The largest algal blooms in 20 years have invaded the lower Chesapeake Bay and its tributaries, causing at least two major fish kills and discoloring the water from Sandy Point to Parker's Creek in Calvert County on the Western Shore.
The algae -- called "mahogany tide" because of their color -- have been building during the last few weeks and may have reached their peak, said Pat Glibert, a professor at the University of Maryland's Center for Environmental Science at Horn Point in Oxford who has been studying the algae.
"Or it could just be a lull between the storms -- we don't know yet," Glibert said.
Blooms have shown up in the Choptank, Miles, Tred Avon and Corsica rivers on the Eastern Shore; in the Patuxent River from its mouth to the bridge at Benedict connecting Charles and Calvert counties; and in the lower Potomac River.
The blooms have been responsible for the deaths of more than 100,000 mummichog, silverside and white perch in Breton and St. Clement bays, about 24 miles upstream from the mouth of the Potomac, according to the state Department of Natural Resources.
While the number of fish deaths sounds large, it's "a drop in the bucket" compared with the numbers of those species of fish in the bay, and won't produce any "long-lasting effects," Charles Poukish, an environmental specialist with the state Department of the Environment, said yesterday.
Most of those fish are small, short-lived "shore zone species" that reproduce very quickly, he said.
The algae, Prorocentrum minimum, contain reddish pigments and give the water a brownish to mahogany hue. Unlike the better-known "red tide," caused by toxic algae that live in the salty water of the open ocean, mahogany tide grows in brackish and fresh water.
In most cases, Prorocentrum is not toxic or harmful to humans. It poses a danger to fish when large quantities of the algae remove dissolved oxygen from the water.
Mahogany tides dissipate as the water gets warmer and filter feeders such as oysters and menhaden eat the algae. "Water temperatures in the mid-70s appear to be the upper limit for this species," Poukish said.
The last widespread mahogany tide bloom in the bay was in 1992, but it was not as large as this one, Rob Magnien of the Department of Natural Resources said. Usually, they dissipate by the end of May.
Poukish said the water in Breton and St. Clements bays recorded zero parts of oxygen per million on MDE's instruments this week. Five parts of oxygen per million is the minimum needed to sustain marine life, and anything less than two parts per million is considered lethal, he said.
Prorocentrum occurs naturally in Chesapeake Bay and coastal waters around the globe. It is among many harmful algal blooms occurring with increasing frequency throughout the world. The increase has been attributed to increased nitrogen levels from fertilizer runoff, polluted air and septic tanks. Tighter controls on fertilizer runoff and demands for more efficient septic systems have been the subject of fierce political debate in recent years in Maryland.
"There is no doubt that harmful algal blooms are increasing in frequency and distribution," said Glibert. "This is a global phenomenon and there are global efforts to try to understand it."
`This year is worse'
While Prorocentrum has "been present in the Chesapeake Bay for decades, often causing mahogany tides" during April and May, she said, "this year is worse than others."
Bill Goldsborough, a senior scientist with the Chesapeake Bay Foundation, said this year's algal bloom can be traced to the drought of last summer and the relatively wet winter and early spring.
"All that water in April represents a pretty good spurt of nutrients being washed into the water," he said. "We were saying during the drought that all that fertilizer is still there, and now it's coming at us all at once."
Watermen keep watch
But while state officials are taking note of the large concentrations of algae, some Eastern Shore watermen said yesterday they haven't seen any sign of it.
"I've never seen red water in Broad Creek, and I've been working out of there for years," said David Kemp, a St. Michael's crabber. "Sometimes in the summer, they'll have them up in the Miles."
And Alan Poore, owner of Big Al's Seafood Market in St. Michael's, said he hadn't heard anyone talking about algal blooms, "and crabbers usually holler about that."
Normally, the mahogany tides show up in the northern part of the bay this time of year, said P. T. Hambleton III, owner of a seafood company in Bozman, Talbot County. "I've seen eels die right in the pot."
Sun staff writer Chris Guy contributed to this article.