Because of its size, price and abundance of illustrations, "The Living Chesapeake" qualifies as a "gift book" as the term is generally used. But the term also has pejorative overtones (pretty pictures, lousy text) that most emphatically do not apply in this case; not merely is "The Living Chesapeake" a lovely book; it is also an uncommonly interesting and informative one.

Both the pictures and the text are by J.R. Schubel, who is director of the State University of New York's Marine Sciences Research Center at Stony Brook but who has 14 years of previous experience with the Johns Hopkins University's Chesapeake Bay Institute. Forty double-columned pages of text are at the front of the book, followed by 65 black-and-white photographs; but this is not a case of duty coming before pleasure, for the text is every bit as good as the pictures.

Schubel is writing science for the layman, and he does it very well. In seven compact essays he traces the formation and history of the Chesapeake, describes its daily workings, depicts "four estuarine animals that use the bay in very different ways," and contemplates the bay's future. To the trained scientist, his text might seem once-over-lightly; but to the layman in search of a clearer understanding of the bay's mysteries and complexities, it offers a concise and enjoyable education.

Twenty thousand years ago the bay did not exist; sea level was some 350 feet below its present height, and the floor of what is now the bay was above ground. Then, in a process that lasted about 17,000 years, the temperature of the earth slowly warmed and the ice caps slowly melted, raising the sea level and filling in the valley that had been carved out by the Susquehanna River. And by 3,000 years ago, "The Chesapeake Bay was nearly complete; it had stopped growing. Dotted with islands, tall trees extending to its shoreline, its drainage basin covered with lush vegetation, its tributaries free of fine sediments, clear and deep. Never again would the modern Chesapeake Bay be as grand as at that moment."

The bay's fundamental nature is shaped by the unceasing struggle between the Susquehanna to the north and the Atlantic to the southeast. The fresh water flowing southward in the river collides in the bay with the salt water being shoved in by the ocean's tides, the result being "a giant mixing basin for sea water and fresh water." Schubel writes:

"The struggle between river and sea has created something that is neither river nor sea. We call this new thing an 'estuary,' meaning 'a semi-enclosed coastal body of water, freely connected to the ocean and within which sea water is mixed with and measurably diluted by fresh water from the land.' An estuary is like nothing else. Within this dynamic environment, only a relatively small number of organisms can thrive, but those that do are found in greater abundance, area for area, than anywhere else on earth. Around it, by choice, cluster people in their millions."

It is a place of equally astonishing beauty, as captured in Schubel's striking photographs, and productivity. For most forms of sea life, its constantly changing environment is inhospitable; relatively few species can survive its sedimentation, its fluctuating levels of salinity, its abrupt collisions of sea and river waters. "But what the bay lacks in variety," Schubel writes, "it more than makes up for in sheer numbers. Species that can tolerate bay conditions have often left behind in the ocean their natural predators, who are unable to survive in the estuary. Food is abundant in this giant cornucopia; for those who can thrive in the bay's harsh environment, it proves to be the promised land."

Hence the blue crab, the oyster, the striped bass -- all the notable and savory denizens of what Mencken called "the immense protein factory of Chesapeake Bay." Within the marshes of that bay, productivity is staggering: "A productivity one hundred times greater than that of the open sea; five times greater than that of the adjacent bay; three times greater than that of the most productive terrestrial grasslands. The rates of primary production in marshes are at least equivalent to those of the world's most productive grain fields; fields that are carefully tended and nurtured with the best diet technology can provide and money can buy."

As for the future of the protein factory, Schubel is concerned but not alarmed. Essentially he argues that the bay must be more scrupulously "managed," in order "to protect its biological resources and to conserve its recreational and aesthetic values for human use." He is not an environmental extremist; he recognizes that the bay serves legitimate industrial, commercial and military functions as well as natural and recreational ones, and he proposes simply that decisions be made about which functions are most important and what can be done to ensure their continuation.

For any of the many thousands whose interest in the life of Chesapeake Bay has already been piqued by William W. Warner's classic study of the blue crab, "Beautiful Swimmers," "The Living Chesapeake" will provide further illumination. For those wanting a succinct account of how and why the bay works as it does, it stands by itself. No one who reads it can fail to see the bay in a new and revealing light.