Red tides and health risks for recreational water users
Photo credit: Pacific Salmon Foundation, Citizen Science Oceanography Program.
Marine algal blooms are natural global phenomena caused by elevated concentrations of phytoplankton cells appearing as dense coloration in ocean waters. These are sometimes called red tides, but are more accurately termed harmful algal blooms (HABs). The phytoplankton species that cause red tides, or marine HABs, can sometimes release harmful phycotoxins, which can cause shellfish poisoning, presenting a significant food safety risk to consumers. The visual impact of seeing a marine HAB can also raise concern amongst beachgoers and other recreational water users, leading to inquiries to public health units. However, little information is available on the risks, if any, of recreational exposures. While the human health effects associated with contact or inhalation exposure to marine HABs are expected to be low in Canada, there are many knowledge gaps. This blog sets out some additional information on marine HABs, the predominant species that cause them in Canada, and the possible health risks to recreational users.
What is a red tide, or HAB?
Red tides, or more accurately, marine HABs, are caused by various species of phytoplankton that become visible as coloured seawater when ocean conditions (e.g., low salinity, elevated temperature, nutrient availability, light intensity) favour a rapid proliferation of cells. There are several thousand marine phytoplankton species, hundreds of which can form blooms, especially diatoms and dinoflagellates. The blooms they produce can occur in various colours, including red, pink, brown, orange, and purple. Marine HAB events have been common across Canada’s east and west coasts for several decades, as detailed in the Harmful Algae Event Database (HAEDAT), and summarized in a 2021 review of the spatial and temporal trends.
How do HABs affect aquatic life?
Phycotoxins produced by some species of marine phytoplankton that form HABs can poison shellfish and other aquatic life, causing mass mortalities of marine species. HABs can also harm aquatic life in other ways. When a bloom decays, it can cause bacterial growth that rapidly depletes dissolved oxygen in water, causing sudden death of fish species inhabiting those waters. Phytoplankton cells can also clog the gills of fish, causing direct harm. Dense blooms can damage important marine habitats, such as eelgrass beds, by blocking out sunlight needed for growth. Species reported to have caused events of water discoloration and mortality of aquatic life in Canada are listed in Table 1. A mass mortality event associated with a HAB caused by Heterosigma akashiwo (commonly found in Canadian waters), recently occurred in San Francisco Bay in August 2022, affecting many aquatic species including large fish such as sturgeon, bass, and sharks.
Table 1 Species associated with harmful algal bloom (HAB) events in Canada as recorded in the HAEDAT database
|Species||HAB causing water discoloration||HAB causing mass marine mortalities (e.g., of fish, invertebrates, shellfish)|
|East Coast||Alexandrium spp.||✓||✓|
|West Coast||Alexandrium spp.||✓||✓|
How do HABs affect shellfish food safety?
During a HAB caused by phycotoxin-producing species, bivalve shellfish can concentrate harmful toxins in their tissues and cause poisoning in humans that consume them. The toxins are colourless, odourless, and cannot be removed by cooking. Shellfish poisoning events on both the east and west coasts of Canada have been caused by the dinoflagellates Alexandrium spp. (paralytic shellfish poisoning) and Dinophysis spp. (diarrhetic shellfish poisoning), and the diatom Pseudo-nitzschia (amnesic shellfish poisoning). These species can sometimes cause poisoning even if the cell concentrations are too low to form a visible bloom (e.g., Alexandrium sp.); therefore, absence of colour is not always a sign of safety.
How do HABs affect recreational water users?
Human health risks of recreational exposure to marine HABs in Canada are typically related to recreational shellfish harvesting and associated shellfish consumption. Recreational harvesters are advised to consult information on shellfish harvesting closures provided by Fisheries and Oceans Canada or the BC shellfish-harvesting map, and to avoid consumption of shellfish collected from affected harvesting sites.
There have been no reports of adverse human health effects associated with recreational exposure to marine HABs in Canada arising from direct contact during swimming or wading, or via inhalation of aerosolized toxins from marine HABs. In other parts of the world, however, exposure to some types of phytoplankton or phycotoxins have caused health effects via these routes:
- In the Gulf of Mexico (e.g., Florida and Texas), HABs caused by the dinoflagellate Karenia brevis, which produce brevetoxins, can cause neurotoxic shellfish poison and gastrointestinal illness if ingested. Respiratory irritation or neurotoxic effects and headaches can occur if aerosolized brevetoxins are inhaled. Swimming during a K. brevis HAB is generally considered safe for most people, but could cause skin or eye irritation for some. K. brevis has been detected in Canadian waters but, to date, there have been no reported HABs caused by this species.
- Reports of health effects following dermal exposure to phycotoxins are rare, but some toxins can cause irritation or dermatitis among swimmers or waders. For example, 60 cases of dermatitis were reported among beachgoers in Cuba in 2015 where a bloom of the dinoflagellate Vulcanodinium rugosum, resulted in exposure to phycotoxins (pinnatoxins and portimines). HABs caused by V. rugosum do not occur in Canadian waters, although pinnatoxins have previously been detected in Canadian shellfish.
- Some phycotoxins have been detected in ambient air near HAB-affected beaches. These include okadaic acid, associated with diarrhetic shellfish poisoning, domoic acid, associated with amnesic shellfish poisoning, and saxitoxin, associated with paralytic shellfish poisoning. Beachgoers may inhale very small amounts of aerosolized toxins in sea spray, but the dose via this route of exposure is generally too low to pose a health risk. Most reported cases of human illness following inhalation of phycotoxins in sea spray are mild and associated with respiratory irritation only. Less is known about health effects of chronic inhalation exposures to low levels of individual phycotoxins or mixtures.
How do we monitor marine HABs?
In Canada, there is no formal monitoring for marine HABs or phycotoxins in recreational waters, and no specific mechanisms to alert beachgoers. Areas without active commercial or recreational fisheries are not well covered by monitoring activities. Monitoring for phycotoxins in commercial shellfish is conducted by the Canadian Food Inspection Agency. Observations of HABs are also recorded by various groups including Fisheries and Oceans Canada, non-profit environmental associations, or citizen science programs.
Some US organizations such as National Oceanic and Atmospheric Administration (NOAA) or Northwest Association of Networked Ocean Observing Systems (NANOOS) provide monitoring and forecasts that border Canadian waters. These can provide some indication of toxin concentrations (e.g., domoic acid in the Strait of Juan de Fuca), or phytoplankton forecasts (e.g., A. catenella near the Bay of Fundy). Improved monitoring and data sharing could allow for greater understanding of where hazards may be present and inform early warning systems. Advancements in the use of satellite imagery to detect changes in chlorophyll-a and colour, the use of real-time environmental data, and citizen science initiatives, which crowd-source reporting of disperse environmental events, could improve HAB alerts.
What public health guidance exists on recreation during a HAB event?
There is currently no specific public health guidance on avoiding swimming or recreating during a marine HAB in Canada. Likewise, there is no specific evidence that recreational exposure to the dominant phytoplankton species present in Canadian marine waters will result in health effects similar to those experienced in areas such as Florida or Cuba. However, a precautionary approach may be taken to avoid swimming when visible blooms are present as there may be additional hazards associated with swimming or recreating in bloom-affected waters, unrelated to toxin exposure. During marine HABs, concentrations of bacteria may be elevated, particularly as blooms begin to decay. This can increase the likelihood of bacterial infections following immersion in affected waters. One example includes a scuba diver from Monterey County California who developed bilateral mastoiditis, a serious ear infection, after swimming in the ocean during a HAB. Other organisms such as Vibrio vulnificus or Shewanella algae can cause serious infections. Persons with compromised immunity or open wounds should follow the general advice to avoid swimming or wading in waters where there is the potential for microbial contamination, including marine waters affected by HABs.
Could climate change affect the occurrence of marine HABs?
There is concern that climate change could lead to increased occurrence of HABs and a change in the distribution of phytoplankton species, or the timing and intensity of blooms. Temperature is an important variable that affects rates of feeding, metabolism and growth of many microorganisms, and warming could influence the timing or intensity of blooms, or dominant species. Heavy precipitation has been found to precede bloom events, potentially due to the beneficial effect of lowered salinity and nutrient loading. Changing precipitation patterns could affect bloom intensity or frequency. Other environmental variables such as light availability, wind patterns (affecting currents or coastal upwelling), and chlorophyll-a concentrations may also be important in predicting future HAB events.
Modelling of the future distribution of Dinophysis acumimata, D. norvegica and Pseudo-nitzschia seriata, on Canada’s east coast using salinity, temperature, and wind-speed variables based on the RCP 8.5 (high emissions) scenario, predicted a change in the frequency of observed blooms due to climate change. The study predicted that blooms of all three species could start earlier and occur over a longer duration than they do currently. Climate change is thus likely to affect the distribution and occurrence of HABs in Canada, and warming waters may affect the range of species observed in Canadian waters over time.
Knowledge and practice gaps
Several knowledge and practice gaps remain related to the recreational risks associated with exposure to marine HABs. Little is known about the effect of low-level chronic dermal or inhalation exposure to individual phycotoxins or mixtures. Lack of monitoring for phycotoxins also limits the ability to identify areas of elevated risk, or identify linkages between exposures and health effects. Further study is thus needed to inform more detailed public health guidance. Further study is also needed to identify how climate change may affect the human health hazards associated with changing patterns of marine HABs.
For further information on identifying and managing public health risks in coastal waters, see our other NCCEH resources on:
- Marine shellfish poisoning [Topic page]
- Recreational coastal, freshwater, and other untreated natural waters [Topic page]
- Climate change, coastal communities, and food from the sea [Blog]