How climate change is making harmful algal blooms worse

Harmful algal blooms (HABs) are increasing across lakes and reservoirs worldwide, driven not only by nutrient pollution but also by climate change. Historically, management strategies have focused on reducing external phosphorus inputs from the watershed. However, growing evidence and recent research show that managing inputs from the watershed is no longer enough. As the climate warms, internal phosphorus loading, or the release of phosphorus from the lake itself, has become an overlooked driver of HABs.

When the bottom of a lake runs out of oxygen (hypoxia), the chemistry of the sediments changes in ways that release phosphorus that was previously locked in place. This legacy phosphorus, accumulated over decades, then rises into the water column, where algae can utilize it immediately. Climate change is increasing the frequency, duration, and intensity of the conditions that drive this release.

As lakes warm, they develop stronger thermal layers, with warm water on top and cold water below. This stratification blocks mixing, preventing oxygen from reaching the bottom. As oxygen declines, phosphorus is released from the sediments in a process known as internal loading. Lakes that once remained ice-covered through winter now lose their ice earlier or, like Owasco Lake, sometimes no longer freeze at all. This shifts phosphorus release from winter to summer, precisely when harmful cyanobacteria are actively growing.

Drought and slower water movement further warm and stabilize lakes, promoting stratification and additional release from the bottom. Climate change is also reducing wind speeds over many lakes, a phenomenon called atmospheric stilling, which limits mixing. With less mixing, less oxygen reaches the bottom sediments, allowing hypoxia to develop even in systems once considered well-oxygenated. When intense storms do occur, they can rapidly mix deep, phosphorus-rich water into the sunlit surface layer, delivering sudden nutrient pulses that fuel blooms.

The key point is simple: Climate change helps lakes unlock their stored phosphorus and deliver it into the exact zones and seasons where cyanobacteria thrive. Internal phosphorus can be even more problematic than watershed inputs because it is highly bioavailable and can continue fueling blooms long after external pollution has been managed.

A series of misconceptions still prevents lake managers from adequately addressing internal loading. These misunderstandings lead to insufficient monitoring, misdiagnosis of bloom causes, and sometimes ineffective management strategies.

The belief that only external load matters can cause lake managers to overlook internal loading entirely, even when it may be the dominant nutrient source. Even treating external and internal phosphorus as equivalent misses the reality that internal phosphorus is often more bioavailable, more concentrated, and better timed to support HABs. Assuming internal loading in stratified lakes only matters after turnover ignores how short-term mixing, thermocline erosion, and even cyanobacterial vertical movement can transport sediment phosphorus upward throughout summer.

The notion that shallow, well-mixed lakes cannot experience internal loading is also misleading. Anoxia can still happen during heat waves or periods of high algal productivity, triggering release even when the overlying water remains oxygenated. Similarly, assuming that low-productivity or oligotrophic lakes lack internal loading ignores the substantial legacy phosphorus stored in their sediments. Deep, low-productivity lakes can also experience prolonged hypoxia capable of releasing phosphorus. Believing climate change directly causes HABs misses the necessary intermediate step: Climate change enhances internal loading, which then fuels blooms. And assuming internal loading is irrelevant or nonexistent disregards how sediment-derived phosphorus can favor cyanobacteria adapted to various nutrient conditions.

The Finger Lakes represent a critical case study for understanding internal loading under climate change. These deep, glacially carved lakes have accumulated substantial legacy phosphorus in their sediments, yet management and research efforts remain focused entirely on watershed-based controls. While important, this misses potentially more than half the problem. Comprehensive research is urgently needed to quantify internal phosphorus release across the Finger Lakes, examining how warming temperatures, altered stratification patterns, changing ice cover and meteorological changes are mobilizing unmeasured nutrients and potentially driving HABs.

Such work could include continuous water-column profiling, and climate-informed modeling to forecast current and future loading scenarios. Only by understanding the magnitude and mechanisms of internal loading can lake managers develop effective, climate-adapted strategies to protect water quality, public health, and local economies.

In an era of warmer, more stable, and more extreme lake conditions, ignoring internal loading means solving only a fraction of the problem. The time to act is now, before legacy phosphorus becomes an even more powerful driver of lake degradation across the Finger Lakes and all of New York state.

Ally Berry is a member of the board of directors of the Owasco Watershed Lake Association.