Taking the Long View

The Seward Line begins at the head of Resurrection Bay. NPS Photo

Long-term data sets provide perspective on changes in the marine ecosystem in the Gulf of Alaska

Transect lines for the Seward Line and others in the Gulf of Alaska. Red circles denote CTD-nutrient-phytoplankton-zooplankton stations; blue circles denote CTD-nutrient-phytoplankton stations. Image credit: http://research.cfos.uaf.edu/sewardline/overview.html

Identifying trends is no easy task. Detecting trends in the making, especially those that are invisible to the naked eye, is even more of a challenge. Access to good data helps, of course, and the farther back in history that data reaches, the more reliable and informative the results. Long-term sets of data provide critical insight into what is normal in a natural system, and what is not. Ultimately, they better equip the scientific community to predict how an ecosystem will respond when those trends become the norm.

Enter the Seward Line. This series of oceanographic survey stations in the Gulf of Alaska has been in operation, in various forms, since the early 1970s. The Seward Line begins with the GAK-1 monitoring station at the mouth the Resurrection Bay and proceeds south 150 nautical miles to the outer edge of the continental shelf. The University of Alaska, Fairbanks established a long-term observation program along this transect in 1974 in order to fill in data gaps about the hydrology on the Gulf of Alaska shelf. Since that time a series of agencies have assumed management of the monitoring program, each with a unique focus for the information gathered.

The U.S. Global Ocean Ecosystem Dynamics (GLOBEC) program, sponsored by the National Science Foundation and National Oceanic and Atmospheric Administration began monitoring the stations in 1997, to examine the effects of climate change on ocean ecosystems, particularly on important fisheries. From 2005 to 2010, the North Pacific Research Board (NPRB) continued the research along the Seward Line an effort to better understand how the environment influences the success of fisheries in the Gulf of Alaska.

Today the Seward Line is part of a comprehensive long-term ecological research program funded by a consortium of agencies, including NPRB and the Alaska Ocean Observing System. The monitoring itself is conducted by Gulf Watch Alaska, an offshoot of the Exxon Valdez Oil Spill Trustee Council. Gulf Watch Alaska collects ecosystem data from multiple regions of the gulf affected by the 1989 spill, including Prince William Sound, the lower Cook Inlet and Kachemak Bay, and at point locations in Cordova and Seldovia Harbors. Their research cruises along the Seward Line focus on environmental drivers – those biological and physical factors that have the greatest influence on an ecosystem. In the marine environment, these include currents, water temperature, salinity, dissolved oxygen, nutrients, chlorophyll and plankton communities. They influence not only the gulf’s recovery when things go awry, (as in an oil spill), but its ability to adapt to a changing climate.

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How is the Sampling Done?

Twice a year, in May and September, Gulf Watch Alaska researchers board the research vessel, Sikuliaq, to collect water samples from each station along the Seward Line transect. A CTD (conductivity, temperature, depth) instrument is lowered down through the water column to measure salinity and water temperature relative to the depth. Crews may also attach additional sensors to the CTD to measure water clarity, light penetration, dissolved oxygen content and phytoplankton.

Two different zooplankton-collecting systems are employed to gather a broad range of sizes and species. During the daytime, a simple vertical net coupled with two additional nets with smaller mesh sizes (.05 mm and .15mm) is lowered 100 meters into the water to collect the smaller zooplankton. At night, a more sophisticated "multinet" system with .5 mm mesh, is towed slowly behind the ship to catch the larger and faster zooplankton, such as krill.

Researchers also monitor for ocean acidification by evaluating water samples gathered at varying depths for carbonates. Biological and chemical samples are taken to the laboratory for analysis, while physical data are read directly from the instruments. In total, these data provide significant details on seasonal patterns and trends, as well as changing environmental conditions in the gulf.

What are They Finding?

Persistence, in terms of consistent, multi-decade studies, is beginning to pay off. The combined results of all of Gulf Watch Alaska’s monitoring sites, along with the efforts of multiple research teams and nearly 50 years of data, have revealed some significant trends in recent years. These trends are having a profound impact on marine life in the gulf.

Temperature readings taken in May and September from 1998 to 2016. Station 1 is the closest to shore. Note cold period in May between 2007 and 2009, and extreme warm period beginning in 2014. September shows a major difference in inshore and offshore temps, but less inter-annual variation.

http://research.cfos.uaf.edu/sewardline/Physics_time-series.html

The collective data, for example, reveal a region-wide increase in water temperatures between 2013 and 2016. These unusually warm ocean temperatures appear to be driving dramatic changes that cut across the marine food web, from microscopic plankton to massive whales. More precisely, researchers discovered a corresponding decrease in the number of large, fatty, nutrient-rich zooplankton and an increase in warm water tolerant species that tend to be smaller and nutrient poor. They have also detected an increasing proportion of toxin-producing diatoms that thrive in low-nutrient conditions, and subsequently, more frequent toxic algal blooms.

Along the Seward Line, specifically, researchers have noted a 3°C (5.5°F) temperature variation during the May cruises from 1998 and 2016. While seemingly minor, these changes have influenced a host of critical factors. They found, for example that in warm years, the growth rate and timing of the spring phytoplankton bloom was smaller and occurred earlier than in colder years. Likewise, they detected that peaks in zooplankton production came earlier, and were smaller and shorter during warm-year periods. Results have also revealed that, in general, macro-nutrients, such as nitrate, phosphate and silicate, which help fuel the phytoplankton bloom, are depleted earlier in warm years and later in cold years.

Seabirds, such as this crested auklet, have experienced record die-offs throughout Alaska, beginning in 2015. Studies show the birds are primarily dying of starvation. While not conclusive, these deaths are believed to be linked to the warming ocean trends. Credit: NPS Photo/Stacia Backensto

Not surprisingly, these changes have come at a cost to other marine wildlife. As water temperatures have increased, Gulf Watch Alaska scientists have observed changing distributions of forage fish, and large die-offs of multiple species, including sea stars, sea birds, sea otters and even baleen whales.

Results for the Long-term

Monitoring along the Seward Line has generated one of the most detailed, long-term oceanographic data sets in the Gulf of Alaska. Such consistent efforts have improved researchers’ overall understanding of natural cycles in the gulf. These, often subtle, details provide insight into the normal range of variability in the marine ecosystem. Perhaps more significantly in light of a warming climate, they help the scientific community predict the gulf ecosystem’s capacity to adapt, when pushed beyond those limits.