Three decades ago, I was working alone one flat, calm June morning on Barnegat Bay, an estuarine embayment on the coast of New Jersey (site of the nation’s most recent National Estuary Program). Wanting to photograph my research, I set a camera and tripod on the cabin top, tripped the self-timer and returned to my experiment. Some miles distant, and unnoticed by me, a clammer finished his morning’s and work sped off, leaving a wake which traveled silently over the intervening waters. When it arrived, my boat rocked violently and I looked up to see the tripod and camera topple overboard and sink into the depths, self-timer still whirring.

I was on a graduate student’s budget, and seeing my year’s discretionary income plummet into the depths was unacceptable and I seized my diving mask and frantically followed it to the bottom.

Barnegat Bay is a turbid estuary and I despaired at finding my valuable tool. I dove deep, and my face and upper body thrust into a layer of remarkably cold water. The tide was sweeping in through the inlet from the springtime Atlantic. Because of its high salt content and low temperature, this layer of ocean water was much heavier than the turbid, sun-warmed Bay above and had slid in across the bottom for miles. I had dramatically discovered, and penetrated the pycnocline, that gradient region between surface and bottom waters which oceanographer Donald William Pritchard has instructed us is a major feature in estuaries the world over. This cold and saline water was also remarkably clear and there, at the limit of vision, was my camera on its tripod. With heart pounding and lungs about to burst, I grabbed it and thrust for the surface. (The lens, full of salt water was a total loss but the camera body, disassembled and soaked in fresh water, later dried out in the bright sun. After a few hours, I heard the whir, and click, as the self-timer finished its task and tripped the shutter. The camera continued working for two years until I had a real job and could replace it.)

The pycnocline, of course, has been a feature of estuaries as long as salt has been in the sea but, in historic terms, it is only in the past five decades that the role of salinity and circulation has been appreciated. This story tells how that significance developed, and the central role which our colleague Don Pritchard played in this process of scientific discovery.

Twenty years after my introduction to the pycnocline, I stood on a windy shoreline at Kealakekua Bay on Hawaii, largest island in the archipelago of that name. The volcano Mauna Loa rose nearly 14,000 feet behind. This part of the coastline is where, in 1779, Captain James Cook, seeking to resolve the thievery of a ship’s boat, was killed by native Hawaiians in what amounted to an accidental clash of cultures. Cook’s death was a great loss to science and England, because at the age of 50, he was not only establishing a humanist imprint for the British Empire, but his naturalists were gathering a store of knowledge about this marvelous planet.

Among the many collections returned to Europe were bottles of water, meticulously collected from all over the world’s oceans, sealed to prevent evaporation and labeled by collection site. These found their way to Denmark and the laboratory at Charlottenlund, outside Copenhagen. At Charlottenlund, in the 1840–50s, oceanographer Georg Forchthammer was looking for some way to usefully analyze and distinguish waters from the world’s seas. He had, among others, Captain Cook’s samples from the previous century.

With his relatively simple chemistry, he found that by titration with silver nitrate, he could accurately analyze one component of common salt — the chloride ion. He called his measure chlorinity and found that it varied widely. It was lower near coasts, and higher in the tropics than in mid-latitudes.

But the “saltiness” we taste in seawater is made up almost entirely from 13 chemical components, and in the 1880–90s, Danish chemist Martin Knudsen found that chlorinity bore a nearly exact proportion to the total dissolved solids, which cause seawater to be heavier than freshwater.

This principal became the Law of Constancy of the Constituents of Sea Salt, such that: Salinity = 0.03 + 1.085 (Chlorinity), Thus we arrive at salinity (abbreviated and expressed as S o/oo, or parts per thousand), the measure we still use today. (The story is more complicated for estuaries.) With this revelation came the gradual understanding among physicists that density or weight differences, accentuated by whether the water is cold or warm, are the driving force behind much of the observed circulation in the world’s oceans.

Knudsen published his landmark tables for conversion and interpolation of chlorinity to salinity in 1901. Even today, the World Standard of Salinity is kept and distributed as a reference material in sealed, glass ampules of seawater from the Hydrographic Laboratory in Copenhagen.

Donald William Pritchard was born in 1922, 20 years after Knudsen published the salinity tables, which would become a primary tool in the former’s profession as a physical oceanographer.

When Donald was in the sixth grade, Adolf Hitler was elected chancellor of germany, setting in motion all the pain and destruction of World War II. A little more than a decade later, Pearl Harbor was bombed, and Pritchard, who was studying chemical engineering at Caltech, was soon accepted as an Army Air Corps meteorological cadet. Pritchard said goodbye to his newlywed wife, Thelma, and was shipped overseas. In England, he struggled from a mobile weather station to predict flying conditions for Allied pilots during the air war in Europe and, was landed on the Normandy beachhead during the subsequent invasion.

As the end of this conflict came into sight, Pritchard wrote a soul-searching letter to his sister in which he looked both back and, with uncanny prescience into the future. His father, Pritchard said, had both virtue and fault. The virtue, which the son sought to emulate, was that he was universally liked and admired. The fault, which Pritchard would avoid, was that he worried about money. He wrote that he looked forward to three things in the years to come: to be a good father, to receive real acceptance in his chosen field of physical oceanography, and to follow faithfully his religious beliefs. Pritchard did all three, and did them well.

Pritchard went to work on the West Coast and in the fall of 1948, while he was at sea on a research cruise off Vancouver Island, a mailboat delivered a letter inviting him to head a new facility at Johns Hopkins University that would become the Chesapeake Bay Institute (CBI). Starting with a budget of about $90,000, Pritchard founded the organization, hired staff, built a research vessel, the Maury (named after Matthew Fontaine Maury, first hydrographer of the U.S. Navy) and began running oceanographic data cruises around the Bay.

By 1951, Pritchard and his colleagues were using electrical measurements of salinity and temperature rather than the cumbersome and costly mercury thermometers and Knudsen titrations of the past. The ability to cover large areas of the Bay and to make detailed profiles of data with depth rapidly began to reveal to scientists how the Bay worked. Over the years CBI and this database became major foundation tools upon which our understanding of estuaries is based.

In 1952, Pritchard published “Salinity Distribution and Circulation in the Chesapeake Bay Estuarine System” in the Journal of Marine Research, Vol.11. In this, and later in a broad spectrum of scientific journals, he proposed a system of classification for all of the world’s estuaries. He based this on how density — largely salt or salinity — was distributed from top to bottom and explained for the first time how partially mixed estuaries, like the Chesapeake, worked. His system is still taught to oceanographers today and in trade jargon, the Chesapeake — his Type B Estuary — is known as the classic “Pritchard estuary.” Pritchard’s contributions were of such significance that over the decades he became known as “Mr. Estuary.”

Meanwhile, in 1955, Reginald Van Trump Truitt left his post as founder and director of the Chesapeake Biological Laboratory. A young scientist, L. Eugene Cronin from the University of Delaware, replaced him as director. He would continue in this capacity for nearly two decades, and over the decades would earn the title, “Mr Chesapeake Bay “

Pritchard and Cronin, both struggling to support their respective institutions with limited resources competed fiercely, both philosophically and in actuality. Their differences in the early decades are clear in a quote from a Cronin letter dated March 6, 1969, reprinted in “Chesapeake Waters,” (2nd edition, 1997 by Davis, Merwin, Capper, Powers & Shivers, Johns Hopkins Press) about the controversy which once swirled over power plant discharges of heated water into the Bay: “You and I differ principally in philosophy, and this should not be misconstrued. You are inclined to encourage full use of the Bay unless available knowledge proves that human uses will be impaired. I am more conservative and prefer not to risk damage to the Bay until reasonably good estimates can be made of all effects."

Don Pritchard was of another opinion: “[Many statements have] led to the general public impression that the Chesapeake Bay and its tributary estuaries are in imminent danger of catastrophic damage to commercial fisheries, sport fisheries, recreation and other uses. It is my firm opinion that such is not the case. … In my opinion, the public as well as their elected representatives have been misled as to the scope of the problem, and the degree to which existing knowledge can be utilized in appraising the problem.

Both of these eminent scientists, of course, were right in many ways but their knowledge at the time was imperfect. The knowledge science has is still imperfect. It is the nature of the beast!

Pritchard, in later years, characterized this relationship drawing an analogy from the classical philosopher, Pliny. “When there are few facts.” Pritchard said, “there are many stories that fit them. As we have gathered more data and learned more about the Bay, Gene’s stories and mine have gotten closer together.”

They spent their twilight years as good friends. I watched them a couple of years ago walking slowly across a large foyer at an international conference in Rhode Island. Pritchard’s hand was on Cronin’s shoulder and they were deep in conversation; Don at the business of making Gene a physicist, Gene at the business of making Don a biologist. Cronin died Dec. 18. 1998 at the age of 81.

On April 23, 1999, I gave a talk about Cronin at the spring meeting of the Atlantic Estuarine Research Society. He had founded that Society, which is now entering its second half-century. Literally, while I was preparing to speak, Pritchard, also viewed as an AERS founder, was driving a friend to their Rotary meeting, excited about a project where Rotarians were growing Chesapeake oysters, Crassostrea virginica, to help filter and clean up the Bay’s waters. During the drive, Pritchard remarked to his passenger, “Something’s not right ...” pulled to the side of the road, applied his emergency brake with characteristic efficiency and keeled over from a massive stroke.

Though hospitalized within minutes, Pritchard died, surrounded by his loving family, almost as I was speaking about the contributions he and Cronin had made to oceanography and estuarine science.

We have lost two of our greatest mentors within four months, but their wisdom will live on as long as the marine sciences remain disciplines in the service of human knowledge.