Studying the Big Impact of Small Animals in a Changing Ocean

“The importance of zooplankton cannot be overstated.” So says Senior Research Scientist David Fields.

“If you take a plankton net tow at certain times of year, you’ll find the larvae of everybody in the water column from baby swordfish and tuna to microscopic organisms,” he said. “It’s an amazing window into the diversity of life in the ocean.”

Unlike many of the scientists at Bigelow Laboratory who study phytoplankton, the ocean’s flora, Fields focuses on its fauna. He’s spent his career looking at the outsized role of these small creatures in the marine food web — and their future in a rapidly changing ocean.

Zooplankton refers to all of the animals floating in the upper water column, from those that are there for only one stage of development, such as fish and lobsters, to microscopic animals that spend their entire existence there, such as the vital copepod.

A type of crustacean found in just about every aquatic habitat, copepods are among the most abundant and diverse type of animal on the planet.

“Copepods are more different from each other than mammals are,” Fields said. “They all look alike from the human perspective, but their evolutionary divisions are deep.”

Because of their abundance, copepods help form the foundation of the marine food web. They graze on plant-like phytoplankton and then funnel that energy up the food web. Almost every species, from fish to whales, relies on copepods for food at some point in their development. They’re also essential to nutrient cycling, and copepod pooping contributes to the largest pump of organic matter into the deep ocean.

But, as the climate changes, their future in the Gulf of Maine is uncertain.

Recently, Fields and a team of partners, including Senior Research Scientist Nick Record, received NOAA funding from the Northeastern Regional Association of Coastal Ocean Observing Systems to study the impacts of climate change on communities of copepods and other zooplankton. They’ll be doing net tows regularly at two important sites off the Maine and New Hampshire coasts to identify the types of species and how their populations are changing over time and space.

Similarly, for the past year, Fields has been funded by the Maine Department of Marine Resources to collect baseline data on zooplankton at a future wind energy test site about 45 miles offshore. The state is hoping to understand what the population looks like before, during, and after construction of the offshore turbines.

This kind of regular monitoring will allow the scientists to separate out regular seasonal changes from the “superimposed changes” from human activity and climate change — changes that could have far-reaching implications for the whole ecosystem.

“Copepods, like all animals, occupy a particular niche in the ecosystem, but as you change things more and more, and add more stressors, their capacity to live in the marginal areas of that niche go down,” Fields said. “If you start to heat things up, you narrow the range they can live in. And then you add ocean acidification, and it pulls that in a little more bit. And then you add their food moving and other kinds of problems, and things become tighter and tighter until they can’t make a living.”

On top of that, warming will likely cause copepods to generally be smaller. Those kinds of changes could cause problems for the animals that rely on copepods for food. In the Gulf of Maine, that includes lobsters.

Calanus finmarchicus is among the fattiest species of copepods. In winter, they go into hibernation and sink, settling into a thick layer that stretches across the North Atlantic. Fields describes it as a “butter layer” — an easy, fatty buffet that provides essential food for whales. But studies of stomach contents Fields and his colleagues have done suggest that Calanus is a favored meal for baby lobsters as well.

As summers get longer, Fields says they’re already seeing the Calanus hibernation season get shorter. This could cause what’s called a phenological mismatch where the timing of natural events that evolved together, like the lobster and Calanus life cycles, gets disconnected. Fields and his colleagues think this mismatch could explain a mystery that’s plagued the lobster fishery for the last several years.

Even as the abundance of big, adult female lobsters has been at an all-time high, managers are seeing fewer young lobsters settling out of the water column to enter their next stage of development on the seafloor.

“The disjunct suggests that somewhere between all these babies being born and the first ones settling, there’s a breakdown,” Fields said.

He and colleagues have been doing experiments looking at ocean acidification, changes to diet, and other factors to explain this “great disconnect.” In a recent paper Fields co-authored in the ICES Journal of Marine Science, they’ve shown that the timing of when the population of larval lobsters peaks in the Gulf of Maine is no longer aligned with the timing of the Calanus peak. That disconnect, coupled with a general decline in Calanus due to warming, could help explain the missing baby lobsters.

The team is also now looking at how lobsters at different stages can digest different sized copepods, which would help them understand whether lobsters could adapt their feeding habits in response to the Calanus changes. It’s an important question given the immense impact that copepods have on the delicate ecosystem of the Gulf of Maine.

“Because of their sheer number, small changes in what happens to copepods matter. Small disruptions at this level could cascade across the food web,” Fields said. “They really are the true charismatic megafauna of the sea.”

Photo Captions

Photo 1:   A microscopy image of two copepods, both female Calanus finmarchicus. The one on bottom is smaller with significantly less fat, which is representative of a copepod raised in warmer water. (David Fields).

Photo 2:  Senior Research Scientist David Fields examines zooplankton in the lab.