Trying to save Lake Okeechobee, south Florida's water supply. Lake's biology not well understood; a serious problem is phosphorus

The strange subtropical lake that feeds the Everglades, and the source of much of urban south Florida's drinking water, became more strange this summer. For well over a decade, interest groups have argued over the health of Lake Okeechobee, often dubbed the ``liquid heart of Florida.'' But when a red algae bloom spread over roughly 100 square miles of the more than 700-square-mile lake last month, Floridians snapped to attention.

There still is not a consensus on the plan the state is drafting to save the lake. But even some of the dairymen who have been blamed for the problem are now beginning to agree that action is warranted.

The concern is that the vast, shallow lake is dying -- following the pattern of another Florida lake, Apopka, that suffocated in the 1950s. Okeechobee is far more important, however, than Lake Apopka.

The largest tropical lake in the Western hemisphere (even though it is actually north of the Tropic of Cancer), Okeechobee supports major fisheries and feeds the slow, shallow, sometimes 70-mile-wide ``river of grass'' called the Everglades. Through the bottom of the Everglades, water seeps into the aquifer that quenches metropolitan Miami, Fort Lauderdale, and Palm Beach County.

In droughts, such as one in 1981, the lake is the only reservoir for these cities.

The most serious problem in the lake is an overabundance of phosphorus, an organic nutrient. It runs off into the lake from dairy farms, from other agriculture, and it builds up from dying weeds and fish. The bottom muck is thick with it.

With too many nutrients, the water loses the oxygen it needs to support fish. The algae takes over.

``We're on the brink,'' says Thomas Crisman, professor of environmental engineering at the University of Florida. ``We're always on the brink with a lake like this.''

The state is putting together a plan to cut the nutrient levels in the lake by 40 percent. Gov. Bob Graham has asked for a complete plan design and budget requests from state agencies and the South Florida Water Management District by Nov. 1.

The district completed a draft plan last weekend and is circulating it to environmental groups, state officials, and agricultural interests for responses this week. The tools under consideration include diverting some of the most nutrient-laden streams away from the lake, deep-well injection of pollutants, mechanical weeding of aquatic plants, changing farm practices to cut runoff, and encouraging fishermen to harvest non-game fish.

A major effort already underway is the de-channelizing of the Kissimmee River. The once-meandering river is the major feeding stream into Okeechobee, but for flood control it has been straightened into cement-lined channels. This more efficient flow does not allow the settling out of nutrients that the old languid oxbows allowed, so the river is being returned to its old channels.

Dr. Crisman stresses that the biology of Okeechobee is little understood. The August algae bloom and the poor fishing lately may be a belated result of the low water levels during the drought of the early 1980s.

Whatever action the state takes now, likewise, is apt to have a long lag time before the effect is felt.

So much phosphorus is trapped in the bottom muck already, he says, that controlling the flow of new nutrients is ``just going to be a drop in the bucket.''

One concern is that a serious storm that churned the bottom could release the phosphorus and kill the lake. A hurricane did in Lake Apopka that way.

If the lake is choked, the Everglades are likely to filter out the water before it travels far, says Crisman. But in a drought situation, the water quality in the lake itself is critical.