Main Pollutants in Aquaculture Water and Their Effects on Aquatic Animals

For aquaculture, managing pollutants in rearing ponds is a critical concern. Common pollutants in aquaculture water include nitrogenous substances and phosphorus compounds. Nitrogenous substances encompass ammonia nitrogen, nitrite nitrogen, nitrate nitrogen, dissolved organic nitrogen, among others. Phosphorus compounds include reactive phosphates and organic phosphorus. This article explores the primary pollutants in aquaculture water and their impacts on aquatic animals. Let's first look at a simplified diagram for easier memorization and understanding.

Below we will provide specific explanations.

Ammonia nitrogen is one of the main pollutants in aquaculture water, mainly produced from the decomposition of residual feed and metabolic products of aquaculture animals in the water. Accumulation of ammonia nitrogen in the system can damage the epidermal tissues and gills of fish, disrupting the biological enzyme activity system. Even low concentrations of ammonia nitrogen (>1 mg/L) can have toxic effects on aquaculture animals, especially the highly toxic non-ionized ammonia, which can cause damage at very low concentrations. Increased concentrations of ammonia nitrogen in the environment also lead to reduced nitrogenous excretion by aquatic organisms, reducing their ingestion of ammonia-containing substances, ultimately affecting the normal growth and development of aquatic animals. High concentrations of ammonia nitrogen in the environment can also affect the osmotic balance of aquatic animals, leading to reduced oxygen transfer capacity and inability to excrete toxic substances from their bodies. Most domestic and international research on the treatment of aquaculture water focuses on the treatment of ammonia nitrogen.

Nitrite in aquaculture is mainly an intermediate product generated during nitrification or denitrification processes. It can enter the body through the gills of aquaculture animals and reduce the oxygen-carrying capacity of hemoglobin in their blood, causing hypoxia and death in aquatic animals. It is important to note the accumulation of nitrite in water bodies, especially in newly operated systems, which can have significant toxic effects on aquaculture organisms.

Nitrate has relatively low toxicity to fish, hence there is no specific concentration limit, but high concentrations can affect the taste of aquaculture products. Nitrate nitrogen during denitrification processes can also produce nitrous nitrogen, which can be toxic to aquaculture organisms. Literature reports have shown that accumulation of nitrate nitrogen can lead to slow growth and diseases in aquaculture organisms. It is generally believed that during salmon aquaculture, nitrate levels in the water should be kept below 7.9 mg/L. Therefore, in the process of treating aquaculture water, various nitrogen transformations should not blindly convert to nitrate nitrogen alone, and consideration should also be given to the removal of nitrate nitrogen.

Dissolved organic nitrogen in aquaculture water mainly originates from residual feed, excreta, and metabolic products of aquaculture organisms. Dissolved organic nitrogen in aquaculture water has a relatively simple structure, good biodegradability, and can be easily utilized by microorganisms, achieving good removal efficiency through conventional biological treatment processes. When the concentration of organic nitrogen in water is not high, it has little impact on aquatic organisms. However, when organic nitrogen accumulates to a certain extent, it can promote the proliferation of pathogenic and harmful microorganisms, deteriorating the water quality and causing diseases and death in aquaculture organisms.

Active phosphates in aqueous solutions may exist in forms such as PO3- 4、HPO2- 4、H₂PO- 4和 H₃PO₄, with their relative proportions (distribution coefficients) varying with pH. They can be directly utilized by algae, bacteria, and plants. Active phosphates have minimal direct harm to fish but can promote extensive growth of algae and bacteria in water, consuming oxygen and impairing fish growth. The removal of phosphates from aquaculture water mainly relies on chemical precipitation and adsorption. Chemical precipitation involves adding chemical agents to the water to form phosphate precipitates through chemical precipitation processes, followed by flocculation and solid-liquid separation to remove phosphates from the water. Adsorption utilizes adsorbents with large surface areas and numerous pores to allow phosphorus in wastewater to undergo ion exchange, coordination complexation, electrostatic adsorption, and surface precipitation reactions, thereby removing phosphorus from the water.

Total phosphorus refers to the sum of soluble phosphorus and particulate phosphorus. Soluble phosphorus in water can be further divided into soluble organic phosphorus and soluble inorganic phosphorus, with soluble inorganic phosphorus mainly existing in the form of active phosphates. Particulate phosphorus refers to phosphorus forms present on the surface or inside suspended particles in water, which are usually difficult for aquatic animals to directly utilize. Particulate organic phosphorus mainly exists in cellular tissues and organic debris of aquatic animal tissues, while particulate inorganic phosphorus mainly adsorbs onto suspended clay minerals.

In summary, the most important task in aquaculture is to regulate the aquaculture water environment, considering various factors to create a balanced water environment, thereby minimizing losses and maximizing economic benefits. How to regulate the water environment will be analyzed in future articles.