Virginia Bay Plagued by Algal Blooms

VIMS reports intense and widespread algal blooms

Water sampling and aerial photography by researchers at William & Mary's Virginia Institute of Marine Science show that the algal blooms currently coloring lower Chesapeake Bay are among the most intense and widespread of recent years.

VIMS Professor Kimberly Reece reports that water samples collected near the mouth of the York River on August 17 contained up to 200,000 algal cells per milliliter, the densest concentration she has seen in nearly 10 years of field sampling. A sample with a concentration of even 1,000 algal cells per milliliter is visible to the naked eye and considered dense enough to be called a bloom.

The current blooms are dominated by a single-celled protozoan called Alexandrium monilatum, an algal species known to release toxins harmful to other marine life, particularly larval shellfish and finfish. Since mid-August, VIMS has received sporadic and localized reports of small numbers of dead fish, oysters, and crabs from the lower York River and adjacent Bay waters associated with nearby blooms, although a direct cause/effect relationship has not been established for any of these events.

Alexandrum is one of a group of microalgae called dinoflagellates, many of which can form blooms, and a good number of which are violently toxic. It has long been dogma that the dinoflagellates in Chesapeake Bay are non-toxic, although there have been a few exceptions. We often have red-tides caused by dinoflagellates here, but we rarely have toxicity issues. However, as they say in the investment community, past performance is no guarantee of future

Aerial photography and water sampling by VIMS Professor Wolfgang Vogelbein between Aug. 17 and 27 confirmed the blooms’ intensity in the lower York River, and revealed that they extended much farther up the York River and out into Chesapeake Bay than previously reported. The flyovers were facilitated by the Virginia Marine Resources Commission.

“This is new and important information,” says Vogelbein, “as we have never appreciated that Alexandrium extends so far into the mainstem of the Bay or so far up the York River.” Bloom patches in the mainstem reach from the York River to the mouth of the Rappahannock River, across the Bay to within three to four miles of Cape Charles, and as far south as the Chesapeake Bay Bridge-Tunnel. The bloom patches are most dense on the western side of the Bay, with other areas experiencing less activity. “The main body of the bloom is several miles off shore,” says Vogelbein, “and thus wasn’t appreciated prior to the recent flyovers.”

Alexandrium monilatum is one of several species of harmful algae that are of emerging concern in Chesapeake Bay. It was first conclusively detected in Bay waters in 2007, when Reece and colleagues used microscopy and DNA sequences to identify it as the dominant species of a bloom that persisted for several weeks in the York River. There are generic reports of Alexandrium in the Bay from the mid-1940s, and specific reports of A. monilatum in the mid-1960s, but none in the intervening decades. 

Most dinoflagellates, including Alexandrium, are marine and prefer a higher salinity than we have in our neck of the Bay. Hopefully we won't be seeing them any time soon.