The Great Lakes, long one of the largest freshwater sources in the United States, are steadily becoming saltier.
Feeding the rising sodium chloride levels are industrial waste, winter road salt, and salt used to soften city and home water supplies.
A first sign of this increased salinity is showing up in shifts in the kind of algae that are the core of the lakes' food chain system.
Already growing rapidly in the Great Lakes, according to University of Rhode Island botanist Dr. Robert Sheath, are red marine algae - or seaweed of the kind that thrives in waters off the Atlantic coast. They were first noticed in the mid-'60s after the St. Lawrence Seaway opened. Dr. Sheath says the marine algae now are equal in amount with the common green variety in three lakes.
Changes in salt levels--and in the kinds of algae growing in the lakes--undoubtedly will affect the fish living in those waters. Indeed, some common Great Lakes fish, such as the alewife, smelt, and lamprey eel, were originally ocean fish.
''Freshwater fish and saltwater fish have very different requirements, and certainly native freshwater fish (such as lake trout and whitefish) would not be able to survive over the long run if chloride levels increase,'' says Lee Botts, former chairman of the Great Basin Commission and now a research associate at Northwestern University's Center for Urban Affairs and Policy Research.
But for the moment, few researchers are concentrating on what might happen to the fish population. Their first concern is monitoring levels of salt in the lakes, the rate of saline increase, and the effect on algae.
The highest levels of sodium chloride are found where human population and industrial activity are heaviest, particularly closer to shore. Lakes Erie and Ontario have the highest salinity readings, while Lake Superior is lowest with only l or 2 milligrams of salt per liter of water. Even in areas where the highest concentrations have been found, such as the mouth of the Saginaw River in Lake Michigan, the salinity level is only about one-tenth that of ordinary sea water.
''The levels are low enough now--it's the rate of increase that's the concern ,'' says Lee Botts. ''The question is how high are the levels getting and how fast. The rate at which the change is occurring is alarming and the rate of change in the species of algae is an early indicator of that.''
While there have been some efforts to curb the Great Lakes' salt inflow--some industries, for instance, have reinjected leftover salt brine into existing rock salt formations--many experts say the lakes may be in imminent danger before any corrective action is ordered. Then it is often too late, they say.
Natural dilution with fresh water is considered the only way to reverse or reduce salinity levels. But the process is slow, and scientists do not agree on whether or not rising salinity levels in some areas will overtake any natural dilution.
Eugene Stoermer, a research scientist with the University of Michigan's Great Lakes research division, says that the high salinity levels of Lakes Erie and Ontario are probably peaking and will stay about where they are now. By contrast , in the upper Great Lakes sodium chloride is being added at a faster rate than it is being discharged, he says.
''It's a question of whether or not you're adding in at a greater rate than a lake is turning over,'' explains Rhode Island's Dr. Sheath, who says he doubts there will be a reversal of the present trend and that the Great Lakes yet could become ''the Great Salt Lakes.''