Algae grow in lakes when excess nutrients, warm temperatures, and calm water conditions combine to fuel rapid reproduction. Every lake contains some algae naturally, and in small amounts it forms the base of the aquatic food web. Problems start when nutrient levels spike and environmental conditions tip in favor of explosive growth, producing the dense, sometimes toxic blooms that turn lake water green, murky, or covered in scum.
Phosphorus and Nitrogen: The Two Key Nutrients
Phosphorus and nitrogen are the primary fertilizers behind algal blooms. Algae need 10 to 40 times as much nitrogen as phosphorus to grow, so the ratio between the two nutrients determines which one acts as the bottleneck. When nitrogen is relatively scarce compared to phosphorus, nitrogen limits growth. When nitrogen is abundant, phosphorus becomes the controlling factor. In most freshwater lakes, phosphorus is the nutrient in shorter supply, which is why it gets the most attention in water quality management.
What makes the combination so potent is how they interact. Research involving nutrient manipulation in 20 freshwater lakes found that adding phosphorus alone or nitrogen alone roughly doubled or tripled the amount of algae. But when both nutrients were added together, algal growth increased tenfold. This means that controlling just one nutrient while ignoring the other leaves a lake vulnerable. Effective prevention requires reducing both.
Where the Nutrients Come From
Agriculture is the dominant source of phosphorus and nitrogen loading in many major lake systems, including Lake Erie and Lake Winnipeg. Fertilizers applied to cropland don’t stay put. Rain washes them off fields and into streams, rivers, and eventually lakes. Livestock operations add to the problem through manure runoff. The more intensively farmed a watershed is, the higher the nutrient load reaching its lakes.
Urban and suburban areas contribute their own share. Lawn fertilizers, pet waste, leaking septic systems, and stormwater runoff from roads and parking lots all carry nutrients into waterways. Wastewater treatment plants discharge phosphorus and nitrogen directly, though many facilities have upgraded to reduce these outputs. Industrial sources add to the mix in certain regions, but agriculture consistently ranks as the largest contributor across most freshwater systems studied in North America.
Storms amplify the problem dramatically. During heavy rainfall events, concentrations of total phosphorus in runoff can be more than 44 times higher than during normal water flow. For certain forms of nitrogen, storm concentrations run about two times baseline levels. Surface runoff carries phosphorus bound to soil particles off the land, while subsurface drainage delivers dissolved nitrogen. As climate patterns shift toward more frequent and intense storms, these nutrient pulses into lakes are expected to increase.
Nutrients Already Trapped in the Lake
Even when you cut off external nutrient sources, a lake can keep feeding its own algae problem for years. Phosphorus that settled into bottom sediments over decades doesn’t just stay locked away. When oxygen levels near the lake bottom drop low enough, a chemical reaction releases stored phosphorus from iron-containing minerals in the sediment back into the water column. This process, called internal loading, can supply enough phosphorus to sustain blooms long after the original pollution source has been addressed. It’s one reason lake restoration efforts sometimes take much longer than expected to show results.
Water Temperature and Seasonal Timing
Warm water is a prerequisite for the worst blooms. Cyanobacteria, the blue-green algae responsible for most toxic blooms, begin growing when water temperatures reach 11 to 15°C (roughly 52 to 59°F) in spring. Growth ramps up significantly above 20°C (68°F) and peaks between 20 and 30°C (68 to 86°F). Once water cools below about 10°C (50°F) in autumn, cyanobacteria essentially go dormant, maintaining only minimal activity through winter.
This temperature dependence explains why blooms are overwhelmingly a summer and early fall phenomenon in temperate lakes. It also explains why warming trends are extending bloom seasons. Lakes that once cooled early enough to shut down growth in September may now sustain blooms into October or beyond. Warmer springs mean earlier starts. The window for bloom formation is widening on both ends.
How Still Water Feeds Blooms
Lakes that develop strong temperature layers between warm surface water and cold deep water create ideal conditions for certain algae. This layering, called thermal stratification, prevents vertical mixing. Nutrients accumulate near the surface where sunlight is abundant, and cyanobacteria, which can regulate their buoyancy to stay in the sunlit zone, gain a competitive advantage over other types of algae that sink and need mixing to stay near the surface.
Stratification also depletes oxygen in deeper water, which triggers the internal phosphorus release from sediments described above. The combination is self-reinforcing: stratification promotes blooms, blooms block light and consume oxygen as they decay, low oxygen releases more phosphorus, and more phosphorus feeds more blooms. Wind, wave action, and cooler temperatures eventually break stratification in fall, but by then the damage is often done for the season.
Invasive Species That Shift the Balance
Zebra mussels, introduced to North American lakes in the early 1990s, have reshaped algal dynamics in ways that initially seem counterintuitive. These filter-feeding mussels clear enormous volumes of water, which should in theory reduce algae. And they do consume many types of algae. But research from NOAA’s coastal science program has shown that zebra mussels actually promote the growth of Microcystis, the cyanobacterium responsible for many toxic blooms.
The mechanism works like this: zebra mussels filter out the algae species that compete with Microcystis while rejecting or even spitting out Microcystis cells. They also recycle nutrients through their waste, making phosphorus more available near the lake bottom where the mussels live. In lakes with even low to moderate nutrient levels, zebra mussel invasions cause increases in both Microcystis biomass and microcystin toxin concentrations. The mussels essentially clear the field of competitors and fertilize the one organism you least want to encourage.
When Algae Become Dangerous
Not all algal blooms are harmful. Green algae blooms can be unsightly and smell bad, but they’re generally not toxic. The real health concern comes from cyanobacteria, which under the right combination of light, nutrients, temperature, and salinity can produce toxins called cyanotoxins. The most common of these, microcystin, can cause liver damage, skin irritation, and gastrointestinal illness.
Most of these toxins are released when cyanobacterial cells die and rupture rather than being continuously secreted into the water, though some species can release toxins from intact cells. This means a bloom can become most dangerous as it’s breaking apart. The World Health Organization sets a safe threshold for microcystin in drinking water at 1 microgram per liter. During major bloom events, concentrations of 2.5 micrograms per liter and higher have been documented, well above that safety limit.
Why Blooms Are Getting Worse
Several trends are converging to make algal blooms more frequent and more severe. Rising water temperatures extend the growing season for cyanobacteria. Heavier and more frequent storms flush larger nutrient loads off the landscape. Decades of phosphorus accumulation in lake sediments create a persistent internal supply that resists cleanup. Invasive species like zebra mussels restructure lake ecosystems in ways that favor toxic species. And in many watersheds, agricultural intensity and urban development continue to increase nutrient inputs.
These factors don’t operate independently. A lake receiving heavy nutrient runoff from farmland, warming earlier each spring, hosting an established zebra mussel population, and developing strong summer stratification faces compounding pressures that make severe blooms increasingly likely. Addressing algae in lakes requires tackling multiple causes simultaneously, reducing nutrient inputs at the source, managing internal loading from sediments, and accounting for the biological and climatic shifts already underway.