While Alexandrium is often considered through the lens of its human health impacts, saxitoxins produced by this species also have extensive effects within marine ecosystems. This dinoflagellate is responsible for fish kills in the northeast US, affecting mainly Atlantic herring, Atlantic menhaden and sand lance. Toxins enter the food chain when larval fish consume Alexandrium, and organisms that consume larval fish can also experience the effects of toxicity and pass it along to higher trophic levels. Fish that are exposed to saxitoxin exhibit erratic swimming behavior, neurologic symptoms, paralysis and death. Severity of symptoms depends upon level of exposure.

Heterosigma akashiwo is a bloom forming photosynthetic raphidiophyte algae found in enriched temperate coastal waters and estuaries around the world. Fish kills from the blooms have been recorded in the United States, Canada, Chile, Scotland, Spain, Japan, China, Tasmania, and New Zealand. In coastal waters of British Columbia, Canada, H. akashiwo blooms have been recorded every year since the 1960’s resulting in substantial economic losses for trout and salmon fisheries in the Pacific Northwest. Heterosigma akashiwo produce a suite of compounds, including reactive oxygen species and brevetoxins, that may cause physiological damage to fish gills and other organs. The damage can lead to respiratory failure, compromised growth, increased susceptibility to diseases, and ultimately to death. H. akashiwo blooms also negatively impact other economically important invertebrates such as oysters, scallops and shrimp, which can further disrupt the ecosystem and food web.

Chronic blooms of Karenia brevis have made the news in recent years due to massive and widespread fish kills and wildlife mortalities, as well as impacts to human health.  The magnitiude, extent, and duration of these blooms have also harmed economies and caused social disruption in affected areas of Florida’s Gulf coast.  The toxins produced by Karenia are released into the water when cells lyse, and are lethal to over a hundred different fish species. Symptoms of poisoning in fish include impaired swimming and schooling behavior, paralysis, regurgitation and death. Hypoxic and anoxic conditions associated with Karenia brevis blooms cause further mortality. In Florida, these blooms can last for months, and toxins will remain in the water even after the algal cells are dead leading to fish kills that continue long after the bloom has terminated. These extended events have caused drastic changes in fish communities, disrupting juvenile recruitment and altering ecosystem structures. Contaminated fish are a vector for toxins and can lead to mortality in birds and mammals that consume toxic fish.

Karlodinium veneficum is an athecate, or unarmored, dinoflagellate that relies on a combination of photosynthesis and predation for energy and growth. This dinoflagellate produce karlotoxins that help it to capture prey and deter grazers, but these toxins also have negative impacts on fish. The toxins disrupt osmoregulation in the gills, causing damage to gill tissue, symptoms of lethargy and eventual death. Karlodinium can reach exceedingly high abundances on the order of one million cells per milliliter, but it is important to note that these high concentrations do not always lead to fish kills depending on the toxicity of the bloom strains. This species is prominent in the Chesapeake Bay but has also been found in the Gulf of Mexico. The state of Maryland experiences frequent blooms and associated fish mortality, with one event in 2005 killing an estimated 30,000-50,000 fish in the Corsica River. Fish affected by Karlodinium veneficum include black drum, bluegill sunfish, menhaden, shad, catfish, perch, carp, cod and killifish.

This small dinoflagellate forms golden or copper-colored blooms that can stretch for hundreds of kilometers. Prymnesium parvum is tolerant of a wide range of temperatures and salinities, allowing it inhabit both coastal and freshwater ecosystems such as rivers and streams. When subjected to physiological stress, such as nutrient limitation, these blooms produce toxins that are deadly to fish. Prymnesium parvum toxins disrupt gill function, preventing oxygen exchange and ultimately leading to suffocation. Fish in affected waters will exhibit bleeding gills, slow or erratic swimming, and may attempt to leap from the water. Fish kills have included game fish such as largemouth bass, smallmouth bass, striped bass, catfish, crappie, and rainbow trout, as well as threatened species such as blue suckers and Rio Grande darters. As of 2017, 23 states have experienced golden algae blooms. While events are most common in the south-central United States, blooms of Prymnesium parvum have occurred as far north as Maine.

Pfiesteria piscicida is a heterotrophic dinoflagellate; it has no chloroplasts or photosynthetic capabilities and relies on prey consumption for energy. First discovered in North Carolina, this species is found in estuaries on the Atlantic coast of the US and includes both toxic and non-toxic strains. Toxic strains of Pfiesteria piscicida will actively attack fish, leading to lesions and neurologic symptoms such as lethargy, erratic swimming, panic, air gulping and narcosis. Major events caused by Pfiesteria piscicida have been on the order of 1,000-1,000,000 fish killed. Affected species include juvenile Atlantic menhaden, American eel, Atlantic croaker, catfish, tilapia, striped bass, sheepshead, flounder, striped mullet and white perch.