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Standard Seawater? Yes, There Is Such A Thing!

July 12, 2021

In celebration of our 150th anniversary, we are highlighting people and activities that helped build the foundation of fisheries and marine science. Most of us have never heard of standard seawater—a worldwide standard reference for ocean salinity.

Close-up photo of 4 glass vials, or ampoules, of standard water from Denmark (1937 and 1954) and Woods Hole (1941)in a styrofoam holder Four glass vials, or ampoules, of standard seawater, two from Denmark dated 1937 and 1954, and two from Woods Hole dated 1941. Credit: NOAA Fisheries/Shelley Dawicki, Woods Hole Data Library and Archives

Spencer Baird established the first federal fisheries lab in Woods Hole for a number of reasons. One of them was the salinity of the seawater, which he felt was suitable for rearing animals and a consistent source for research experiments.

In his history of the Woods Hole federal fisheries laboratory, Paul Galtsoff wrote: “Another biologically important characteristic of Woods Hole water mentioned by Baird remains unchanged. Due to the absence of large fresh-water streams, the salt concentration of Woods Hole sea water is nearly constant throughout the year and does not vary with the alternate changes of the tides...There are very few locations along the eastern coast of the United States and in the Gulf of Mexico where such a stability in the salinity of water can be found... Baird (1884) described them in the following words: ‘the water is exceptionally pure and free from sediment, and where a strong tide, rushing through the Woods Hole passage, keeps the water in a state of healthy oxygenation especially favorable for biological research of every kind and description.’"

Turns out there is more to the story. Not all ocean seawater is the same. The Atlantic Ocean is the saltiest of the five oceans. On average, salinity decreases near the equator and at both poles, but for different reasons. As Baird noted, researchers need consistent measurements of salinity to compare data. Seawater of a specific salinity—known as standard seawater—is used as the reference. Standard seawater is also used to calibrate salinometers, which measure salinity, and for chemical analyses, particulate studies, and other applications.

What is Standard Seawater?

Photo of a 1947 letter from Normal Allen at Woods Hole Oceanographic Institution to Joseph Kahl at GM Manufacturing Company in New York City replying about the basic source of standard seawater in Denmark.
A 1947 letter from Norman Allen, who later established the Archives at the Woods Hole Oceanographic Institution, regarding the source and availability of standard water in Denmark. Credit: NOAA Fisheries/Shelley Dawicki, Woods Hole Oceanographic Institution Data Library and Archives

But what is standard seawater? It’s natural open-ocean seawater, collected from the ocean surface at the edge of the Sargasso Sea in the North Atlantic where there are no land boundaries. Its boundaries are ocean currents. The Gulf Stream lies to the west, the North Atlantic Current to the north, the Canary Current to the east, and the North Atlantic Equatorial Current to the south.

Seawater from this patch of ocean is transported in containers to the Standard Seawater Service facility in England. There, it is filtered and circulated for several weeks and gradually diluted with distilled water to reach a final salinity near 35. That’s Standard Atlantic seawater. 

The World Standard

Baird had a Woods Hole standard for seawater in the 1870s. Perhaps he planted the seed for others. The idea for an international standard for seawater came in 1899, when Danish physicist Martin Knudsen recommended it at the International Council for the Exploration of the Sea (ICES) meeting in Stockholm. Interest in marine sciences was growing rapidly, along with the need for international standards to compare data collected worldwide. 

The first batch of standard seawater was produced in 1902 by the newly created Standard Seawater Service at a central laboratory under director Fridtjof Nansen of Norway. Oversight was provided by ICES and later the International Association for the Physical Sciences of the Oceans (IAPSO).

Nansen was an explorer and scientist who invented the Nansen bottle to collect water samples at depth. He decided he no longer wished to continue as director of the service in 1908. Knudsen took over once again. Preparation and distribution of standard seawater was moved to a new facility in Copenhagen, where it remained for more than 65 years.

A Woods Hole Connection

Photo of brief 1950 letter from Herbert Graham to Woods Hole Oceanographic Institution inquiring about the price of 10 bottles of standard Copenhagen seawater.
Photo of 1950 letter from Herbert Graham to Woods Hole Oceanographic Institution inquiring about the purchase of 10 bottles of standard Copenhagen seawater. Graham served the fisheries as director of the Woods Hole Laboratory from 1951 to 1970. Credit: NOAA Fisheries/Shelley Dawicki, Woods Hole Oceanographic Institution Data Library and Archives

During the two world wars collection of Atlantic seawater was interrupted. The U.S. Coast Guard and Woods Hole Oceanographic Institution briefly partnered to provide standard seawater during World War II. It was used by American researchers at universities and government laboratories, including fisheries laboratories. 

Among those requesting bottles of standard seawater after the war was Herbert Graham, then head of red tide investigations at the federal fisheries lab in Sarasota, Florida. Graham served as director of the Woods Hole Laboratory from 1951 to 1970. For a brief period, GM Manufacturing in New York City also supplied standard water from Copenhagen after the war, but all requests were soon directed back to Denmark. 

In 1974 the Standard Seawater Service was transferred from Copenhagen to the Institute of Oceanographic Sciences in Surrey, England. In 1989 the service was moved again, with IAPSO’s agreement, to a private company, Ocean Scientific International Limited in Hampshire, England, where it is located today. In 1994 standard seawater started to ship worldwide in glass bottles after 90 years of ampoules.

The IAPSO standard seawater remains the world standard for salinity. It allows scientists globally to compare data with the same single source reference standard. Researchers in more than 108 countries order and use Atlantic standard seawater in their work.  


For more information, please contact Shelley Dawicki.

Note: This story began some 25 years ago when a fisheries employee gave several standard seawater ampoules, or sealed glass vials, in a foam container to the writer, who worked at the time at Woods Hole Oceanographic Institution. I donated the glass vials to the Institution's Data Library and Archives for safekeeping. I was intrigued then by the use of a standard for seawater. When I learned of Spencer Baird's early realization of its value to marine science, I was delighted to find the glass vials still existed. With research and a trip to the archives to photograph the 80-year-old samples, this story had finally come full circle for me.