Time Series Studies Illuminating the Oceans

The Industrial Revolution brought with it new manufacturing processes, the rise of factories, and a dramatic increase in the carbon humans pump into the atmosphere. About a quarter of this carbon ends up in the ocean, affecting everything from chemical processes to sea life. 

At the same time, natural climate and seasonal cycles continue to cause variability in the ocean. Untangling all these effects is a major challenge. 

“You have to measure long enough to capture natural cycles and detect underlying change associated with the last 200 years of putting carbon into the atmosphere,” Senior Research Scientist Mike Lomas said. “The challenge is that if you don’t measure frequently or long enough, you can’t answer questions about climate change.”

Coincidentally, Bigelow Laboratory for Ocean Sciences began next door to the site of one of the longest sea surface temperature records on the Atlantic coast, at the Maine Department of Marine Resources in West Boothbay Harbor. The richness of a century of temperature data inspired Bigelow Laboratory scientists to start collecting complementary data from their dock. Michael Sieracki, a former senior research scientist at Bigelow Laboratory, launched a comprehensive study from the dock in the mid-1990s. Research Scientist Nicole Poulton recognized that continuing these measurements had the potential to capture significant changes in the harbor and inform long-term research. She formalized the study into a time series in 2000.

Time series studies provide the consistency and longevity to understand and untangle ocean changes. Through consistent measurements, Poulton’s team has gradually built a comprehensive picture of both stasis and change in Boothbay Harbor. They have tracked its temperature and salinity, the composition of its plankton communities, and nutrient cycling at the base of its food chain.

“This is a labor of love. This project was started so long ago, and I feel it’s really important to maintain,” Poulton said. “By sampling frequently and over decades, we can capture the fluctuations that take place over the course of a year, as well as shifts we believe are related to climate change.”

This is one of many similar efforts around the planet that are overseen or contributed to by Bigelow Laboratory scientists. These long-term studies are providing unique insights into our oceans.

Poulton’s team is learning how phytoplankton communities in Boothbay Harbor are responding to a changing climate. She has observed that some phytoplankton bloom at different times during the year than when the time series began. She believes these shifts may be related to rising surface temperatures in the Gulf of Maine, which is steadily warming by about a hundredth of a degree every year.

As they build this time series, Poulton is careful to preserve her data so it can be useful long into the future — a challenge with ever-evolving technology and storage systems. She uses cell-imaging technology to take photographs of the phytoplankton within her samples. Scientists in the future will be able to use her data to examine trends that they are researching.

“Working with other researchers that are generating similar data makes a time series even more powerful,” Poulton said. “It’s incredibly important to observe the ocean in order to identify trends, and time series studies provide crucial data for understanding ecosystems around the globe.”

Gulf of Maine

The Gulf of Maine is changing quickly, and Maine with it. About thirty years ago, warming waters bolstered the Gulf's lobster population, fueling a booming industry at the heart of Maine’s economy and culture. More recently, acidifying waters have concerned fishery managers and residents alike.

Over the last century, the Gulf of Maine has warmed, become more acidic, and even turned more yellow. These developments come from shifts in an ecosystem so complex it requires decades of observation to understand. In 1998, Senior Research Scientist Barney Balch set out to do just that when he began the Gulf of Maine North Atlantic Time Series (GNATS), a coastal time series inspired in part by the open-ocean Bermuda Atlantic Time-series Study (BATS), which Lomas leads.

“I came up with that tongue-in-cheek acronym to rhyme with BATS," Balch said. “The idea with GNATS is that by sampling the same line over and over again, we can understand critical connections between the land and sea.” 

This September, GNATS completed its 20th year and 200th trip. About 10 times per year, Balch drives a truck that holds a mobile laboratory onto a passenger ferry, temporarily turning it into a research vessel. While crossing the Gulf between Maine and Nova Scotia, his team takes the same measurements at the same geographic points. Over the last two decades, GNATS has built a vast dataset that captures both the influences of seasonality and climate change on the Gulf. 

“The power of any time series is the ability to connect global phenomena with the ecosystem you’re observing,” Balch said. “Having a time series of this length allows us to see the complex effects of temperatures warming and weather becoming more extreme.”

Balch has observed how these climatological trends are shaping the Gulf of Maine, from its water quality to the food chain it supports. As more precipitation falls over the land, more water percolates through the soil — like steeping a tea bag — before flowing down rivers and into the Gulf. 

“As temperatures and the amount of rain go up, the water steeps better and more material is flushed into the Gulf of Maine,” Balch said. “We’ve seen it fifty miles out to sea. It is striking.”

This colored water soaks up light, competing for the sun’s energy with phytoplankton — a foundational food source for the entire Gulf of Maine. Small phytoplankton are becoming more common and large phytoplankton species rarer, reorganizing the food web in a way that impacts animals from tiny zooplankton to whales. 

Integrating historical measurements taken by Charlie Yentsch, a founder of Bigelow Laboratory, and Henry Bigelow, the father of modern oceanography, enables Balch to look back more than a century in the Gulf of Maine. This allowed him to identify the yellowing waters and use weather station data to measure an air temperature increase of 1.4 degrees Celsius during the last century. Most recently, Balch discovered rapid warming in deep Gulf of Maine waters, which he believes is connected to shifting circulation patterns in the North Atlantic Ocean.

“The value of a time series increases exponentially with how long it runs,” Balch said. “While 20 years may seem like a long time, we’re just getting to the point where we have enough data to address the big questions. Being able to use historical datasets to look back at a full century puts us in a fascinating position where we can really examine how the Earth responds to climate change.”

Sargasso Sea

Earth’s only sea unbounded by land, the Sargasso Sea lives large in literature and cultural imagination. Over the last three decades, the Sargasso has warmed, movements of nutrients between seawater and phytoplankton have shifted, and the water itself has become saltier.

These changes are representative of what is happening in other regions of the global ocean, and their impact is huge. Sixty percent of Earth’s surface area has the same key characteristics as the surface of the Sargasso Sea, and areas of the ocean deeper than 1,000 meters have similar processes of mixing and circulation. Studying this region serves as a barometer for the change occurring across much of the planet.

Despite the challenges of accessing such a remote place, it is one of the best studied parts of the ocean. This is due in large part to BATS, which has run monthly since 1988.

“Our objective is to assess change over a very long time, like monitoring the heartbeat of the ocean,” said Lomas, who has been a lead investigator on BATS since 2001. “Programs like this are critically important, because the scales of variability in the ocean are numerous.”

BATS measures ocean characteristics and processes that shape the Sargasso Sea. Every month, scientists board a research vessel at the Bermuda Institute of Ocean Sciences and sail to the same geographic point in the ocean they have studied for three decades. 

The consistency and duration of the time series has allowed Lomas and other investigators to assess what is happening in the Sargasso Sea — as well as the rest of the ocean.

“Even small shifts in the ocean scale up to big change,” Lomas said. “BATS and other time series allow us to build an understanding of how key measurements of ocean health and function change over time.”

One of the most important trends Lomas has measured is rapid warming. Since BATS began, the surface of the Sargasso Sea has warmed almost a full degree — a huge increase in a relatively short amount of time. In addition, the water has become saltier and more oxygen-rich, and the biological community has transformed. As in the Gulf of Maine, populations of large phytoplankton are decreasing while smaller species increase.

As the BATS dataset becomes longer and more powerful, Lomas and the other investigators can ask more questions of it. One persistent mystery is how the Sargasso Sea pulls so much carbon from the atmosphere. Lomas believes it is related to other processes taking place in the Sargasso Sea and around the globe. It could be caused by an increase in the atmospheric dust that blows seaward from deserts and fertilizes the ocean, or it could be related to an uptick in the efficiency with which phytoplankton use nutrients.

“As the amount of carbon in the atmosphere increases, it is essential to understand the processes that transfer carbon into the ocean,” Lomas said. “Time series allow us to observe the world and confidently conclude whether things are changing.”

Final image courtesy of Andrew Collins.