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Earth & Marine Sciences - Fueling the Ocean

Copepods

It’s midnight in Wilkinson Basin in the Gulf of Maine on the research vessel Gulf Challenger and University of Maine graduate student Cameron Thompson is up checking on the Calanus Finmarchicus collected for his research. Thompson, a dual-degree student in marine policy and biology, is studying the cross-shore mortality of the copepods in the Gulf of Maine.

Calanus finmarchicus are succulent little butterballs. Simply scrumptious to a whole host of marine animals.

No bigger than grains of rice, the translucent crustaceans that look like a cross between a crayfish and a flea gorge on spring phytoplankton blooms and microzooplankton in the ocean to bulk up with energy-rich lipids. It’s those lipid reserves that make the planktonic copepods particularly delectable — and power-packed.

In the northern Atlantic Ocean, C. finmarchicus is the primary prey for a range of species — fueling schools of herring and powering pods of endangered northern right whales. This and other Calanus species are such vital intermediary links in the marine food web that changes in their populations could profoundly affect the health of marine animals — from leaner fish of lesser value to fewer whale calves — and the structure of the pelagic ecosystem in northern oceans.

That’s why Andrew Pershing and Jeffrey Runge study them. The two research scientists, who hold joint appointments with the University of Maine and the Gulf of Maine Research Institute, consider C. finmarchicus a linchpin whose role must be better understood in the face of growing ecosystem variability and environmental change.

“Arguably copepods are the most abundant multicellular animals in the world, yet most people don’t know much about them,” says Runge, a biological oceanographer. “Calanus finmarchicus is among the most predominant of the copepods in the North Atlantic, including the Gulf of Maine. If its special capacity to produce large amounts of lipids were substantially reduced here, what would be the impact on species like herring, sand lance, mackerel, and the rest of the system? It could have implications for the region’s fisheries in the future.”

Runge studies ocean ecosystem productivity. He focuses on the physical and biological factors that can affect zooplankton production — from variable ocean currents and temperatures to the growth and survival of fish larvae.

Pershing focuses on what causes changes in the Gulf of Maine ecosystem over time. He uses satellite and other data to develop computer models of marine ecosystems that can reconstruct and forecast population dynamics in C. finmarchicus and other key species.

For both scientists, the implications of climate changes on C. finmarchicus, such as warmer water temperatures and acidification, loom large.

“There are huge changes going on in the marine environment,” says Pershing. “Some of them are natural. We’ve always had changes in the climate. But then on top of that, we’re adding this new signal of global warming and climate change. What effect that’s going to have on ocean ecosystems is really important, both for understanding fisheries and the way humans interact with these systems, and for understanding the ocean’s ability to take carbon out of the atmosphere and lock it away. The big question for me is really all about change: how things shift from one year to the next and what drives that.”

CopepodsC. finmarchicus dominates the zooplankton community in the Gulf of Maine, the southern edge of the large copepod’s subarctic range. Here, the one-eyed crustaceans that grow about 3 millimeters long spend their lives moving vertically in the water column, transporting carbon and valuable nutrients from the surface. As omnivores, they put a big dent in spring blooms, eating diatoms and phytoplankton, and preying on smaller zooplankton.

The spring phytoplankton blooms give the copepods tremendous reproductive capacity. Over a two-month period, a female will release 3,000 or more fertilized eggs into the water column, where they develop and hatch.

The copepod has a complex 12-stage life history, maturing from an egg to six nauplius and five copepodid stages to reach adulthood. During late summer through early winter, fifth-stage copepodids known as C5s constitute the majority of the C. finmarchicus population in the north Atlantic. At that time, the preadults either molt into adults or enter a state of reduced activity — a kind of dormancy or hibernation — known as diapause.

C5s in diapause have adapted to survive months with little food, at depths of around 150 meters in the Gulf of Maine, with the help of the rich lipid stores they packed on in the spring and summer months. The lipids in the form of wax esters stored in an oil sac ultimately make up nearly 70 percent of the copepod’s body weight.

Fish and whale species depend on that lipid source for their own survival. Herring predation, which is highest in the summer, is a big source of mortality for C. finmarchicus. Then there’s the northern right whale, which eats at least 2,000 pounds of copepods daily. Of the nearly million calories a cetacean needs each day to function, the vast majority come from copepods.

In the late winter and spring, C5s emerge from overwintering to molt into adults, feed and reproduce. The population increases rapidly, with a new generation of C5s appearing in mid-April. By early summer, some of those preadults will begin their own cycle of dormancy.

That’s the typical seasonal production cycle.

But when the Gulf of Maine is too warm because of the intrusion of warmer Atlantic Slope water or above-normal surface temperatures, C. finmarchicus breaks its dormancy in late summer and fall and produces another generation, contributing to the overwintering stock. But these outside influences, what scientists refer to as forcings, can not only accelerate but also hinder development of a fall generation if temperatures are too warm.

The concern is that climate change may result in substantial reduction in Calanus populations, according to a research team led by Runge that reported its findings most recently at the 5th International Zooplankton Production Symposium in Chile. What’s needed are models that couple what we know about the copepods’ life cycle and the physical circulation in the ocean to better understand the roles of transport and production. Also needed is a long-term, integrated observation system in American and Canadian waters focused on collecting data on zooplankton abundance and diversity in the north Atlantic.

“It’s an important priority to have the capacity to observe how the system is changing,” says Runge. “We have the pieces — the researchers and physical modelers — and can put together models that are very insightful, not just for understanding climate forcing on copepods, but also on the planktonic early life stages of marine fishes.”

copepodsRunge was introduced to copepods three decades ago as a graduate student at the University of Washington, where he studied their population dynamics, including the effects of food supply, temperature and ocean physics. He has spent his career studying zooplankton, particularly the Calanus species, in Hudson Bay, the Gulf of St. Lawrence and the Gulf of Maine.

“I find it an especially beautiful animal under the microscope,” he says. “The gracefulness of the way it swims. Its behavior is complex for what seems to be a simple crustacean invertebrate — from its suspension feeding on diatoms and other phytoplankton, as well as microzooplankton, discerning particles that are good and bad, to its tremendously sophisticated migration behavior that’s different in the day and night.”

Runge has made some fascinating discoveries about Calanus, including the fact that, near the end of their dormancy, they migrate to the surface at night to feed on ice algae and return to the depths at first light.

“That was one of the more exciting things we documented,” Runge says. “I remember being 30 miles out on the ice in Hudson Bay with northern lights dancing on the horizon and seeing copepods leaving the surface at dawn, probably to avoid visual predators.”

Today, his research continues to examine the role of zooplankton in marine food webs, including the biological mechanisms behind diapause. He collaborates with Pershing and other oceanographers to create 3-D models integrating zooplankton production, larval fish survival and recruitment — the number of fish surviving the larval and juvenile phases to enter the adult population each year.

“The new tools, including computer modeling, are giving us tremendous capacity to address the questions we have (about copepods) and the potential to understand scenarios of climate change and how they will impact plankton populations,” Runge says.

Runge, Pershing and UMaine oceanographer David Townsend were among the topflight marine scientists from around the world participating in the Georges Bank Program of GLOBEC, a decade-long international study of global ocean ecosystem dynamics initiated by the Scientific Committee on Oceanic Research and the Intergovernmental Oceanographic Commission of UNESCO. GLOBEC concluded in January 2010.

The goal of the U.S. GLOBEC Georges Bank Program was to understand the population dynamics of four key species of the region — cod, haddock and two zooplankton, Calanus finmarchicus and its smaller cousin, Pseudocalanus — in order to predict changes in their distribution or abundance.

As part of one of their GLOBEC research initiatives, Runge and Pershing helped develop a life cycle model that examines the controls on diapause and can be used to investigate population responses to climate change scenarios for species of copepods. The Individual-Based Model (IBM), as it’s called, helps track growth and development of individuals, including their lipid accumulation and utilization rates — the triggers of dormancy entry and exit.

IBMs for Calanus have led to improved models of copepod life cycles. Pershing and Runge, with UMaine postdoctoral associate Frederic Maps and UMaine Research Associate Rebecca Jones, as well as researchers from NOAA, the University of Maryland and East Carolina University, collaborated on an NSF project studying life histories of Calanus species and their response to climate forcing, looking in particular at the role of dormancy in both the north Atlantic and north Pacific.

Most recently, Runge received a nearly $700,000, three-year grant from the National Science Foundation to study the impact of ocean acidification on three dominant species of high-latitude Calanus — C. glacialis and C. hyperboreus in the Arctic Ocean, as well as C. finmarchicus in the northern Atlantic. He and ocean chemist John Christensen will study the impact of increases of carbon dioxide, higher temperatures, and lower surface and deep pH on population dynamics on the copepods.

In the next century, ocean acidification and temperatures are predicted to rise. Previous studies have shown that the two factors could affect copepods’ reproductive success and early-life stages.

That’s why Runge is so passionate to combine computer modeling with the multiple, disparate ocean observing and data systems in the gulf. The pelagic ecosystem in the Gulf of Maine is complex and subject to environmental forcing over seasons, months and years. Despite the complexity, scientists are developing the capacity to integrate observational data and models to interpret and forecast change relevant to fisheries and coastal ecosystem questions.

“It’s important to set up long-term, dependable, transnational observing systems that can to understand zooplankton diversity and plankton system changes,” Runge says. “That’s the data we need to validate models and to test predictions.”

research vesselPershing is key to developing and integrating those models. He was an undergraduate when he saw his first copepod, albeit a freshwater species, under a microscope. That was 1994, and it was love at first sight.

“They’re really beautiful creatures,” he says. “Because they’re trying not to get eaten, they’re clear and look like the water around them. But they have these long antennae that they use to suspend themselves in the water.”

Four years later, as a graduate student, Pershing was involved in a research project to better understand the ecosystem of Georges Bank, where Calanus is the keystone species.

“We were supposed to be looking at Calanus when it was in diapause, but in the deep basins of the Gulf of Maine in 1998, they were very rare,” he says. “That got us interested in why they weren’t there and what effect that had on the ecosystem.”

Pershing is currently looking at how the number of copepods has changed in the last half-century. To do that, he uses data from the Continuous Plankton Recorder (CPR) Survey, a marine monitoring program of the National Marine Fisheries Service that has been collecting information on plankton since 1961.

Pershing is particularly interested in the monthly CPR data collected across the Gulf of Maine from Boston to Yarmouth, Nova Scotia. The continuous record offers an unparalleled perspective of year-to-year changes and patterns across four decades. In the case of C. finmarchicus, the CPR record has shown that the copepod was abundant in the gulf in the 1980s, but its numbers were low through the 1990s and took a nosedive around 1998, then rebounded.

Armed with that data, Pershing then looks for any changes in the physical environment that may explain the sea change in the copepod population. He also studies the effect of those changes on other species in the food web that depend on copepods for sustenance. Those species include right whales.

“In particular,” says Pershing, “we looked at the number of calves right whales produced and found that during the 1990s, when Calanus was low, right whales had fewer calves. They had more variable reproduction and were, actually, in poorer health. They tended to be skinnier. But in 2001, when Calanus rebounded, all of a sudden the right whale population was able to put out 20 or 30 calves per year and were in much better health.”

The decline in large zooplankton, especially C. finmarchicus, also led to leaner herring that, in turn, led to blue fin tuna of lesser value.

Pershing is now looking at the role played by herring and other small pelagic fish in the Gulf of Maine, including the nutrients they supply to larger animals like tuna and whales, and their effect on zooplankton. The project was one of eight funded in 2010 by the Comparative Analysis of Marine Ecosystem Organization (CAMEO), a program of NOAA and NSF.

Pershing is joined on the research team by two scientists from UMaine, two from GMRI, three from NOAA, the one each from the University of Massachusetts and Ohio State University. The scientists are comparing the physical changes in the Gulf of Maine during three decades, beginning in the 1980s when herring stocks were low.

The researchers hope to better understand the critical link between copepods and herring, and how trade-offs between fish abundance and fish weight are linked in fishery ecosystems.

“In addition to helping uncover the mechanisms driving big changes in systems like the Gulf of Maine, our work will help improve forecasts of fisheries,” Pershing says.

How environmental conditions, including the degree of stratification and production of phytoplankton, determine species composition is now driving Pershing to develop a new class of copepod model. In 2003, he used an NSF Information Technology Research grant to develop a zooplankton model that is now the basis for forecasting C. finmarchicus distributions that are right whale feeding areas. His newest NSF-funded research is expected to help scientists make even better predictions concerning the effects of climate change on this critical trophic level.

“Once we have an estimate of what will happen to the copepods, I think we can have a much better estimate of how the fisheries in a particular region will respond, as well as the birds, the whales and the other animals the people really care about,” Pershing says.

copepod

University of Maine graduate student Phoebe Jekielek washes copepods collected from a plankton tow into sample jars for preservation. The C. finmarchicus copepods are the size of grains of rice; the larger zooplankton also present (at far left) are krill. Specimens are collected in ring nets towed off the stern of the research vessel and kept alive in seawater in coolers for experiments back in the laboratory on shore.


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