WHEN one thinks of environmental degradation at sea, the main image that comes to mind is the 1989 Exxon Valdez oil spill in Alaska and similar marine disasters. Or the sailor who jumped ship last year to protest the United States Navy's dumping of trash in the oceans.
Bad as they are, such high-visibility occurrences are a relatively small part of what's behind a troubling decline in the health of the world's oceans. Most of that decline comes from us landlubbers who may love the beach but rarely get over the horizon. In the case of nutrients, sediments, and air pollutants, the source of the damage in fact can be far inland.
Two recent studies make clear the scope of the problem and the need for national, international, and (especially) individual action. ``Abandoned Seas: Reversing the Decline of the Oceans,'' is a new paper by Peter Weber, a researcher with the Worldwatch Institute.
The second and lengthier work, ``Global Marine Biological Diversity,'' was published last month by Island Press. Edited by renowned biologist Elliott Norse, this is a joint project of the Center for Marine Conservation, the United Nations Environment Programme, the World Bank, the World Conservation Union, and the World Wildlife Fund.
There are several root causes to the problem: over-exploitation, mainly through commercial fishing; physical alteration by dredging and filling, mining, oil and gas development, the logging of mangrove forests, and other means; pollution - chemicals, solids, nutrients from agricultural runoff; the introduction of alien species, principally through the transfer of ballast water; and global atmospheric change through ozone depletion and the buildup of greenhouse gases.
Again, much of this occurs inland. ``Of all the pollutants entering the oceans worldwide,'' Mr. Weber writes, ``33 percent come via air emissions from land-based sources, and 44 percent via rivers and streams.''
For example, 46 percent of the oil that ends up in the oceans comes from autos, heavy machinery, industry, and other land-based sources. That's 3-1/2 times as much as comes from accidental oil spills like the one that soiled Prince William Sound.
The competition for commercial fish stocks has become so great (more than doubling in the last 30 years) that the UN Food and Agriculture Organization estimates that nine of the 17 major fisheries around the world are in serious decline - and all have reached or passed their natural limits. One problem here is government subsidies for the fishing industry - $124 billion spent annually to bring in $70 billion worth of fish.
Regarding climate change, oceans usually are thought of in a passive sense - rising and flooding if polar ice melts. But oceans - healthy oceans - produce one-third to one-half of the global oxygen supply and absorb 20 times as much carbon as all the forests and other biomass.
The destruction of habitat and consequent loss of biological diversity is typically associated with rich areas like tropical rainforests. But the same is just as true - perhaps more so - for oceans. Weber points to several indicators: half the world's salt marshes and mangrove swamps cleared, drained, diked, or filled; as much as 10 percent of all coral reefs destroyed and another 60 percent threatened; 70 percent of all beaches eroding.
``Ecosystems have been eliminated, and, from the loss of populations, one can assume that there has been great loss of genetic diversity,'' states the Island Press work edited by Dr. Norse. ``As a result, we are losing both products and services from the sea, losses no less important than they are on land.''
Overpopulation and overconsumption are two key factors here. But even more important may be the lack of general knowledge about oceans and also the assumption that they are so vast as to be infinitely bountiful or able to absorb any amount of pollution or manipulation.
``It is a curious situation that the sea, from which life first arose, should now be threatened by the activities of one form of that life,'' Rachel Carson wrote in ``The Sea Around Us.'' ``But the sea, though changed in a sinister way, will continue to exist; the threat is rather to life itself.''
Ms. Carson issued that warning more than 30 years ago. It's even more relevant today.