Why Is Algae So Bad in Florida Lakes During Summer?
If you've spent any time around a Florida lake in August, you've probably seen it: thick green or blue-green scum floating on the surface, sometimes smelling musty or even like rotten eggs. It's not your imagination — algae blooms really do get worse in summer, and they're showing up in more lakes and ponds across the state.
But here's the thing: while some algae is natural and even healthy for a lake, the explosive blooms we see in summer are usually a sign that something's out of balance. And more often than not, the root cause traces back to what's washing into the water.
Let's break down why summer turns Florida lakes green — and what can actually be done about it.
Why Summer Makes It Worse
Algae need three things to thrive: sunlight, warm water, and nutrients (mainly nitrogen and phosphorus). Florida summers deliver all three in spades.
Longer, stronger sunlight — More daylight hours and higher UV intensity give algae more energy to grow.
Warm water temperatures — Lake temperatures in the 80s and 90s are prime conditions for rapid algae reproduction. Cooler winter water naturally keeps blooms in check.
Afternoon thunderstorms — Summer storms wash fertilizer, pet waste, grass clippings, and other nutrient-rich runoff from lawns, farms, and roads into lakes and ponds. That's like dumping a energy drink into the water — algae feast on it.
The result: what starts as a faint green tint in June can turn into a thick, surface-covering bloom by August.
Not All Algae Is Bad
It's worth noting that algae are a natural part of a healthy lake ecosystem. Microscopic algae (phytoplankton) form the base of the food web, feeding tiny organisms that fish and other wildlife depend on. A little bit of algae means the lake is alive and functioning.
The problem is when algae grow out of control — especially when it's cyanobacteria (often called blue-green algae). Some cyanobacteria produce toxins that can harm pets, wildlife, and people. Even nontoxic blooms can kill fish by consuming oxygen at night, cause taste and odor problems in drinking water, and make lakes unpleasant for swimming, fishing, and boating.
The Fertilizer Connection
This is the big one. Florida's sandy soils don't hold onto nutrients well. When you fertilize your lawn — especially with nitrogen or phosphorus — and a heavy rain hits within a day or two, a significant portion of that fertilizer can wash straight into the nearest lake or stormwater pond.
That's why many Florida counties have summer fertilizer restrictions or blackout periods (typically June 1 through September 30). During these months, you're not supposed to apply fertilizers containing nitrogen or phosphorus to your lawn. The idea is simple: if the grass can't use it before the next storm, the lake gets it instead.
Even if your county doesn't have a formal ban, skipping summer fertilization is one of the most effective things you can do for local water quality.
What Can Be Done?
For homeowners and residents:
Skip summer fertilizer — Hold off on nitrogen and phosphorus fertilizers until fall. Your lawn will be fine, and the lake will thank you.
Keep grass clippings out of the street and drains — Blow or sweep clippings back onto the lawn, not into the storm drain. They're full of nutrients.
Pick up pet waste — Dog poop is a surprisingly big source of phosphorus and bacteria in urban lakes.
Create a buffer zone — Leave a strip of native plants or unfertilized grass along the lake edge. That buffer absorbs runoff before it reaches the water.
Wash your car on the grass, not the driveway — Soap and dirt runoff ends up in the same storm drains that feed the lake.
For lake and pond managers:
Aeration systems — Circulating oxygen through the water helps beneficial bacteria break down excess nutrients and discourages cyanobacteria blooms.
Beneficial aquatic plants — Plants like water hyssop, herb-of-grace, and even water lettuce (where managed) soak up excess nutrients that would otherwise feed algae.
Alum or phosphorus-binding treatments — These can lock up phosphorus in the sediment, making it unavailable to algae. But they're a treatment, not a cure — without reducing incoming nutrients, the problem comes back.
Dredging — In lakes with decades of accumulated nutrient-rich sediment, removing that muck can provide long-term relief.
The Bottom Line
Algae blooms in Florida lakes during summer are partly natural — the combination of heat and sunlight makes some growth inevitable. But the explosive, lake-smothering blooms that close beaches and kill fish are almost always fueled by excess nutrients washing in from the surrounding land.
The good news is that the solution starts in our own yards. By keeping fertilizer, grass clippings, and pet waste out of the runoff, we can give our lakes a fighting chance to stay clear through the dog days of summer.