Abstract:This paper provides a review of some biological control or mitigation strategies for HABs, a problem worldwide in coastal waters. Bacteria, viruses and parasites are abundant in marine ecosystems and their abilities for rapid replication and host-specificity make them attractive HAB controlling agents; however, potential ecological impacts need to be taken into account. Species interactions between bloom microalgae and other algae play essential roles in affecting the phytoplankton sequence either by competing for the available nutrient supply or by secreting extracellular organic substances (allelopathy) into the environmental medium. This control strategy is not well understood due to a lack of experimental data, especially under natural conditions. Grazing of phytoplankton by zooplankton and suspension-feeding benthos is also considered a promising control agent, but this approach has many logistical problems and is a long way from the application stage. Collectively, some of the biological strategies discussed are decades from possible implementation, but others are further developed and thus worth considering in the immediate future.