What is a bloom? A commentary

There are at least eight different modes and mechanisms by which harmful. phytoplankton species can cause mortality, physiological impairment, or other negative in situ effects. Their distinction from nonharmful phytoplankton taxa is clearly warranted. Increasing use of bloom descriptors such as ''exceptional,'' ''unusual,'' ''nuisance,'' and subjective reference to specific occurrences as blooms reveal widespread confusion concerning: what is a bloom?, how is it to be defined?, what is harmful?, and what distinguishes a harmful bloom from other blooms? Such efforts have been influenced by comparison with perceived spring diatom bloom characteristics. Examples, selected from among harmful bloom events, are presented to support the view that the classical focus on the spring diatom bloom has significant conceptual and operational biases as to what constitutes a bloom. This and the inexact criteria used to define a bloom compromise research on both harmful algal blooms and phytoplankton blooms in general. The subjective, differing, and arbitrary criteria used to define blooms and their presumed ecological consequences need to be replaced by a quantitatively based, ecological classification of the various types of phytoplankton blooms. The issue of what constitutes a bloom is more than simply a biomass issue.