Although these thresholds have clarified what constitutes HABs
for certain groups, the specific choices that had to be made, and the diversity
among the definitions selected by the WHO, IJC, and Ohio EPA, further
highlight that, although intelligent decisions as to operational definitions
can be made, a single clear scientific definition is elusive. This diversity in
metrics, harms, and forms of linkages between them can be problematic
for addressing more complex questions if the choice of metrics leads to
contradictory conclusions. For instance, if inferences based on measurements
of Microcystis biovolume differ fromthose based on cyanobacterial
biomass, then mechanisticmodels validated against one or the other metric
could yield substantively different results. This is especially true if different
metrics disagree on seemingly straightforward questions of bloom
occurrence, extent, and timing. This topic is explored in the following
sections.