The discovery of Karlodinium micrum in the Chesapeake brings to at least 17 the number of harmful algae species present in the Bay that have shown toxic effects someplace in the world.

As recently as 1996, a survey by Harold Marshall, a phytoplankton expert at Old Dominion University, found only a dozen potentially toxic phytoplankton species in the Bay — and none were known to produce toxins in the Chesapeake.

Not only has the number of potentially harmful species grown, but at least four — K. micrum, Pfiesteria piscicida and Dinophysis acuminata and the blue-green algae Mycrosistis aeruginosa — have shown signs of toxicity in the Bay.

D. acuminata, a marine species, produced toxins that resulted in the closure of shellfish beds in part of the Potomac River this spring, and P. piscicida was linked to fish kills on the Pocomoke River in 1997.

Many of the other species, Marshall said, may not have shown signs of toxicity in the Bay either because they have not encountered the right conditions to spur toxin production, or because only nontoxin-producing strains are present in the Bay.

“Each of these species have toxin-producing and nontoxin producing strains,” he said. “We are fortunate that we apparently have many that are the nontoxin-producing forms, or they are in low enough concentrations that we have not had any toxic events associated with most of them.”

The likely cause of the increase, officials say, is that scientists are more actively looking for potentially harmful species, and such species are likely being moved around the world by ocean-going ship

“We are in a major seaport area and I’m sure many of these species are introduced into our water through ballast water exchange that has taken place,” Marshall said.