Reconstructing Food Webs to Reveal a Dynamic Gulf of Maine

Reconstructing Food Webs to Reveal a Dynamic Gulf of Maine

When most people think about corals, they imagine a tropical reef with crystal blue water, teeming with colorful fish. But, in the depths of the cold, murky Gulf of Maine, deep-sea corals thrive, feasting on a steady supply of organic matter raining down from the surface ocean.

Like tree rings, corals preserve a record of their growth in their skeletons. Combining that information with advanced chemical tracer approaches, scientists can unravel what kind of food is sinking to the seafloor to feed these corals and, critically, how that's changing over time.

A team of scientists, jointly led by University of Rhode Island and Bigelow Laboratory for Ocean Sciences, is reconstructing a multidecade record of changing food webs in the Gulf of Maine. They're measuring how the biological communities of the surface and seafloor are connected and how that relationship is influenced by large-scale shifts in oceanography.

The National Science Foundation-funded project, which recently wrapped up, has showcased the importance of tiny plankton called copepods in serving as conduits for energy through the water column. It's challenged assumptions about the Gulf's biology and highlighted the limitations of using historic data to understand the future.

"Our goal has been to reconstruct past food webs, measure how organic material is moved to the seafloor, and bring it together to understand what's driving major changes in the Gulf of Maine," said Bigelow Laboratory Senior Research Scientist Karen Stamieszkin. "We're trying to develop this holistic picture of the ecosystem, to use what's happened in the past to understand the future. What we're finding is that it's all more complicated than it looks."

Complex Methods for Complex Questions

Export production is the amount of organic matter produced by plant-like phytoplankton at the surface that sinks (rather than being decomposed and recycled by bacteria in the upper ocean in what scientists call the "microbial loop"). It's an important process for regulating the cycling of nutrients and Earth's climate, as well as feeding animals that live on the seafloor, including the Gulf's deep-sea corals.

Scientists have long appreciated the importance of this surface-seafloor connection but have struggled to accurately measure how export production changes with shifts in the food web and environmental conditions. Researchers from Bigelow Laboratory, URI, and Dalhousie University, have been working to fill that gap.

In 2023, the team undertook a week-long research cruise to Jordan Basin aboard the R/V Endeavor. Using an arcade game-like claw, they collected corals - some hundreds of years old and hundreds of feet below the surface - to dissect growth rings. They used surface nets to collect copepods and then extracted their fecal pellets in a device Stamieszkin has coined the "cope-a-pottie" (these dense pellets are one of the main ways that organic matter sinks to the seafloor). The final piece of the puzzle was an archive, going back 50 years, of the Gulf's most common copepod species collected and preserved by NOAA.

To connect those pieces, the team used a novel approach called compound-specific stable isotope analysis, pioneered in the lab of Kelton McMahon, an associate professor at University of Rhode Island and the project's lead.

"Scientists have used whole tissue isotope analysis for nearly a century to understand how organic matter moves through food webs," McMahon said. "But in the last 20 years, we have developed powerful new tools to look at the isotope signals of every individual compound that makes up an organism, which allows us to get this really holistic view of the food web structure."

"Isotopes act like a fingerprint of an organism's diet so you can use them to map how energy moves up the food web," added Catrina Nowakowski, a former PhD student in McMahon's lab. "It's a really exciting method for measuring something you can't directly observe."

Different types of primary producers (the organisms like phytoplankton at the base of the food web) incorporate different kinds of carbon from the environment, creating a signature that's unique to that producer. Because compounds like essential amino acids aren't altered as they move up the food web, that carbon signature is passed up the food chain, virtually unchanged.

Nitrogen isotopes, in contrast, accumulate up the food chain. So, they provide a measure of how many times energy has been transformed - in essence, where in the food web a particular organism sits.

"Essentially, we're trying to figure out where the coral is in line behind the copepods. Are whole copepod bodies sinking down, and the corals are eating that, or are they eating fecal pellets, which reflect what the copepods are eating?" Nowakowski said. "Those scenarios are very different in terms of how much energy makes it to the seafloor."

By running isotope analysis on the samples from the surface (the copepods), the seafloor (the corals), and the pellets that move between the two, the researchers can reconstruct the food web to determine how active the microbial loop is, who is eating what, which producers are driving the system, and, ultimately, how that's all changing.

"With this chemical link between the base of the food web and our samples, we can tie that to the environmental conditions in which these organisms are living," McMahon said.

A Dynamic Natural Laboratory

The Gulf of Maine is a hub for oceanographic research with a fount of available data. Combined with the region's geography at the boundary between two ocean biomes, and its strong warming trend, it's an ideal place to understand the drivers of ecosystem change.

The existing paradigm, Stamieszkin explains, is that the Gulf has two distinct states. Sometimes, cold, freshwater from the Labrador Current dominates, creating a food web with large copepods (like Calanus) consuming large phytoplankton (like diatoms) and producing heavy fecal pellets that quickly sink. Other times, warm, salty Gulf Stream water results in small phytoplankton (like dinoflagellates), small copepods (like Centropages), and an active microbial loop, with more material being recycled rather than sinking.

Yet, the team's isotope results suggest the microbial loop is active even when the system is in that cooler state. The big plankton, like Calanus, are consuming organic matter that's been heavily recycled by microbes, and their pellets are carrying that chemical signature down to the corals.

"Diatoms versus dinoflagellates. Calanus versus Centropages. Cooler, subarctic conditions versus warmer, subtropical waters. Those are the assumptions that have been baked into the way we think about the Gulf," Stamieszkin said. "They're not wrong. They're just vast overgeneralizations."

The findings may inform how the Gulf adapts in the future.

The expectation is that, as the ocean warms, this microbially active food web will become the norm, meaning fewer nutrients will sink to the deep ocean. If there is already a microbial connection between the surface and the seafloor, though, that would suggest that the biological community might be more adaptable to warmer conditions.

"People have set up this juxtaposition where only certain ocean conditions allow for strong connections between the surface and seafloor, but we're showing that large copepods are connecting the microbial world to the deep," McMahon said. "It provides a glimmer of hope in some ways. Even if we do move into this new state, we know these big copepods can continue to provide this critical ecosystem service."

Looking Back to Look Ahead

Last fall, the team published their first results in the journal Progress in Oceanography.

The researchers identified four periods over the last 40 years where the Gulf of Maine flipped between its two recognizable states. For three of those periods, there was one set of environmental variables (particularly water stratification) that drove the transition.

The fourth transition in 2006, though, looked completely different in the models and was driven largely by rapid warming. The findings highlight just how much the environment has changed in recent years.

"We're trying to make statements about what direction the ocean is going, but we're working with a new set of conditions that doesn't look like our historic data," Nowakowski said. "The paper highlights this breakdown of historic relationships to an essentially new regime with warming."

It's an important reminder that the patterns of the last 50 years provide no guarantee for the future. That has motivated the team to continue unraveling this complex ecosystem and share their results. They have several academic papers in progress, and have been working with a filmmaker on a short documentary. Nowakowski is also putting her sculpture skills to use in partnership with a student at Rhode Island School of Design, all for the purpose of making the project more accessible to everyone with a stake in this ecosystem.

"The Gulf is this fascinating place to do this work, and we have these exciting tools that are providing all kinds of new information to challenge longstanding assumptions," Stamieszkin said. "This has been one of my favorite projects I've been involved in, and we're really getting a sense of the ecosystem as a whole."

Photo Captions

Photo 1: Using an remotely operated vehicle manned by Brennan Phillips, an associate professor at URI, and his students, the research team collected samples of Primnoa resedaeformis, a species of coral found on the seafloor in the Gulf of Maine (Credit: Karen Stamieszkin).

Photo 2: Stamieszkin prepares to examine some of the preserved copepod samples the research team collected back at Bigelow Laboratory (Credit: Bigelow Laboratory).

Photo 3: Nowakowski (right), McMahon (left), and Breanna Motsenbocker (center), a former PhD student at URI working with Phillips, examine a sample of coral recently brought up from the seafloor (Credit: Karen Stamieszkin).

Photo 4: Researchers aboard the R/V Endeavor in 2023 deploy a remotely operated vehicle for collecting coral samples from the seafloor of Jordan Basin (Credit: Karen Stamieszkin).

Photo 5: A micrograph of preserved copepods collected during the 2023 research cruise shows an abundant community of pink Calanus finmarchicus among a handful of white Metridia lucens (Credit: Karen Stamieszkin).

Photo 6: Sculptures created by Nowakowski and Eunhyung Chung, a former masters student at the Rhode Island School of Design, visually communicate how the Gulf of Maine food web has changed, with the glass color and length of the sculptures corresponding, respectively, to ocean temperatures and the number of steps in the food chain (Credit: Catrina Nowakowski).