Friday, June 7, 2013

Aha! It's Algae, Not Sewage Causing the "Dirty Sea".

In early June, boaters and beach goers from Nahant to Nantasket reported streams of brown matter floating in the water.

In this photo taken by our BayWatcher Bruce Berman, you can clearly see the streams off the bow of his vessel "The Veranda" off Graves Light.

In some places the material had accumulated around docks or along the shore and was so thick that it looked like "brown crud" according to many observers. 

Many people assumed that a broken pipe or combined sewage overflow (CSO) was to blame, but this was not the case. According to our BayWatcher Bruce Berman and the scientists at the MWRA, the "brown crud" floating in the Harbor is a diatomacious algae bloom.

Here's what it looked like in Boston's Inner Harbor

According to Dr. Andrea Rex of the MWRA, the diatom is a kind of phytoplankton found in temperate latitudes during late spring and summer. 

In a recent email, Dr. David Borkman of Batelle wrote "This diatom can exist as a single cell, but more commonly forms chains of up to a dozen cylindrical cells. This diatom thrives at higher temperatures, as population growth rate increases during the rapid warming period of late spring/early summer. While algal blooms are often green in color, they can appear yellow-brown or even red, depending on the species of algae."
A microscopic image of the diatom, Guinardia delicatula

Dr. Borkman explained that the brown froth comes from a diatom bloom where the diatoms produce mucus that clumps together to form the "floc" people witnessed in the Harbor. Borkman explained, "Often times diatoms exude excess carbon during unbalanced growth in which there is plenty of nutrient and light for photosynthesis, but not enough silicon to divide and make new cells. The 'excess' carbon is dumped out of the cells as long-chained carbon molecules--mucilage like material--that can result in "Mare sporco" or "dirty sea" in which the mucilage accumulates and fouls beaches." 

The occurrence of the algae bloom was likely the result of multi-day rains followed by calm, warm weather. Borkman explains, "My working theory would be the classic freshwater input--calm weather bloom scenario. In this, the weekend rains resulted in delivery of freshwater to the harbor, followed by a few days of calm, sunny weather. The phytoplankton present (Guinardia delicatula in this case) bloom in the stratified nutrient enriched surface layer, then accumulate in tidal fronts to water discoloring abundance levels." 

"Algae blooms are fairly common, though this one seems larger than many I have seen. Some can be a nuisance - others, like red tide - which we seem to have largely avoided this year, can be quite harmful. This diatom bloom will provide lots of food for zooplankton, which in turn will feed fish and others creatures higher up the food chain" said Bruce Berman. "I expect that with the warmer weather and predicted rain, this bloom will end fairly soon."

If you are concerned about something you see in the sea around Boston Harbor, send us an email to and we will see what can see!

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