Potential impact of an exceptional bloom of Karenia mikimotoi on dissolved oxygen levels in waters off western Ireland

• Published in: Harmful algae 2016-03, Vol.53, p.77-85
• Main Authors: O’Boyle, Shane, McDermott, Georgina, Silke, Joe, Cusack, Caroline
• Format: Article
• Language: English
• Subjects: algae; anaerobic conditions; Atlantic Ocean; Bays - chemistry; biochemical oxygen demand; carbon; Carbon - metabolism; Cell biovolume; coasts; cytotoxicity; Dinoflagellida - metabolism; dissolved oxygen; Environmental Monitoring; Eutrophication; Exceptional bloom; Harmful Algal Bloom - physiology; Hypoxia; Ireland; Karenia; Karenia mikimotoi; Marine; Models, Biological; mortality; Nitrogen - metabolism; oxygen; Oxygen - metabolism; Oxygen depletion; Reproducibility of Results; Seawater - chemistry; stoichiometry; summer
• ISSN: 1568-9883; 1878-1470
• DOI: 10.1016/j.hal.2015.11.014

•An extensive bloom of Karenia mikimotoi occurred in 2005 off Ireland's west coast.•Impacts were rapid with sudden deaths of large quantities of marine life.•Not all bays exhibited the same level of devastation.•Levels of oxygen demand did not explain the pattern of marine mortalities.•Cause of mortality was likely to be cellular toxicity and/or mucilage production. In the summer of 2005 an exceptional bloom of the dinoflagellate Karenia mikimotoi occurred along Ireland's Atlantic seaboard and was associated with the mass mortality of both benthic and pelagic marine life. Oxygen depletion, cellular toxicity and physical smothering, are considered to be the main factors involved in mortality. In this paper we use a theoretical approach based on stoichiometry (the Anderson ratio) and an average K. mikimotoi cellular carbon content of 329pgCcell−1 (n=20) to calculate the carbonaceous and nitrogenous oxygen demand following bloom collapse. The method was validated against measurements of biochemical oxygen demand and K. mikimotoi cell concentration. The estimated potential oxygen utilisation (POU) was in good agreement with field observations across a range of cell concentrations. The magnitude of POU following bloom collapse, with the exception of three coastal areas, was considered insufficient to cause harm to most marine organisms. This indicates that the widespread occurrence of mortality was primarily due to other factors such as cellular toxicity and/or mucilage production, and not oxygen depletion or related phenomena. In Donegal Bay, Kilkieran Bay and inner Dingle Bay, where cell densities were in the order of 106cellsL−1, estimated POU was sufficient to cause hypoxia. Of the three areas, Donegal Bay is considered to be the most vulnerable due to its hydrographic characteristics (seasonally stratified, weak residual flow) and hypoxic conditions (2.2mgL−1 O2) were directly observed in the Bay post bloom collapse. Here, depending on the time of bloom collapse, depressed DO levels could persist for weeks and continue to have a potentially chronic impact on the Bay.