The fish kills that have plagued Baltimore's Inner Harbor and nearby creeks over the past two weeks may have eased with the dip in temperatures, but scientists caution that's not the last we've seen of potentially toxic and even deadly algae blooms in area waters.
As summer heats up again, potentially dangerous microorganisms could bloom in the wake of algae blamed for suffocating fish from Dundalk to Annapolis. Officials are keeping tabs on the growth of algae around the Chesapeake Bay that have poisoned tens of thousands of fish — including one type found blooming in a Cecil County river that has in the past killed two dogs and can also be fatal to humans.
Algae flourish every spring and summer in the bay, scientists say, most of the time without producing the stench and rafts of belly-up fish seen recently. But the blooms seem to have started earlier and grown thicker than usual this year, with dramatic results so far. Scientists have attributed that to a mild winter, early spring and a flood of nutrient pollution into the bay from last summer's tropical storms — plus a massive sewage spill a few months ago.
More intense blooms have taken hold in years past, experts note, but the Baltimore area outbreaks provide a pungent reminder that the Chesapeake's pollution woes remain unsolved — and can hurt fish, pets and even people.
"It goes in cycles, but the problem is still there," said Judy O'Neil, a scientist at the University of Maryland's Horn Point Environmental Laboratory, who likens them to the springtime awakening of noxious ticks and insects. "These blooms are kind of like the pests of the water."
Like other plants, algae thrive on a diet of nitrogen and phosphorus, which get into the water through sewage discharges, runoff of fertilizer and pet waste and air pollution, among other sources. And warm weather stimulates algae growth, as it does for land-based plants.
"When you get warm temperatures and nutrients, it's just perfect for them," O'Neil said.
The alga behind the recent fish kills, Prorocentrum minimum, regularly appears every spring in the bay and other coastal waters, where it's often called "mahogany tide" because it turns the water brown.
Though Prorocentrum has killed oysters in other locales, scientists say they've never seen that in the bay. Instead it kills indirectly, they say, by growing until it consumes all the nutrients in the water and begins to die back. As it decomposes it consumes all the oxygen in the water, so fish have nothing left to breathe.
In the past two weeks, state investigators found dissolved oxygen levels near zero around dead and dying fish in the algae-infested waters of Baltimore harbor and creeks in Anne Arundel and Baltimore counties.
The fish kills seem to have eased as temperatures slipped from the 90s in the daytime back to the 80s. But officials say it's too early to say the worst is over. Storms, with heavy rains and strong winds, can disperse algae and help re-oxygenate the water, they say. But brief or mild rains can do just the opposite, simply washing more nutrients into the water to feed the algae.
"It can be a double-edged sword," says Thomas Parham, who oversees tidal water conditions for the Department of Natural Resources.
Eventually, as summer wears on, this particular bloom will dissipate. But that could simply make room for others, including some that attack fish more directly, Pfiesteria piscicida and its less-heralded but more lethal co-conspirator, Karlodinium veneficum.
Pfiesteria were blamed for a series of fish kills in Eastern Shore rivers in 1997, which triggered a media frenzy after fishermen and others who came in contact with the water complained they were being made sick as well. Alarm over Pfiesteria also prompted state officials to take the first steps toward regulating farmers' use of fertilizer, a major source of the nutrient pollution in the bay.
But the real fish killer behind those and other die-offs, it turns out, was Karlodinium, according to Allen Place, a professor and biochemist at UM's Institute for Marine Environmental Technology at the Columbus Center in the Inner Harbor. The "little dinoflagellate with a big bite," as Place and colleagues dubbed it in a recent journal article, produces toxins that essentially strangle a fish, by wrecking its gills.
"Karlo," Place said, often lurks in the water in numbers too low to pose much threat to fish, but if conditions are right it can proliferate, with lethal effect. Six years ago, a bloom in the Corsica River in Queen Anne's County killed an estimated 55,000 menhaden, he says.
It bloomed in Baltimore harbor the same year, but without a similarly large fish kill, Place says. That's probably because the organisms tend to grow near the surface and the harbor is deep enough for fish to swim away.
State natural resources officials have spotted Karlodinium recently amid blooms of Prorocentrum growing in the Severn and Magothy rivers, and in a tributary of the Choptank River. Their numbers appear to be too low to be a threat, says Place, but that may change, because they thrive in warm water temperatures that are not uncommon in late summer.
"I'm waiting for Karlo's head to pop up," Place says.
Meanwhile, he and other scientists are keeping a close watch on the Sassafras River in Cecil County, where a potentially deadly blue-green algae, Microcystis aeruginosa, was found this week to be blooming. Tests show it is at low levels for now, but officials will keep monitoring.