Defining Phytoplankton and Harmful Algal Blooms (HABs)

Microalgae, or phytoplankton, are the base of the marine food web and support filter-feeding shellfish, are critical for marine nutrient cycling, and produce 50% of the oxygen on Earth. Therefore, phytoplankton are important locally and globally, despite their microscopic size. A small percentage of microalgae are classified as harmful algal bloom (HAB) species. Although HAB species are only visible under a microscope, they can form blooms that are evident to the naked eye as discolored water (shown below). HABs are deemed harmful because they are associated with toxin production and have detrimental effects on human health and the environment. Marine and/or freshwater HABs impact every single state in the United States.

Paralytic Shellfish Poisoning

When a phytoplankton bloom turns the water red or brown, the event is commonly referred to as a "red tide." Toxic and non-toxic species have caused red tides around the world, although red tides are typically synonymous with toxic species. Additionally, multiple toxic species are associated with red tide in the United States. The confusion surrounding the term "red tide" can cause concerns about water quality and local seafood, miscommunications about public health issues, and economic losses due to reductions in recreational activities and tourism.

New England red tide is caused by the dinoflagellate Alexandrium. Alexandrium produces a potent neurotoxin called saxitoxin. Paralytic shellfish poisoning (PSP) is the syndrome associated with the consumption of shellfish contaminated with saxitoxin. PSP symptoms may include, tingling, numbness, burning in the extremities or mouth, lack of coordination, drowsiness, nausea/vomiting, diarrhea, fever, and rash. In extreme cases, PSP symptoms can include respiratory arrest and death if not provided supportive care at a hospital. New England red tide is naturally occurring and has been documented for hundreds of years. Native Americans even developed a method for identifying when shellfish were not safe to consume before the cause of shellfish poisonings was known.

New England red tide has become predictable, with extensive monitoring occurring from March to October. While northern New England states such as Maine began to have annual PSP closures in the 1970's, southern New England states such as Connecticut and Rhode Island, as well as New York historically had sporadic, smaller-scale closures. Despite Alexandrium's documented presence in New York since the 1980's, extensive, near-annual closures did not occur until 2006. Although Alexandrium has been documented in Connecticut since the 1980's, shellfish beds in Groton, CT have only experienced closures in 1985, 1992, 2003, 2020, and 2023.

Similar to how plants produce hardy seeds that can germinate the following spring, Alexandrium produce hardy cysts that lay dormant in the sea sediment. Once conditions become ideal, typically in the spring, Alexandrium emerge from their cysts and can potentially form toxic blooms as planktonic (free-swimming) cells. There have been no PSP-related illnesses in Connecticut.

Amnesic Shellfish Poisoning

The diatom Pseudo-nitzschia produces a potent neurotoxin, domoic acid. Amnesic shellfish poisoning (ASP) is the syndrome associated with the consumption of shellfish contaminated with domoic acid. ASP symptoms may include dizziness, headache, disorientation, and short-term memory loss. There are also gastrointestinal ASP symptoms such as nausea, vomiting, abdominal cramps, and diarrhea. In extreme cases, ASP symptoms can include seizures, respiratory difficulty, coma, long-term neurological damage (including memory defects and weakening or death of muscle in the extremities), and death. In addition to shellfish, planktivorous (low-level, plankton feeding) fish and crabs are potential vectors for ASP in humans. Pseudo-nitzschia have caused substantial marine mammal and seabird illness and mortality events, particularly on the west coast of the U.S.

The first ASP closure in New England occurred in 2016. Part of the Gulf of Maine was mandatorily closed in September 2016 and five tons of mussels and clams were recalled. The Pseudo-nitzschia australis bloom and its associated toxicity had a rapid onset, with shellfish accumulating 3-4 times the FDA domoic acid closure limit within just 22 days. Pseudo-nitzschia australis also reached Massachusetts and Rhode Island, which instituted precautionary closures. Then in March 2017, a Pseudo-nitzschia australis bloom was detected in Rhode Island and prompted a mandatory closure due to the detection of domoic acid over the FDA closure limit. Maine saw a resurgence of Pseudo-nitzschia australis in September 2017 and again had to recall shellfish product. After the rapid onset of shellfish toxicity and two major recall events (2016, 2017), Maine started to institute precautionary closures prior to domoic acid levels reaching the FDA closure limit.

Pseudo-nitzschia, including some toxic species, are native to Long Island Sound. However, no human or animal ASP events have been reported in Long Island Sound.

Diarrhetic Shellfish Poisoning

The dinoflagellates Dinophysis and Prorocentrum produce the toxin okadaic acid and its derivatives, the dinophysistoxins. Diarrhetic Shellfish Poisoning (DSP) is the syndrome associated with the consumption of shellfish contaminated with okadaic acid and its derivatives. DSP symptoms may include nausea, vomiting, abdominal pain, and incapacitating diarrhea. Individuals with DSP typically recover within three days, but consumers with predisposed conditions may require hospitalization to treat fluid and electrolyte imbalance. While DSP is one of the least severe shellfish syndromes, it can be underreported due to misdiagnosis as viral or bacterial contamination.

The first local bloom of Dinophysis acuminata was discovered in Northport Harbor, Long Island, NY in 2008. In Northport Harbor in 2011, D. acuminata reached over one million cells/L, making this the largest recorded bloom in North America, and some shellfish tested at over seven times the FDA closure limit.

Fredudenthal and Jijina (1988) documented over 10 different Dinophysis species in Long Island Sound and its surrounding areas. Dinophysis acuminata and Dinophysis norvegica are the most common Dinophysis species in Connecticut, and both are known to produce toxins.

While DSP events are typically associated with Dinophysis, some Prorocentrum species also produce okadaic acid and its derivatives. There are more non-toxic Prorocentrum species in Connecticut and they are more frequently observed; Prorocentrum micans (shown above) is an example of a non-toxic species. Prorocentrum lima (also shown above) is an example of a toxic species. 

No DSP illnesses have been reported in Connecticut.

Summary

Neighboring states New York and Rhode Island are managing emerging toxic HAB events that have not yet caused closures in Connecticut. The emergence of HABs strains local industries such as the shellfish industry and requires additional monitoring efforts by regulatory agencies such as Connecticut DoAg.

There is a general consensus among scientists that the intensity and frequency of HABs are increasing around the world. With the increasing threat of HABs globally and the reoccurrence of many harmful species around Long Island, DoAg's monitoring programs help advance understanding of HABs in Connecticut and act as an early warning system to protect shellfish consumers.