Chemical interactions between species have been widely studied in terrestrial ecology but are less well known in marine ecosystems, especially among planktonic protists. In the marine plankton, temporal and spatial changes of biomass and species composition have traditionally been thought to be mainly regulated by resource availability and abiotic factors. However, there is increasing evidence that interspecific interactions in the plankton play a major role in succession, food web structure and bloom development. Many HAB species are regarded as rather poor exploitation competitors in terms of growth rate and/or resource uptake capabilities. There is some evidence for the hypothesis that a number of HAB species may gain dominance by the production of bioactive compounds, particularly secondary metabolites that affect growth or elicit other physiological responses in other organisms. Such allelochemicals may be targeted to exclude competitors from exploiting limited resources (interference competition), as well as to avoid/reduce predation.