In the present study we tested the hypothesis that environmental concentrations of the antifouling agent Irgarol 1051, as measured in coastal Western European waters, affect marine phytoplankton performance. The impact of Irgarol was investigated in the phytoplankton species Thalassiosira weissflogii, Emiliania huxleyi, Tetraselmis sp. and Fibrocapsa japonica. EC50 concentrations for growth, effective quantum yield of PSII and viability were calculated from dose response relationships established during 72 h exposures to six Irgarol concentrations. Furthermore, the biological recuperation from a temporary exposure to a high Irgarol concentration (39.47 nM l− 1) was monitored. Growth rates and effective quantum yield were strongly affected by Irgarol, however viability loss was never observed. EC50 values differed five fold between species and ranged from 0.43 to 2.38 nM for effective quantum yield and from 0.46 to 2.44 nM for growth rate. For all species, complete biological recuperation was shown within 3–4 days after the Irgarol treatment, both for effective quantum yield and growth rate. All calculated EC50 values and EC20 values fall within the Irgarol concentration range measured in Western European coastal waters. We therefore conclude that present day Irgarol 1051 levels may affect the in situ performance of marine phytoplankton in this area.