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Algal mats in Lake Ontario. (G. Boyer)

Other Freshwater HABs

Non-cyanobacterial freshwater HABs cause harm by generating excessive biomass or through production of compounds that are toxic to fish or other aquatic organisms.  These include some species of haptophytes, dinoflagellates, green algae, raphidophytes, euglenophytes, and cryptophytes.  Some of these species bloom in inland waters of higher salinity, such as the Salton Sea. 


Nuisance species of the green algae Chlorophyta include both macro- and microalgae.  The macroalga Cladophora has had a recent resurgence in the Great Lakes.  Cladophora forms foul-smelling nuisance blooms that are deposited on beaches, clog water intakes, and potentially harbor pathogens, such as E. coli.  Economic impacts also result due to lowered beach use.  It is hypothesized that the resurgence in the Great Lakes may be due, in part, to increased water clarity caused by the introduction of the zebra mussel.  Other potential causes include possible increased nutrient inputs, increased water temperature, and changing lake level.


Euglenophytes can be found in fresh, estuarine, and marine waters and are most common in slow moving or still waters with high nutrient concentrations.  Blooms are most likely to occur during summer in freshwater ponds and ditches that receive nutrient-rich waste or runoff, and many species of euglenophytes are considered indicators of organic water pollution.  Euglenophyte blooms may color the water green, reddish brown, or red and are generally not toxic, but some freshwater euglenophytes have recently been implicated in fish kills in North Carolina, presumably due to the production of an ichthyotoxic compound. 


The marine raphidophyte Chatonella, which has caused fish kills in Japan and Australia, has also been linked as a potential cause of fish kills in the Salton Sea, which is a saline but inland water body in California.  The freshwater raphidophyte, Gonyostomum semen, and the freshwater dinoflagellate, Peridinium polonicum, form nuisance blooms in other countries, with the latter causing fish kills. These species have the potential to cause problems in the U.S.  For example, Peridinium polonicum blooms have caused water discoloration in Oklahoma, but have not been associated with fish kills in the U.S.  Cryptophytes often form blooms that discolor the water but are not toxic and generally are not harmful.

Last updated: July 11, 2016