Algal Bloom and Its Effects in Aquaculture

An algal bloom occurs in freshwater or marine ecosystem when the population of phytoplankton rapidly increases. You can easily recognize the algal bloom by observing the water color. In this case, pigments of phytoplankton produce the water discoloration.  In the aquatic ecosystem, many photosynthetic organisms such as seaweed and unicellular organisms such as cyanobacteria grow rapidly by taking excess nutrients from the water and form algal bloom.

A suitable phytoplankton population enhances the aquatic ecosystem through photosynthesis during the day and maintains low levels of gases like carbon dioxide (CO2), ammonia (NH3), nitrite (N2), hydrogen sulfide(H2S), methane(CH4) etc. Phytoplankton can take ammonia, nitrogen and heavy metals as food. Therefore, a healthy phytoplankton bloom can reduce toxicity.

A healthy algal bloom maintains proper turbidity resulting in stability in shrimp farming and reduction in cannibalism. It stabilizes water temperature by preventing winter heat dissipation. As food density increases, phytoplankton competes for food with other microbes and lower class pathogenic bacteria for nutrients. Phytoplankton play a major role in water quality management by providing a suitable color to the water.

Water color indicates the abundance of plankton species. Sometimes the color of the water changes as a result of extensive outbreak of phytoplankton. When the phytoplankton reaches the end of its reproductive cycle, the physical chemical environment of the water body suddenly becomes unfavorable for the phytoplankton.

These happen when there is a drastic change in salinity or temperature or when nutrient deficiencies are observed or when the presence of zooplankton is greatly increased. The number of plankton increases rapidly during extreme heat when the intensive aquarium has adequate nutrients.

Excessive phytoplankton mortality on hot days causes problems for the survival of prawns, shrimps and fish.  Dead plankton cells decompose rapidly as a result of high temperatures. O2 is used in this case as a result deficiency of oxygen occurs.

In the anaerobic sediment, ammonia and sulfide are produced which do detrimental effect on benthic shrimp and prawn. In this situation, the water of the fish farm should be changed quickly.

Steps to Control Algal Bloom

30-50% of the water in the traditional farming ponds should be changed. When water is brought into it from a good pond, the water condition of the pond improves. Then the nutrients are applied. In case of semi-intensive pond, dead algae should be controlled in the sediment by keeping the aerator running.

Applying algae rich nutrients and exchanging 30-50% of the bottom water will improve the color of the water. There are two types of water color, real color and external color. Color is visible in sunlight and is made up of microorganisms, soluble substances and minerals, clay particles, organic particles, pigments and degraded colloids or others.

In general, microorganisms, phytoplankton, zooplankton and bacteria produce visible colors. The reason for the change in water color is that microorganisms, especially phytoplankton, vary and decrease. The rate of nutrient uptake is related to the rate of production as water color is related to productivity. Farmers use water color to measure water condition without equipment.

Phosphorus and nitrogen are the main nutrients of plants. Plants receive nitrogen as NO3 and phosphorous as PO4 from the water as result NO3 and PO4 are removed from the water.

Nitrogenous wastes and compounds are produced from the decomposition of plant and animal substances by the bacterial activities.  Nitrogenous wastes and compounds are also produced from the excretion of aquatic animals. 

These compounds are gradually converted to ammonia by the action of anaerobic bacteria which is converted to nitrite and nitrate by nitrification. Phosphorus is an important nutrient as it plays an important role in the formation of nucleic acids and proteins. Available carbon is also important. When there is a shortage of carbon, the production of the water bodies also decreases.

The ratio of carbon: nitrogen: phosphorus requirement for most species of phytoplankton is 106: 16: 1. Ammonium nitrate, ammonium sulphate, calcium metaphosphate, calcium nitrate, ammonium phosphate, muriate of potash, potassium nitrate, potassium sulphate, sodium nitrate, super phosphate etc. are some of the commonly used inorganic fertilizers.

Sodium, Calcium, Magnesium, Phosphorus, Sulfur, Potassium, Ammonia, Cirrhoate etc. are considered as inorganic macronutrients. The micronutrients are aluminum, boron, cobalt, copper, iodine, iron, manganese, molybdenum, selenium and zinc.

In addition some organic secondary nutrients such as thiamine, nicotinic acid, calcium pantothenate, methionine, P-aminobenzoic acid, biotin, I-insitol, folic acid, cyanocobalamin, lysine, colin, ascorbic acid,  vitamin A, D and K etc. are added with fertilizer for better initial production.

Aquatic ecosystems have higher levels of nutrients if they have high levels of organic matter. Excessive growth of phytoplankton is observed from such nutrients. As a result, more oxygen is produced in the afternoon. When water is over-saturated with oxygen, shrimp become infected by gas bubbles. They float on the surface of the water with bubbling gills. Phytoplankton cannot produce their oxygen in the evening or on a cloudy day. Moreover, if there is a large bloom of phytoplankton in a pond, the pH value during the day and the level of dissolved O2 will decrease drastically.

High pH, DO and ammonia are found in ponds that have high levels of phytoplankton in the afternoon. Due to the high pH quality, if the amount of non-ionized ammonia increases in this condition, it becomes risky for shrimp/prawn and fish. Low levels of phytoplankton keep pH and dissolved oxygen levels more stable. However, this situation is not good for fish farming.

Feed supply and turbidity control the growth of formula-shaped algae at the bottom of the pond. The idea is that the middle condition of plankton is better than that of more plankton. The density of phytoplankton is usually measured by Sekkidisk readings. Phytoplankton species and their density can be monitored through salinity control. Low salinity helps in the growth of green algae. The diatom community increases with increasing of salinity.

Outbreak of Algal Bloom

Massive outbreaks of Plankton occur in the following four stages:

First, the color density of the water is significantly the same throughout the water column. If only a few plankton species exist in a community and they continue to reproduce rapidly.

Second, a bunch of pigments can be seen on the surface of the water. At this stage some plankton still does not explode.

Thirdly, a cloudy layer like the milk of a water column can be seen. The result is glue and scams are created. In this condition, when the paddle wheel is operated, foam accumulates on the water. This condition is caused when the cell wall of the phytoplankton ruptures. The cellular material and pigment are removed and the phytoplankton loses its color.

In the fourth stage, the water becomes clear and the clarity increases rapidly. Dead phytoplankton no longer remain underwater as sediment. During the massive molting of shrimp, the organic sediment is rapidly depleted as the shrimp take it as food. Nutrients are lost during defecation due to unused food or being opaque or taking extra food.