How Temperature Fuels Coastal Blue-Green Algae Proliferation

Global sea surface temperatures have risen by approximately 0.7°C over the past three decades, with projections indicating an additional 4°C of warming this century. For coastal ecosystems, this thermal shift is not merely a number on a thermometer—it is a biological game-changer. 

Among the most conspicuous responses is the accelerated proliferation of blue-green algae (cyanobacteria) and, in many regions, green macroalgae. This article examines the mechanisms through which rising temperatures drive these blooms and the cascading consequences for coastal environments.

The Cyanobacterial Advantage

Cyanobacteria possess a suite of traits that make them exceptional beneficiaries of warming conditions. Many bloom-forming cyanobacteria and green algae have temperature optima above 25°C, frequently exceeding those of competing groups such as diatoms and dinoflagellates. 

As waters warm, these organisms simply grow faster. Studies from the Baltic Sea confirm that cyanobacteria abundance increases predictably with rising temperature, independent of seasonal succession, while diatom abundance declines. Similar patterns emerge in the Yellow Sea, where high-temperature, low-nutrient conditions favor cyanobacterial dominance.

Beyond simple growth rate advantages, warming enhances cyanobacterial buoyancy regulation—a critical competitive edge. Many species possess gas vesicles that allow them to rise to the surface, where they access abundant sunlight and effectively shade competing organisms. Rising temperatures accelerate these buoyancy changes, modifying vertical migration patterns and directly promoting bloom formation. Thermal stratification of the water column, intensified by warming, further aids this surface accumulation.

Green Tides: The Macroalgal Response

While cyanobacteria dominate the microscopic realm, green macroalgae such as Ulva and Cladophora are also expanding under warming. Field studies in the Baltic Sea show that the green alga Cladophora increases strongly with higher water temperatures, while the red alga Ceramium declines. 

Along China's coasts, green tides driven by Ulva prolifera have intensified, with ocean warming enhancing the competitive advantage of green tide species over other bloom-formers. These macroalgal blooms—often massive enough to be visible from space—fundamentally alter coastal habitats, smothering seagrass beds and altering nutrient cycling.