four fold increase in the concentration of mycosporine-glycine (maximum 2 pg cell−1) as UV-B:PAR ratio increases from 0·0011 to 0·0014. Dinoflagellates, although contributing to <10% of biomass, are found through the correlation of MAAs to the biomarker peridinin, to contribute to baseline levels of MAAs throughout the year. Our MAA:chl-a values for the English Channel are similar to those measured in coastal areas of southern USA. Similarities with studies on Antarctic phytoplankton are also found with the dominance of porphra-334 and the presence of mycosporine-glycine in P. pouchetti.