The data do not show the magnitude of each event. In general, there are limited ground-based monitoring initiatives with regular data collection on HABs events around the world. Global occurrence of hypoxic zones Diaz, R.J. and R. Rosenberg. A Review of its Ecological Effects and the Behavioural Responses of Benthic Macrofauna. Occurrences are compiled from literature and therefore may be biased toward areas where better reporting mechanisms exist.
Most observations are from industrialized countries.
The data do not include the duration and size of each event. The 1995 National Shellfish Register . There is insufficient data coverage for temporal trend analysis and, often, inconsistent criteria for bed closures. Various country programs exist, mostly in developed countries, but the data are not comparable. Beach closures Natural Resources Defense Council.