Understanding Normal Standards for Blue-Green Algae in River Water

Blue-green algae (cyanobacteria) are natural components of aquatic ecosystems, but excessive growth can indicate pollution and pose risks to water quality, ecosystems, and human health. Here’s a concise overview of their normal standards and monitoring practices:

1. Concentration Thresholds

Non-bloom conditions:

Algal density: Below 2×10? cells/L (Grade I, "no bloom")4.

Chlorophyll-a: Under 10 μg/L, indicating low algal biomass45.

Early warning levels:

Grade II (mild bloom): 2×10?–1×10? cells/L or chlorophyll-a of 10–15 μg/L4.

Grade III (moderate bloom): 1×10?–5×10? cells/L or chlorophyll-a >15 μg/L, signaling nutrient enrichment46.

2. Supporting Water Quality Indicators

Nutrient limits:

Total phosphorus (TP): Ideally <0.02 mg/L; higher levels (>0.1 mg/L) promote blooms56.

Nitrogen-to-phosphorus ratio (N:P): Imbalances (e.g., N:P >30) often make phosphorus a limiting factor for algal growth5.

Other parameters:

pH: >9.0 during blooms due to photosynthetic activity410.

Dissolved oxygen (DO): Fluctuates widely (>10 mg/L during blooms, <5 mg/L during decay)4.

3. Integrated Assessment Methods

Trophic State Index (TSI):

Values <40 indicate oligotrophic (low-nutrient) conditions; >70 confirm eutrophy (excessive nutrients)13.

For example, a TSI of 72.6 in Yinchuan’s canals indicated severe eutrophication1.

Biodiversity indices:

Low Shannon-Weaver index (<1.0) and Margalef index (<2.0) correlate with algal dominance and pollution16.

4. Monitoring and Management

Routine surveillance: Track chlorophyll-a, TP/TN, pH, and algal composition weekly10.

Early response: Trigger "encrypted monitoring" during temperature spikes (≥25°C) or nutrient surges45.

Control strategies: Reduce nutrient inputs (e.g., wastewater treatment) and use biomanipulation (e.g., filter-feeding fish)8.

Normal blue-green algae levels in rivers are defined by densities <2×10? cells/L and chlorophyll-a <10 μg/L, supported by balanced nutrients (TP <0.02 mg/L) and stable pH/DO. Exceeding these thresholds requires prompt intervention to prevent ecological and health impacts. Regular monitoring using integrated biological, chemical, and physical indices is essential for effective management.