Between Ammonia Nitrogen Concentration and Cyanobacterial Bloom Outbreaks

Cyanobacterial blooms (often referred to as blue-green algal blooms) are a major global threat to freshwater ecosystems, causing water quality deterioration, ecosystem imbalance, and risks to human and animal health. While influenced by multiple factors like temperature, light, and hydrodynamics, nutrient availability, particularly nitrogen (N) and phosphorus (P), is a primary driver. Among nitrogenous compounds, ammonia nitrogen (NH?-N / NH??-N) has gained significant attention due to its dual role as a preferred nitrogen source for many cyanobacteria and its potential toxicity, which can shape phytoplankton community structure.

Ammonia Nitrogen as a Key Trigger

Ammonia nitrogen exists in water as unionized ammonia (NH?, toxic) and the ammonium ion. Many prolific bloom-forming cyanobacteria (e.g., Microcystis, Anabaena) possess a competitive advantage in utilizing ammonia nitrogen. They can efficiently take up NH?? via specialized transport systems, requiring less energy than assimilating nitrate Under conditions of elevated ammonia, these species can outcompete other phytoplankton, leading to dominance.

Furthermore, some cyanobacteria can fix atmospheric nitrogen (N?), but this process is energetically costly. When bioavailable ammonia is present at sufficient concentrations, it suppresses nitrogen fixation, allowing even non-fixing species like Microcystis to thrive without competition from N-fixing genera. Thus, an elevated ammonia nitrogen concentration acts as both a direct nutrient stimulus and an ecological selector for harmful cyanobacterial taxa.

The Critical Threshold Concept

Identifying a single universal "critical threshold" for bloom initiation is complex, as it interacts with phosphorus levels, temperature, and hydrological conditions. However, research and observational data suggest concentration ranges that significantly elevate bloom risk.

Low-Level Stimulation: Concentrations as low as 0.05 - 0.1 mg/L of ammonia nitrogen can be sufficient to support cyanobacterial growth, especially when phosphate  is above ~0.01-0.02 mg/L (approaching N limitation).

Significant Risk Range: A sustained ammonia nitrogen concentration above 0.2 - 0.5 mg/L is frequently associated with a high probability of cyanobacterial bloom proliferation, given adequate phosphorus and warm, stable conditions.

Dominance & Toxicity: Levels exceeding 1.0 mg/L create a highly favorable environment for ammonia-loving cyanobacteria. At these levels, the toxic form (NH?) may also begin to inhibit other aquatic organisms (e.g., zooplankton grazers, fish), further reducing top-down control on cyanobacteria and exacerbating the bloom.

It is crucial to note that the ratio of Total Nitrogen (TN) to Total Phosphorus (TP) is a classical indicator. A low TN:TP mass ratio (often < 20:1 or even < 10:1) is thought to favor cyanobacteria. High ammonia nitrogen contributes directly to a low TN:TP ratio if phosphorus is present, signaling a system primed for a cyanobacterial response.