HABs produce biotoxins which can bioaccumulate in the marine food web similar to mercury. Bioaccumulation happens when toxins build up in an organism at a rate faster than they can be broken down. Phytoplankton form the base of marine food webs and provide vital energy to each trophic level. When they bloom, any toxins they produce are consumed by, and can bioaccumulate in, higher level organisms like shellfish, copepods, krill, anchovies, and mackerel.
Sometimes organisms are not affected by the toxins themselves, but act as vectors and transport the toxins up the food web into higher level organisms—like fish, seabirds, manatees, sea lions, turtles, and dolphins. In addition to impacts of single HAB events, the effects of chronic exposure to HAB toxins on health, reproductive failure, and behavior are only beginning to be understood. Impacts from this type of exposure could be significant on protected and endangered species, as well as humans.
Human illness or death can also result from HABs when people consume contaminated fish and shellfish. Coastal state governments closely monitor select toxin-producing species of algae to provide as much advance notice to, and regulation of, the seafood industry as possible.
Other types of harmful algal blooms are not toxic but cause harm to marine life in other ways: by damaging or clogging fish gills or blocking sunlight for beneficial algae and seagrasses. Some HABs bloom so densely that the death and decay of the algae leads to oxygen depletion in the marine environment, suffocating animals or forcing them to migrate.
In California, the two main illnesses of concern are paralytic shellfish poisoning (PSP) and amnesic shellfish poisoning (ASP). PSP is caused by neurotoxins produced by a dinoflagellate algal bloom, and its symptoms are neurological—causing paralysis or death by slowing respiration. PSP-producing blooms have been sporadic in history and location, but tend to occur annually along the coast north of San Francisco Bay and along the San Luis Obispo County coast. Tens of thousands of shellfish deaths have been attributed to PSP poisoning.
ASP is caused by harmful blooms of diatoms, specifically from the Pseudo-nitzschia genus. Several species produce domoic acid, the toxin responsible for ASP. Amnesic shellfish poisoning results in gastrointestinal and neurological disorders after consumption of toxic shellfish by humans—leading to memory loss or even death. Domoic acid-producing blooms occur nearly every year in California, and ASP has been linked to hundreds of marine mammal and bird deaths since 2003. Domoic acid is regularly detected in harvested shellfish during peak HABs season, crossing seafood safety thresholds for human consumption.
California’s Department of Public Health closely monitors toxin levels in seafood that are associated with PSP and ASP and if toxins accumulate above regulatory limits, both commercial and recreational harvesting is closed to prevent the spread of these illnesses to humans. Sport-harvested mussels are routinely closed from May-October to prevent human cases of PSP and ASP. The Department of Fish and Wildlife also tracks exposure of marine birds and mammals to these toxins.
In addition to PSP and ASP, there are several new illnesses that are emerging as threats to marine ecosystem and human health in California. Several species of the dinoflagellate Dinophysis produce okadaic acid which can bioaccumulate in shellfish, causing diarrhetic shellfish poisoning (DSP) in humans. Toxic effects from Dinophysis algae have only recently become an issue on the west coast. Yessotoxins are also produced by some dinoflagellate blooms and were first detected in 2004. A yessotoxin is suspected to have caused massive abalone and sea urchin deaths in a 2011 event off the Sonoma County coast. Another emerging issue is the increase in HABs in waters at the land-sea interface such as estuaries and rivers. Since the 1990s, blooms of blue-green algae (also called cyanobacteria) have increased in frequency and abundance, sometimes producing toxins called microcystins. These have had lethal effects on fish, shellfish, and even sea otters.
Researchers estimate that blooms of harmful algae cost the United States at least $80 million per year. The economic damage arises from:
- Wildlife mortality: The death of wild and farmed fish, shellfish, submerged aquatic vegetation, and coral reefs
- Medical treatment: of exposed human and animal populations
- Fishery closures: Short- and long-term closure of harvestable shellfish and fish stocks
- Reduction in seafood sales: including the avoidance of “safe” seafood due to over-reaction to health advisories
- Monitoring costs: The costs of conducting monitoring programs to detect and quarantine toxic shellfish and other affected marine resources
- Reduction in tourism and related businesses
- Impacts to desalination systems: HABs can contaminate seawater that is used to produce freshwater for human use. Climate change is expected to worsen this problem.
Recently, the world's coastal waters have experienced an increase in the number and type of HAB events. This is especially true in the United States, where virtually every coastal state is now threatened. As to the causes of this trend, scientists say there is no single factor that accounts for blooms across regions and HAB species. A number of hypotheses have been proposed to explain the increase in HABs including: increased scientific awareness of toxic species, increased runoff of nutrients from land, increased recreational use of coastal waters and fisheries, and/or changing oceanographic conditions.