Extreme irgarol tolerance in an Ulva lactuca L. population on the Swedish west coast

•Effects of irgarol on settlement and growth of Ulva lactuca zoospores.•Settlement and growth of zoospores was not inhibited, even at high concentrations.•Irgarol causes greening and possibly and increased attachment.•Irgarol constitutes a selection pressure in the marine environment. The herbicide irgarol 1051 is commonly used on ship hulls to prevent growth of algae, but as a component of self-eroding paints it can also spread in the surrounding waters and affect non-target organisms. The effect of irgarol on settlement and growth of zoospores from the marine macro algae Ulva lactuca from the Gullmar fjord on the Swedish west coast was investigated in the present study. The zoospores were allowed to settle and grow in the presence of irgarol, but neither settlement – nor growth inhibition was observed at concentrations of up to 2000nmoll−1. This is between 10 and 100 times higher than effect concentrations reported earlier for algae. Irgarol also induced the greening effect (4-fold increase in chlorophyll a content) in the settled zoospore/germling population, typical for photosystem II inhibitors like irgarol. This study support previous findings that irgarol constitutes a selection pressure in the marine environment.