Coastal eutrophication and freshening: Impacts on Pseudo-nitzschia abundance and domoic acid allelopathy

Species of the diatom genus Pseudo-nitzschia can produce domoic acid (DA), a neurotoxin responsible for amnesic shellfish poisoning. This release of DA by toxic Pseudo-nitzschia species in the water column may be a way to help the cells to outcompete other phytoplankton species (i.e. allelopathy). In this study, the influence of salinity and nutrient inputs on the effect of DA on natural phytoplankton community compositions was investigated using bioassays to measure phytoplankton responses to 400 ng DA ml(-1). Dissolved DA (dDA) was added to natural phytoplankton assemblages from a low salinity site dominated by river runoff and from a relatively pristine, high salinity site in which nutrients were added simultaneously with dDA. At the low salinity site, the percent inhibition of cryptophytes and diatoms induced by the addition of dDA was negatively correlated with salinity. However, at the high salinity site with nutrients added, no correlation with salinity was observed. This study highlights how environmental factors such as salinity and nutrients can play important roles on toxin allelopathy by alleviating or exacerbating its effect.