Lake Okeechobee Temperature

I’ve written a couple of pieces on what caused the extreme amount of algae at Lake Okeechobee this year. Every year some algae forms in the lake (it’s in South Florida after all) but this year the amount was extreme. The first piece dealt with the history of Lake Okeechobee and how it took its current shape. The next piece dealt why the extreme amount of water in the lake this year (it’s about 3 feet above last year at this writing). This piece will deal with the temperature of Lake Okeechobee and why it’s so warm. I want to write another on the fertilizer load this year and a final one on options going forward.

Always remember that warm water rises.

Algae primarily grows on the surface of warm water. Since the outflows of Lake O primarily flow through gates it comes from surface water. Therefore the warmest, “algae-est” water is released. This lowers both the average temperature and the algae load in the lake.

This is primarily how the South Florida Water Management District (SFWMD) controls the temperature of the lake. Each summer when they start lowering the lake level to prepare for hurricane season they release the warmest water. In the attached graph you can see how that worked in past years. The red line is the average lake temperature of the past 10 years. Note how it increases until they start releasing warm water in June and then decreases. But this year (blue line) is different. The water temperature actually stayed cooler than average for the first half of the year. But the water releases slowed in June due to the algae and the temperatures continued to rise through June. Now the temperature is much warmer than it should be.

Lake Okeechobee Temperatures

Next question: This has been a very hot year. Why did the water temperature seem to stay below average for so long? That comes from 2 factors. The first is the heavy rainfall for this year. Normal rainfall for the lake is about 18 inches through the first 6 months of the year. This year we’re at about 29 inches. And rain water is cool as it enters the lake. That was the first factor that kept the lake cool.

Or maybe it just looked cooler to the temperature probes. The lake probes (sondes) are hanging from buoys, normally 1 meter above the bottom of the lake. As cool rain water enters it goes to the bottom. The sun heats the top. In this environment the difference between the top layer and bottom widens. As long as the level of the water is consistent the temperatures are comparable year to year. Since the lake is averaging about 4 meters this year vs 3 meters last they may be getting data further from the top than last year. I was unable to find exactly how the SFWMD placed their probes, but if they followed NOAA guidelines they were getting temps that were deeper than last year.

I think SFWMD didn’t fully realize how hot the lake surface was because of the dependence on sonde data. When the algae started forming they were behind the curve in releasing water. The downstream folks objected to pea soup coming their way and there was no Plan B.

I want to write one more piece on the current status of the lake. I’m looking at the fertilizer load but good data on the levels of phosphates is tricky. I’ll try to work through that. But if I don’t find the data I may skip that one. Between the high water levels and very warm water I now understand how the water levels and water temperatures got out of hand due to the combination of heavy rains, high surface temperatures reliance on sonde data. Those 2 factors are sufficient to explain the bad state of the lake without referencing fertilizer loads. But they exacerbated by high fertilizer loads.