Читать книгу Fundamentals of Aquatic Veterinary Medicine - Группа авторов - Страница 46

Overabundance of nutrients like phosphorus, particularly when the water is warm and the weather is calm, can lead to excessive growth of algae. According to the World Health Organization standard, when the population of algal cells exceeds 100 000 cells/ ml, the condition is termed an algal bloom. Excessive algal growth can lead to nocturnal hypoxic conditions where fish may start to gasp on the water surface, and mass mortality events may occur. Phytoplankton blooms can also cause large diurnal fluctuations in other water quality variables (e.g. very high pH due to excessive use of free CO₂ for photosynthesis) in mid‐afternoon. Such conditions are stressful to fish. Blue‐green algae can also produce toxic substances that are lethal to some fish (see also Harmful Algal Bloom below). They can also produce compounds that impart a strong off‐flavor to fish. In addition, dead algae can accumulate at the pond bottom and create toxic gases from decomposition.

The best approach to controlling algae is to regulate nutrient inputs by moderate stocking and feeding rates. Phosphorus is one of the major factors responsible for increased levels of phytoplankton in an aquasystem. It is possible to precipitate phosphorus from pond water by applying sources of iron, aluminum, or calcium ions. These ions precipitate phosphorus as insoluble iron, aluminum or calcium phosphates. The use of alum (aluminum sulfate) 20–30 mg/l or gypsum (calcium sulfate) 100–200 mg/l to precipitate excessive concentrations of phosphorus may also be feasible. Alum is acidic and more suitable for use in waters of 500 mg/l total alkalinity and above. Gypsum is better for use in low alkalinity waters.