Algae require nutrients, particularly nitrogen and phosphorus, to grow and reproduce. When nutrients are scarce, so is algal growth. Conversely, when nutrients are in excess, a condition known as eutrophication can result, spurring algae to bloom in over-abundance. Such blooms can cause problems as vast swaths of algae die and deplete oxygen levels as they decompose. Areas with low oxygen (hypoxia) or no oxygen (anoxia) are inhospitable to most aquatic organisms and can lead to suffocation or force animals to relocate.
Problematic blooms occur in the Chesapeake Bay’s waterways nearly every spring and summer as rains send nutrients downstream, fueling algal growth. Fish and shellfish die-offs often accompany these blooms as oxygen levels cannot sustain life.
A recent report by the National Oceanic and Atmospheric Administration finds that the Mid-Atlantic region, including Chesapeake Bay, suffers from overnutrification more than any other coastal area in the nation, and that conditions may worsen. For efforts to restore the Chesapeake to succeed, addressing the input of nutrients that can lead to eutrophication and harmful algal blooms will be critical. To read the report, Effects of Nutrient Enrichment in the Nation’s Estuaries: A Decade of Change, visit http://ccma.nos.noaa.gov/publications/eutroupdate/
Learn about Maryland Sea Grant Extension's efforts to educate citizens about nutrients and algae.
Some species of algae — though actually a small percentage — are toxic and may harm grazing sea life and their predators (including humans), especially when abundant. When toxic, these algae produce chemicals (called toxins) that can cause a range of problems — from lesions and die-offs in fish, to respiratory and neurological problems in humans.
Around the world, microscopic marine plankton called dinoflagellates are the most common culprits in toxic blooms — even though less than two percent of them contain toxins. Botanists often classify these single-celled organisms as algae or phytoplankton since many — though not all — contain chloroplasts and can photosynthesize. All dinoflagellates have whip-like tails (flagella) that allow them to move through the water and feed on other microorganisms, making them predators as well.
Along with other algae, dinoflagellates provide a main source of primary production — entire ecosystems depend on them — but when producing toxins, they can cause big problems. When in abundance, they often appear to change the color of the water, an event sometimes called a red, brown, or mahogany tide, depending on the species.
In the U.S., especially along the Gulf coasts of Florida and Texas, red tides caused by the blooming dinoflagellate Karenia brevis are a near annual occurrence. The blooms can kill fish and marine animals and sicken people. Noxious brevetoxins from the algae disperse into the air causing respiratory irritation, and accumulate in the tissues of shellfish causing neurological problems for those who consume the contaminated shellfish.
Coral reef areas experience outbreaks of ciguatoxin — the most frequently reported marine toxin in the world. Species of Gambierdiscus produce the toxin, which gets passed on to fish who feed on the dinoflagellate. As those small fish are eaten by bigger fish — and those by still bigger fish — the toxin moves up the food chain, accumulating each step of the way until it reaches high-level consumers such as grouper, snapper, and barracuda. When humans dine on contaminated fish they may experience Ciguatera Poisoning which can include nausea, vomiting, head and muscle aches, numbness, and memory loss. In extreme events, those infected may exhibit a reversal in temperature sensation where hot feels cold and cold feels hot.
As filter feeders of algae, shellfish are particularly susceptible to ingesting toxic dinoflagellates. The presence of neurotoxins in bivalves such as oysters, clams, and mussels has been implicated in several types of illnesses. Saxitoxins produced by the dinoflagellate Alexandrium cause Paralytic Shellfish Poisoning (PSP), a condition marked by tingling, dizziness, nausea, and vomiting. Red-tide causing Karenia brevis’ toxins can lead to Neurotoxic Shellfish Poisoning (NSP), a disease similar to, but less severe than, Ciguatera Poisoning.
The memory loss and confusion of Amnesic Shellfish Poisoning (ASP) are brought on by toxic domoic acid released from Pseudo-nitzchia — a diatom. Like dinoflagellates, diatoms are single-celled marine plankton. Unlike dinoflagellates, almost all diatoms have chloroplasts and can photosynthesize. Encased by a silica wall that resembles glass, diatoms take on beautiful and varied shapes. Unlike the whip-tailed dinoflagellates, they float in water with no means of propulsion, like oceanic snow flakes.
Toxic Algae in the Chesapeake Bay
Fortunately, toxic algae have not been prevalent in the Chesapeake Bay. For the most part, the Bay has avoided widespread fish mortality or human health issues — although there have been a few incidents of concern. Mahogany tides of dinoflagellates Prorocentrum minimum and Karlodinium veneficum have resulted in isolated fish kills and shellfish deaths in hatcheries. Additionally, paint-like blooms of the blue-green algae Microcystis — more accurately classified as cyanobacteria — have led to several beach closures.
And in the summer of 1997, one dinoflagellate single-handedly raised the profile of toxic algae in the Chesapeake region — Pfiesteria piscicida. Pfiesteria drew significant media attention and caused public concern when blamed for significant fish kills and human health problems. Scientists often connect toxic algal blooms to a specific toxin, but in the case of Pfiesteria no toxin could be found. When lab researchers observed the dinoflagellates physically moving toward and eating fish tissue, it led some to hypothesize that no toxin existed — Pfiesteria simply acted as predators. Other scientists contended that an unidentified toxin could still be involved, perhaps as a way to stun prey. The uncertainty spurred decade-long investigations that continue today as researchers hunt for evidence of the elusive toxin or other mechanism by which Pfiesteria kills.
In February 2007 a NOAA scientist reported that his team had isolated a toxin produced by Pfiesteria. According to these findings, under certain conditions of light availability and water chemistry, the toxin quickly breaks down into free-radicals — unstable molecules with the potential to be lethal. This rapid breakdown could explain the ephemeral nature of the toxin. Hailed as a breakthrough, the research still needs to be verified through repeated laboratory and field tests. As this and other work continues, scientists are gaining a better understanding of the science behind toxic algae and what causes their harmful blooms.
View a selected bibliography of scientific journal articles spanning the broad scope Pfiesteria.
Whatever happened to Pfiesteria?
Chesapeake Quarterly, Volume 6, Number 1, 2007
Maryland Sea Grant's Emmy award-winning film
Harmful Algae Blooms in Maryland
Maryland Department of Natural Resources
Harmful Algal Blooms
NOAA’s National Ocean Service
The Harmful Algae Page
Woods Hole Oceanographic Institute
In Harm’s Way? The Threat of Toxic Algae
Maryland Marine Notes Vol. 15, No. 4, 1997
Harmful Algal Blooms: Maryland Status & Trends (pdf)
Integration & Application Network Newsletter, December 2006
Fish Health in the Chesapeake Bay
University of Maryland