Annapolis area creeks and rivers have been hit with recent outbursts of algae blooms known as mahogany tides — with one report of a bloom running the length of the Severn River.
The species found in abundance, prorocentrum minimum, contributes to dead zones as the bloom dies and consumes oxygen in the water column. In high enough concentrations, it can cause fish kills although none have been reported this year.
South River Federation staffer Nancy Merrill noticed the mahogany bloom off her dock in Church Creek.
“It was an orange-coffee color,” Merrill said. “I know it was not good.”
South Riverkeeper Jesse Iliff reported the mahogany tides in Pocahontas and Church creeks early last week. Samples were taken at both locations and sent to the Maryland Department of the Environment.
MDE reported Pocahontas Creek “contained a very large bloom with 104,040 cells per milliliter,” Maryland authorities consider a county of 10,000 cells to be worrisome and worthy of action.
The sample also contained a species more toxic to fish and other water species, Karlodinium veneficum, finding 1,010 cells per milliliter.
“A bloom of this biomass is something to watch,” the MDE report said.
The Church Creek sample was worse — 113,686 cells of prorocentrum minimum and 1,616 of Karlodinium veneficum.
MDE shellfish monitoring in the middle of the South River also found the two species. Though at lower levels they were concentrated enough to still constitute a bloom.
The Maryland Department of Natural Resources has called Karlodinium the “fish killer” because it can produce five toxins.
“Thankfully, no evidence of fish kills has surfaced on the South River so far,” Iliff said.
The blooms were noticed elsewhere. In the Severn River watershed the Severn River Association’s Tom Guay saw a heavy bloom in Chase Creek last week.
“The waters are already murky,” he said. “This is so disappointing.”
Chesapeake Bay Foundation naturalist John Page Williams noted the NOAA buoy at the mouth of the Severn has reported blooms with seven chlorophyll spikes above 50 micrograms per liter in the past two weeks.
He noted that the mahogany bloom is normal for this time of year, in May, but this is a particularly intense outbreak.
On Thursday, Williams monitored an area of the river called the Winchester Lump.
“It was fascinating to see what I interpret to be a bloom 20-feet deep,” Williams said. “I ran into a Natural Resources Police officer who told me he was seeing the bloom the full length of the river.”
Up on Meredith Creek near the Bay Bridge, other CBF staffers noticed Mahogany bloom last week but said it was worse two weeks ago.
P. minimum tends to spike this time of year when rains wash excess nutrients into area waters, and the first warm days of the spring. According to DNR studies, May typically sees the highest number of mahogany outbreaks during the average year.
Scientific studies have noted the increase in algae blooms in recent years. The University of Maryland Center for Environmental Science underscored “more blooms, more often and lasting longer.” And not only in the Chesapeake Bay but worldwide.
“In many places these trends are consistent with increased nitrogen levels,” said study author Pat Gilbert, a professor at UMCES Horn Point Laboratory.
There are other sources of excess nutrients and not just from construction runoff, others note. This time of year homeowners are fertilizing lawns, and a rainstorm can wash excess nitrogen and phosphorus into creeks, rivers and the Chesapeake Bay.
“There is enough nitrogen and phosphorus out there to keep these blooms fat and happy right now,” Williams said.