Bloom of Karenia mikimotoi in Rossbeigh, Kerry

An algal bloom was identified this week in Rossbeigh, Co. Kerry following reports of seawater surface discolouration observations from lifeguards and the public to Kerry County Council. Following samples received by the Marine Institute’s Phytoplankton unit, the cause of this surface water discolouration has been identified and confirmed as a bloom of the microscopic unicellular planktonic species Karenia mikimotoi. During the week commencing 27th July 2026, it has been reported that this bloom is now dissipating.

Karenia mikimotoi is not uncommon in Irish coastal waters at this time of the year and is notable for its occasional brown discolouration at the seawater surface.

These blooms are part of the natural summer marine phytoplankton community, and most likely form due to combination of ocean currents and optimal environmental conditions. These conditions can potentially result in the rapid growth of this species, which can occasionally accumulate in large concentrations along and near to coastlines as a result of transportation into the coastal zone by offshore ocean currents.

Currently, this species has been observed in multiple inshore bays along the south, southwest and west-southwest coasts. They are a natural part of marine ecosystems and plankton communities, however, they have also been associated with mortalities of marine organisms. It is thought that these mortalities are as a result of depleted or low oxygen levels that can occur when the bloom begins to decay.

Marine algal blooms are naturally occurring and are seen almost every year during the summer months. The Marine Institute routinely analyses on a weekly basis, all year round, water samples from around inshore aquaculture producing areas of Ireland to identify toxin producing, harmful or nuisance phytoplankton species, and provide scientific advice on their impact on shellfish & finfish in particular. 

The results of the Marine Institute's Phytoplankton Monitoring Programme can be seen on https://miprodrp01.marine.ie/HABs/.