Aug 25, 2021 | Charting the Past

Agnes Mittermayr, who has studied “black custard” for the Center for Coastal Studies in Provincetown.

By Seth Rolbein

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“Black mayonnaise” is an evocative phrase used around harbors to describe a muck that builds in shallow waters, a thick carpet of ooze that smothers life and creates dead bottom where we’d all rather see sand, eel grass, and creatures with shells and little fins.

But a good look at the history and meaning of the term suggests we’re not using it as we should.

Real “black mayonnaise” is not what we have on the Cape, and the difference is profound enough that Agnes Mittermayr, who has studied this phenomenon from her research perch at the Center for Coastal Studies in Provincetown, believes we need to use new language for what we see. She has a suggestion:

“Black custard.”

The difference is fundamental:

Black mayonnaise is created by human pollution. Black custard, as Mittermayr’s careful research has proven, is composed entirely from natural materials in the marine environment. “It’s coming from the ocean,” says Mittermayr, no hedge or maybe involved.

Mucking around the inner harbor of Wellfleet, where the custard allows little life to grow, the distinction might not seem all that important. But big picture, it is.

Mittermayr’s deep dive (so to speak) into black mayonnaise led her to conclude that the first general use of the term was in 1974, when a geologist was testifying before Congress about a huge offshore sewage dump site that threatened beaches on Long Island, New York. He described the area as having a bottom resembling “frothy black mayonnaise” with sand mixed in. Tons of sewage was the main cause, why the area had earned the nickname “the Dead Sea.”

The phrase showed up again in the 1990s to describe the Providence River and Harbor in Rhode Island, another big disposal area.

Among the most dramatic examples was Boston Harbor, once considered the most polluted waterway in the world bar none. The entire Greater Boston area, more than 50 cities and towns, had been dumping raw sewage directly into those waters for generations:

“The scum floated to the shore, and the sludge settled to the bottom where it formed a ‘black mayonnaise’ that was devoid of all normal aquatic life,” wrote two scientists in a book that chronicled the building of the Deer Island Wastewater Treatment Plant, a gigantic public works project that actually worked, cleaned the harbor, and reduced the mayonnaise.

The term showed up one more time to describe the Gowanus Canal in Brooklyn, New York, a Superfund site with a “polluted stew” of bottom that included “chips of asbestos, plus some arsenic, copper, lead, and mercury mixed into sewage.” That combination became part of the US Army Corps of Engineers description in 2001, the closest thing we have to a formal, accepted “black mayonnaise” definition.

Enter Mittermayr and her Wellfleet work.

A rectangle of Wellfleet’s inner harbor, located behind a solid town pier that holds dozens of parking spaces and blocks historic tidal flow, has become locally famous for its “black mayonnaise.”

Intrigued, Mittermayr set about studying it. Local artist and ocean-goer named Steve Swain built an auger that helped her drill down for core samples in the mayo area and nearby.

Mittermayr found that the stuff can be as much as 12-feet deep. She looked at it with stable isotope analysis for organics, checking for nitrogen, sulfur, carbon, algae and bacteria, watching how photosynthesis worked.

Her conclusion?

“It’s all marine organisms,” she says, “no doubt.” When it comes to nitrogen, a strong indicator for sewage and other human waste runoff, “there was none of that.”

That said, neither is it a fertile place.

“We looked for life, invertebrates and so forth. We found next to nothing,” though there is a sheen of bacteria on top, and the material is highly organic.

This is good news in a big way, but not a happy ending. Why this black custard builds up and what we could do about it remain questions. Mittermayr’s theory is a diminished tidal flow behind the town pier barrier allows for “flocculation”; smaller sand particles clump with clay and other material, become dense, settle to the bottom rather than coursing around, and then keep building into a stultifying blanket.

Fortunately, this is not happening on the open side of the harbor, where the flow is much stronger – and by the way, millions of valuable shellfish grow each year that would not enjoy (or survive) the custard.

Here’s the other amazing thing Mittermayr found:

Natural as Wellfleet black custard is, she cannot find other examples – anywhere in the world. Barnstable Harbor has some bottom that resembles it, maybe for similar environmental reasons, but she is skeptical it is the same. She has reached out to colleagues and contacts across the globe, having come to the Center for Coastal Studies from the Marine Biological Labs in Woods Hole, arriving there from Austria in 2015 (a landlocked country, not the greatest place to be a marine geologist, she smiles). No comps.

“There’s still a lot of fascinating research to be done,” she concludes. One interesting element: Wellfleet has embarked on ambitious harbor dredging, sure to change tidal flow and bottom compositions. Mittermayr assumes that the black custard began to build soon after the town last dredged, decades ago. What she doesn’t know is whether it was there before the 1950s, when the blocking wall created by the big town pier was built.

Always more to be discovered. Meanwhile, we can take her informed advice, and coin a new term. Pass the custard.


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