HABs contain dense concentrations of organisms and appear as discolored water, often reddish-brown in color. It is a natural phenomenon, but the exact cause or combination of factors that result in a HAB event are not necessarily known. However, three key natural factors are thought to play an important role in a bloom - salinity, temperature, and wind. HABs cause economic harm, so outbreaks are carefully monitored. For example, the Florida Fish and Wildlife Conservation Commission  provides an up-to-date status report on HABs in Florida. The Texas Parks and Wildlife Department also provides a status report. While no  particular cause of HABs has been found, many different factors can contribute to their presence. These factors can include water pollution, which originates from sources such as human sewage and agricultural runoff.
The occurrence of HABs in some locations appears to be entirely natural (algal blooms are a seasonal occurrence resulting from coastal upwelling, a natural result of the movement of certain ocean currents) while in others they appear to be a result of increased nutrient pollution from human activities. The growth of marine phytoplankton is generally limited by the availability of nitrates and phosphates, which can be abundant in agricultural run-off as well as coastal upwelling zones. Other factors such as iron-rich dust influx from large desert areas such as the Sahara Desert are thought to play a major role in causing HAB events. Some algal blooms on the Pacific Coast have also been linked to occurrences of large-scale climatic oscillations such as El Niño events.