Official State of Rhode Island website

State of Rhode Island Department of Environmental Management

Marine Harmful Algal Blooms (HABs)

Marine phytoplankton form the base of most marine food chains; like plants on land, they convert sunlight, carbon dioxide, and minerals into a basic food. By using photosynthesis phytoplankton capture the sun's energy to absorb dissolved carbon dioxide and nutrients from the ocean and convert them into particulate carbon in the form of new phytoplankton cells.  These  phytoplankton cells are the food that powers most marine ecosystems.  Additionally, larger plants such as seagrasses and seaweeds (macroalgae) provide important  primary production in nearshore regions. 

For information on Cyanobacteria (blue green algae) in freshwater visit dem.ri.gov/bluegreen. 

Under certain conditions phytoplankton can grow and accumulate causing a 'bloom' or increase in abundance that may result in water discoloration due to the accumulation of pigmented phytoplankton cells. This is evident when comparing the color of the water in the upper portion of an estuary, typically tinted green or brown due to abundant phytoplankton, to the color of the open sea which typically has less phytoplankton abundance and a more intense blue color. 

The overwhelming majority of the thousands of marine phytoplankton species are benign with no negative impact on human health or ecosystem function.  However, a few types of phytoplankton than may be present in RI waters have the potential to produce compounds, broadly called biotoxins, that if accumulated by filter-feeding shellfish and consumed may cause cause health problems in humans.  Blooms or increases in the abundance of these potentially harmful phytoplankton are a type of  'harmful algae bloom' or HAB.   RI waters have potential for blooms of several marine HAB phytoplankton that are associated with shellfish poisoning syndromes in people:

  • Paralytic Shellfish Poisoning (PSP). Caused by the saxitoxin biotoxins produced by the dinoflagellates in the genus Alexandrium. PSP is the classic northern New England 'red tide' that leads to shellfish closures. PSP symptoms include nausea, vomiting, diarrhea, paralysis in severe cases.  Alexandrium and PSP is not common in RI waters.
  • Diarrhetic Shellfish Poisining (DSP) is caused by the biotoxin okadaic acid produced by dinoflagellates Dinophysis and some Prorocentrum. DSP symptoms are diarrhea, gastrointestinal distress, cramps.  While Dinophysis is present in RI during summer, DSP is not know for RI waters.   
  • Neurotoxic Shellfish Poisoning (NSP) is typically caused by Karenia brevis, a dinoflagellate that is not common in RI waters.  
  • Amnesiac Shellfish Poisoning (ASP) is caused by the biotoxin domoic acid by produced by diatoms in the genus Pseudo-nitzschia. Symptoms of ASP include dizziness, memory loss, seizures.  Pseudo-nitzschia is common in RI waters, but ASP biotoxin production and accumulation is uncommon in RI.  The last RI shellfish closure due to Pseudo-nitzschia domoic acid detection was in 2017.

RI DEM and RI DOH monitor RI shellfish waters year-round for marine HABs that may accumulate in shellfish and shellfish areas are placed in the closed status if HAB phytoplankton and biotoxins are present at levels that may harm human health. 

What Should I Do?

  • Check for updates and advisories on the Bay Water Quality Report or from the Bay Line at: (401) 222-8888 (May 15 to October 15).
  • Shellfish closures due to marine HABs are listed in real-time on the 'Shellfish Hotline'. Phone the shellfish hotline at 401-222-2900 for a list of shellfish areas that are closed due to rain closures or emergency closure conditions such as HABs.
  • In the meantime, help prevent HABs by supporting efforts to improve water quality in the Bay. (See "Introduction to Narragansett Bay")

Additional Resources