The Brink: The water in Boston Harbor was long notoriously dirty, largely due to sewage draining directly into it. The cleanup efforts involved serious changes to sewage treatment practices, reducing harmful bacteria in the water. But what else causes problems in an urban harbor setting?

Fulweiler: Boston Harbor [was full of] excess nitrogen, mostly from sewage, but also from fertilizer runoff and things like that. Nitrogen is a tricky thing: it’s essential for life, but too much nitrogen causes problems. Excess nitrogen leads to excess growth of primary producers; in the ocean, those are things like phytoplankton. Imagine a big bloom of phytoplankton growing at the surface of the sea: it’s going to shade out everything below. Now, sea grasses—which are good for the ocean environment because they stabilize sediment and provide habitat for lots of animals—can’t grow, because there’s not enough light getting to them. So they die, taking that habitat with them.

Another major issue is when the phytoplankton themselves die, the bacteria required to decompose [that amount of] phytoplankton uses up all the oxygen in the water. That leads to hypoxia (low oxygen) or anoxia (no oxygen), in the water column, creating dead zones that can kill any animals trapped in them, causing major declines in biodiversity. So, too much nitrogen can really mess up an ecosystem.

The Brink: Your work involves studying how oysters regulate nitrogen levels. How exactly do they do that?

Fulweiler: Oysters are really cool. Scientists call them ecosystem engineers. Oysters, like other shellfish, are filter feeders. An adult oyster can filter up to 50 gallons of water a day. Essentially, oysters increase the amount of food to the good bacteria on the ocean floor that remove nitrogen for us. They do that by grabbing material like phytoplankton out of the water column and eating it—cleaning up the water in the process—and turning that material into either feces or mucus balls that drop into the sediment. The sediment bacteria turns some of the nitrogen the oyster drops into inert nitrogen gas, naturally removing nitrogen from the water. That process is called denitrification. Clams and mussels also do this, to an extent, but we don’t know as much about their impact on the nitrogen cycle. That’s an emerging area of research.

[The flipside is] that shellfish can also take up metals and plastics, and bacteria that make us sick. One common bacteria, Vibrio, can make you sick to your stomach or even cause a flesh-eating disease, depending on the type you’re exposed to. That’s why we have rigorous testing and very regulated processes for shellfish farming.

The Brink: How did you get interested in oysters in the first place?

Fulweiler: The reason my research group got into oysters was because of the rise of oyster aquaculture. Globally, about 85 percent of oyster reefs—dense clusters of oysters that form 3D structures, and provide habitats for other animals—have been decimated by overharvesting and disease. There are very few natural oyster reefs left, so most oysters now come from restoration efforts to rebuild reefs through seeding areas with oyster larvae and through oyster farms. My research partners and I are really interested in how oyster aquaculture is changing water quality and nitrogen cycling in these ecosystems. So far, it looks like it’s helping: it increases denitrification, as well as provides jobs and food. 

The Brink: The PEARL van is vital to those research efforts. How does it aid your work?

The van started as a late COVID-19–era project, and it’s been so fun. It’s a combo of two things: engaging people with science—because we’re doing our work, doors open, in a van with a big blue oyster on it, and people who walk by are naturally curious—and collecting data more efficiently. Typically, we’d have to get our samples and then drive back to analyze them at BU. But with PEARL, we can do that all on-site, allowing us to sample in locations that would otherwise be hard to get to and return in one day.

The Brink: Finally, why should people care about having oysters and other shellfish in our seas?

For one, you want good water quality so that you have good food products. But, more importantly, humans are so intimately connected with coastal systems: we rely on them for everything from recreation to culture to food. A substantial part of our economy across the world and in the United States is based on coastal systems. And here in New England, we have a very close connection to the water. So having a clean home—and I consider coastal systems part of our home—is really important.