Cyanobacteria blooms, also known as harmful algal blooms (HABs), are a significant environmental concern in freshwater ecosystems globally. These blooms occur when microscopic, photosynthetic bacteria called cyanobacteria multiply rapidly, forming visible accumulations that can appear like pea soup or spilled paint on the water’s surface. Though often called “blue-green algae” due to their color and photosynthesis, cyanobacteria are bacteria, distinct from true algae. Understanding their excessive growth conditions is important for mitigating their impact on water quality and ecosystem health.
Essential Environmental Conditions
Several environmental factors are necessary for cyanobacteria blooms. Warm water temperatures are important, as cyanobacteria thrive and grow faster in warmer conditions than other aquatic organisms. Many bloom-forming types have optimal growth between 27.5°C and 32°C, outcompeting other species above 20°C, especially in summer.
Ample sunlight is also necessary for these photosynthetic bacteria. While most require light, some species can adapt to grow in darkness by utilizing organic carbon sources, although this is less common for bloom-forming types. Surface blooms benefit from well-lit conditions, and some cyanobacteria regulate their buoyancy to move within the water column, accessing light near the surface and nutrients from deeper, cooler layers.
Stagnant or slow-moving water bodies also provide an ideal environment. Calm conditions prevent dispersal, allowing cyanobacteria to congregate and form dense surface scums. Reduced water movement, coupled with thermal stratification where surface water forms a distinct warm layer, promotes bloom formation by keeping the cyanobacteria concentrated in the optimal upper, sunlit zones.
The Role of Nutrient Enrichment
Nutrient enrichment is a primary driver of cyanobacteria blooms, with phosphorus and nitrogen being the two most influential elements. These nutrients act like fertilizers, fueling the rapid growth of cyanobacteria in aquatic systems. The process of excessive nutrient abundance leading to rapid growth of aquatic life, including cyanobacteria, is known as eutrophication.
Phosphorus is often considered a limiting nutrient in many freshwater systems, meaning its availability often dictates the extent of cyanobacterial growth. Excess phosphorus directly stimulates rapid cyanobacteria proliferation, and blooms can occur with phosphorus levels as low as 0.03 ppm. Some cyanobacteria possess strategies to utilize organic phosphorus, often abundant in freshwater systems, by activating special genes to break down unavailable forms when inorganic phosphorus is depleted. This adaptive capability allows them to persist and dominate even under conditions that might limit other phytoplankton.
Nitrogen also contributes significantly to cyanobacteria growth, often in conjunction with phosphorus. Certain cyanobacteria species have a unique advantage as they can fix atmospheric nitrogen, converting it into a usable form even in nitrogen-limited environments. This ability allows nitrogen-fixing cyanobacteria to thrive in waters where other algal species might be limited by nitrogen availability, contributing to their competitive success.
These nutrients enter water bodies from various sources, largely due to human activities. Agricultural runoff is a major contributor, carrying fertilizers, animal manure, and eroded soil rich in nitrogen and phosphorus from fields into rivers and lakes. Wastewater discharge, including effluent from sewage treatment plants and septic systems, also introduces substantial amounts of nutrients into aquatic environments. Urban and suburban runoff, such as stormwater carrying fertilizers from lawns and gardens, pet waste, and other organic debris, contributes to nutrient loading.
Anthropogenic Influences and Global Changes
Human activities and global phenomena intensify conditions favoring cyanobacteria blooms. Land use practices directly impact nutrient loading and water characteristics. Urbanization, for instance, replaces natural landscapes with impervious surfaces like roads and buildings, preventing water absorption and increasing runoff volume and speed into water bodies. This urban runoff carries pollutants, including nutrients from fertilizers and waste, directly contributing to bloom formation. Deforestation and agricultural practices also increase soil erosion and nutrient runoff, further exacerbating the problem.
Altered water flow and management practices, such as dams and water diversions, can create stagnant conditions in rivers and reservoirs. These impoundments increase water retention time, allowing cyanobacteria more time to multiply and accumulate. Reduced flow can also concentrate existing nutrients, creating more favorable conditions for blooms.
Climate change is another overarching factor that directly and indirectly promotes cyanobacteria blooms. Rising global temperatures lead to warmer water bodies, extending periods during which water temperatures are optimal for cyanobacteria growth. Warmer temperatures also enhance thermal stratification, where stable layers of water form, providing buoyant cyanobacteria with better access to light at the surface while sequestering nutrients in deeper layers. Changes in precipitation patterns, driven by climate change, can result in more intense rainfall events that increase nutrient runoff into water bodies or prolonged droughts that concentrate nutrients in shrinking water volumes.