By : Ulises Jaime Lopez Paz
Alkalinity plays a crucial role in the health and productivity of Vannamei shrimp aquaculture systems. This article explores the significance of maintaining optimal alkalinity levels, the effects of alkalinity on shrimp growth and survival, and best practices for managing alkalinity in aquaculture environments. Understanding and managing alkalinity is essential for shrimp farmers to ensure a sustainable and profitable aquaculture operation.
I. Introduction
Aquaculture of the white shrimp or Vannamei (Litopenaeus vannamei) has reached significant global importance, accounting for a large proportion of world aquaculture production. The success of this industry depends to a large extent on maintaining optimal water quality, which directly influences the health, growth and survival of the shrimp. Of the various water quality parameters, alkalinity is a critical factor that deserves particular attention. This report aims to provide a comprehensive understanding of alkalinity in the context of Vannamei shrimp aquaculture, covering its definition, its specific role in farming systems, the recommended optimum range, the consequences of unbalanced levels and common strategies for its effective management.
II – What is alkalinity in aquaculture?
In aquaculture, alkalinity is essentially defined as the ability of water to neutralise acids. This property, also known as buffering capacity, allows the water to resist changes in pH when an acid is added. According to Hach, this buffering capacity is essential for the chemical stability of water in different systems. Hanna Colombia also emphasises this acid-neutralising function, highlighting its importance for biological stability in wastewater treatment and, by extension, in aquaculture environments.
The main alkaline components present in aquaculture water are bicarbonates (HCO₃-), carbonates (CO₃²-) and hydroxides (OH-). Aguamarket points out that, in most natural waters, bicarbonate and carbonate ions are the most important contributors to alkalinity. Total alkalinity is the measure of the total concentration of all these alkaline substances in water and is usually expressed in parts per million (ppm) or milligrams per litre (mg/L) as calcium carbonate equivalent (CaCO₃). Hach defines total alkalinity as a measure of all alkalis present, reflecting the total ability of water to neutralise acids.
It’s important to distinguish between alkalinity and pH. While the two parameters are related, pH indicates whether a solution is acidic or basic, while alkalinity expresses the amount of acid a solution can absorb without a significant change in pH. Hanna Instruments explains that alkalinity essentially represents a solution’s buffering capacity. Low alkalinity levels can cause considerable pH fluctuations in culture tanks. Molinos Champion directly points out that culture water with low alkalinity experiences significant pH fluctuations, leading to stress in shrimp. A graph provided by Hanna Colombia clearly shows that pH fluctuations are much more pronounced in water with low alkalinity than in water with optimal levels, demonstrating the crucial role of alkalinity in stabilizing pH.
Finally, it is essential to differentiate between alkalinity and hardness of water. Hardness refers to the total concentration of divalent cations, primarily calcium (Ca²⁺) and magnesium (Mg²⁺) ions, while alkalinity is a measure of water’s ability to neutralize acids, due to the presence of bicarbonates, carbonates, and hydroxides. Hach clarifies that, although both are often expressed in CaCO₃ concentration for ease of comparison, they represent different properties of water. RMAEITVO briefly mentions this relationship, stating that hardness is given by calcium and magnesium ions, while alkalinity is given by carbonate and bicarbonate ions.
Information gathered from various sources converges on a consistent definition of alkalinity as the ability to neutralize acids, highlighting its primary role in stabilizing pH in aquaculture systems. This buffering capacity is essential for mitigating the effects of natural acid and base production in culture tanks due to biological processes. Although alkalinity and hardness are distinct parameters, the common practice of expressing them in terms of CaCO₃ equivalents can potentially lead to confusion among aquaculture professionals. Therefore, understanding and emphasizing their differences is essential for ensuring good water quality management.
III. Importance of Alkalinity in Vannamei Shrimp Farming
Alkalinity plays a very important role in Vannamei shrimp farming systems, primarily by maintaining a stable pH environment, which is essential for their physiological processes. Sudden pH fluctuations, which often result from low alkalinity, can cause significant stress in shrimp, leading to reduced growth and even mortality. Molinos Champion emphasizes that low alkalinity leads to sharp fluctuations in pH, which stresses shrimp and affects their development. Hanna Colombia emphasizes that maintaining alkalinity within an ideal range of 100 to 200 ppm is crucial to ensuring a stable pH, which promotes high growth rates in aquatic animals, including shrimp.
Alkalinity also plays a crucial role in the shrimp molting process. Shrimp need carbonates to form and maintain their exoskeleton, which is primarily composed of calcium carbonate. Chireno Ecological Fish Farm states that adequate alkalinity is essential for shrimp to form and maintain their bone structures and shell, which are based on calcium carbonate. Low alkalinity levels, as indicated in the excerpt, can even stunt shrimp growth due to abnormal molting behavior. On the other hand, high alkalinity, especially when combined with a high pH, can also negatively impact molting by causing excessive salt loss. Balnova points out that high alkalinity levels can block shrimp molting.
Furthermore, alkalinity influences the availability of essential nutrients in the pond ecosystem. For example, low alkalinity (below 40 mg/L CaCO₃) can render phosphorus-based fertilizers insoluble, limiting phytoplankton production. Molinos Champion highlights this effect, stating that phosphorus availability to phytoplankton is significantly reduced when alkalinity is low. This phenomenon can, in turn, affect the natural food chain and oxygen production in the pond, as phytoplankton are primary producers of oxygen through photosynthesis.
Alkalinity also interacts with the nitrogen cycle, particularly the nitrification process. Nitrifying bacteria consume alkalinity when converting ammonia to nitrite and then to nitrate. Hach indicates that alkalinity is a key parameter for controlling the stability of nitrification. Insufficient alkalinity can hinder this process, leading to the accumulation of toxic ammonia. Hanna Colombia points out that a lack of carbonate alkalinity stops nitrification. The fragment indicates that 7.14 mg of alkalinity is destroyed as CaCO₃ for every milligram of ammonium ions oxidized, highlighting the significant demand for alkalinity in this process.
Furthermore, alkalinity plays a role in maintaining the inorganic carbon balance for autotrophic organisms such as algae and nitrifying bacteria. Balnova suggests that alkalinity values above 100 mg/L are recommended for optimal development of autotrophic communities, which are vital for maintaining water quality. Balnova also explains that alkalinity ensures sufficient inorganic carbon for all living organisms in the system, including algae and autotrophic bacteria.
The role of alkalinity goes beyond simple pH buffering; it has a direct impact on essential biological processes such as molting and nutrient cycling, highlighting its multifaceted importance in shrimp farming. Furthermore, the optimal alkalinity level can indirectly influence the toxicity of other compounds in the water, such as ammonia, whose toxicity depends on pH. Since alkalinity helps stabilize pH, it indirectly contributes to managing the risks of ammonia toxicity.
