BOX 1-5
Pfiesteria piscicida: Implications for Nutrient Over-Enrichment

Prior to 1990, problems attributable to nutrient over-enrichment rarely made national news, but a once little-known species called Pfiesteria piscicida1 gained wide public attention in the 1990s and inadvertently served to increase public understanding of these types of problems. Interest began in May 1991, when a fish kill in the Ablemarle-Pamlico estuarine system in North Carolina was attributed to Pfiesteria piscicida (Burkholder 1997). But wide attention began in earnest in August 1997, when hundreds of dead and dying fish were found in a tributary to Chesapeake Bay, the Pocomoke River near Shelltown, Maryland, prompting state and local officials to close a portion of the river. Subsequent fish kills and observations of Pfiesteria-like organisms led to successive closing of segments of the Manokin and Chicamacomico rivers in Maryland. Soon, Maryland’s Department of Health and Mental Hygiene presented preliminary evidence that adverse human health effects could result from exposure to the toxins released by Pfiesteria piscicida or Pfiesteria-like organisms (Grattan et al. 1998).

With the publicity, and despite the fact that the fish most commonly affected by Pfiesteria piscicida are Atlantic menhaden (a fish used primarily as an ingredient in animal feed), the local seafood industry suffered as restaurants and stores stopped selling Chesapeake Bay seafood (Weinraub 1997). In September 1997, the State of Maryland appointed a Citizens Pfiesteria Action Commission, which convened a forum of scientists to provide advice. The final report is referred to as the Cambridge Consensus (Maryland Department of Natural Resources 2000).

The scientists discussed questions that had been raised in the scientific community concerning the relationships between Pfiesteria-like dinoflagellates (which included P. piscicida) and nutrients. After thorough analysis, they concluded there was a likely connection between nutrients, toxic outbreaks of Pfiesteria-like dinoflagellates, and fish kills. Also, they determined that it is improbable that toxic contaminants (such as pesticides and trace metals) are primarily responsible for outbreaks of Pfiesteria-like dinoflagellates. The scientists noted that while most evidence comes from North Carolina and environmental conditions vary, their findings apply to the mid-Atlantic region in general. Specifically, they found:

  • In laboratory cultures, growth of non-toxic stages of Pfiesteria piscicida can be stimulated by addition of inorganic and organic nutrients.


Neither a true plant nor animal, Pfiesteria is a dinoflagellate within the Kingdom Protista. It has a complex life cycle, which makes identification of Pfiesteria species by nonexperts extremely difficult. It spends much of its life span as a nontoxic predatory organism feeding on bacteria and algae, or as encysted cells existing in a dormant state in muddy substrates. However, when large schools of oily fish (e.g., Atlantic menhaden) swim into an area and linger to feed, their excreta may trigger encysted cells to emerge and secrete potent toxins. These toxins make the fish lethargic, so that they tend to remain in the area where they are susceptible to direct attack by the Pfiesteria cells. This, either alone or as a result of concurrent attacks by bacteria or fungi, may lead to open sores on the fish.

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