The Impact of Eight Common Algae in Aquaculture and Their Prevention and Control Measures

Common freshwater algae:

Cyanophyta, Euglenophyta, Chrysophyta, Pyrrophyta, Cryptophyta, Bacillariophyta, Chlorophyta, Xanthophyta.

1. Cyanobacteria

Microcystis (produces greater toxins after death, inhibits the growth of other algae)

Spirulina (difficult to digest)

Oscillatoria (difficult to digest)

Merismopedia

Anabaena

Anabaena

Microcystis bloom

(1) Occurrence of cyanobacteria

Temperature factor

Cyanobacteria reproduction is sensitive to temperature. When the water temperature is below 17 °C, they will not break out in large numbers, or will not pose a hazard to fish. When the water temperature rises to 28 °C, because the growth of other algae is inhibited and they are consumed and digested in large quantities by fish (at high temperatures, fish feeding metabolism increases), cyanobacteria can easily form a dominant population and break out massively.

pH value:

Algae prefer slightly alkaline water bodies. High pH (pH 8.0 to pH 9.5) will promote the occurrence of cyanobacteria, so the method of improving water quality by spraying limewater alone should be avoided.

Nitrogen-to-phosphorus ratio:

Cyanobacteria can not only utilize nitrogen in water bodies, but also have a higher ability to utilize phosphorus. A low nitrogen-to-phosphorus ratio or eutrophic water bodies with relatively high phosphorus content can both lead to massive outbreaks of cyanobacteria. Appropriately increasing the nitrogen-to-phosphorus ratio can inhibit the growth of cyanobacteria to a certain extent.

Ecological relationships:

Cyanobacteria, together with other algae, constitute the producers of the pond ecosystem and provide more than 89 % of the dissolved oxygen. Therefore, in addition to participating in the material cycle of the ecosystem, these producers also affect the survival of fish.

Hypotheses on the causes of blooms:

The key factors differ at different stages:

The formation of cyanobacterial blooms can be divided into four stages: dormancy, recovery, biomass increase, and floating up. After floating up, blooms form, then water turnover occurs.

(2) Harm of cyanobacteria

Because cyanobacteria can change turgor pressure, they gather on the surface of water bodies in hot weather with strong sunlight, absorb most of the sunlight, and while reproducing massively themselves, inhibit the growth of other algae. The massive reproduction of cyanobacteria continuously secretes toxic metabolites into the water body, affecting the population succession and reproductive cycles of plankton, and can also cause the mass death of some zooplankton.

When cyanobacteria break out massively, cyanobacterial particles are also very difficult for fish to digest, and cyanobacteria gradually become the absolute dominant population. This over-reproduction formed through interspecific competition will inevitably bring intraspecific struggle, and the result of intraspecific struggle will in turn lead to the death of large numbers of cyanobacteria.

The mass death of cyanobacteria sharply reduces the producers in the water body, causing a severe shortage of dissolved oxygen supply in the water. At the same time, the decomposition of dead cyanobacteria also consumes large amounts of dissolved oxygen and releases large amounts of toxic substances such as hydroxylamine and hydrogen sulfide. Under conditions of severe hypoxia and the presence of toxic substances, fish will die in large numbers, or even all die.

(3) Methods for controlling cyanobacteria:

Thoroughly clear and disinfect the pond and add fresh water free of cyanobacteria. Because cyanobacteria have stronger competitiveness than other algae, control measures should focus on prevention, with prevention more important than treatment. Thorough pond clearing and disinfection can effectively kill cyanobacteria, lower the base population, and reduce the possibility of large-scale outbreaks. Avoiding the introduction of cyanobacteria with added water is also of positive significance for control.

Regularly change to fresh water: For ponds containing relatively many cyanobacteria, frequently and massively changing fresh water can dilute the concentration of cyanobacteria. At the same time, it also dilutes the concentration of toxins secreted by cyanobacteria, promotes the growth of other algae, and maintains the dynamic balance of the entire ecosystem.

When cyanobacteria break out massively, algicidal drugs can be selected for localized algae killing, and three days later fishery fertilizer can be applied to recultivate algae. Practice has proved that this has a good effect on the prevention and control of cyanobacteria and green algae.

Stock a certain number of filter-feeding fish:

Although cyanobacteria are not easy to digest, because their particles are relatively large, they are more easily ingested by filter-feeding fish, which to a certain extent delays and hinders the growth of cyanobacteria. Optional fish include silver carp, bighead carp, and white crucian carp. Practice shows that when a certain number of filter-feeding fish are stocked, cyanobacteria blooms basically will not occur.

2. Euglena

(1) Occurrence of euglena

Water environment that produces euglena blooms: Euglena likes to grow in small water bodies rich in organic matter, still, and without flowing water, and when nutrients are sufficient, it reproduces massively to form blooms, becoming the dominant species in the water body and rapidly occupying the entire water surface.

Suitable temperature and season for bloom formation: Euglena has a relatively wide suitable temperature range for survival, and the suitable temperature for bloom formation is 20-35, spanning the three seasons of spring, summer, and autumn, with growth most vigorous especially in June, July, August, and September.

(2) Harm of euglena

Euglena is difficult to digest, has low utilization rate, and fish do not like to feed on it; when euglena dies in large numbers, it easily spoils water quality, and the substances it secretes may also have toxic effects on fish.

(3) Prevention and control of euglena

Change a large amount of water to form slight flowing water: Because euglena likes to live in small water bodies rich in organic matter, still, and without flowing water, changing a large amount of water can change this environment, making it difficult for euglena to reproduce in flowing water or new water. Algicidal drugs can be used for localized algae killing, and three days later fishery fertilizer can be used to fertilize the water, with good results.

3. Chrysophyta

Prymnesium parvum

(1) Occurrence of Prymnesium parvum: It mainly occurs in ponds with relatively high salinity and alkalinity in coastal saline-alkali areas. The water quality of affected ponds is lean and clear, there are no other algae in the water, and the water color is light yellow.

(2) Symptoms of Prymnesium parvum poisoning:

The pond surface is covered with fish, most of which stay at the four corners and shallow pond edges, with heads facing the shore, arranged neatly, motionless under the water surface, not gasping at the surface, and showing no reaction when disturbed. Randomly take 4 silver carp and 4 crucian carp each that are near death, and observe that the fin bases on the body surface are congested, and the color of the rear part of the fish body becomes lighter; open the gill covers, and there is a large amount of mucus in the gills, with slight rot of the gill filaments; dissect the fish body, and the intestine is empty with no obvious lesions.

(3) Prevention and control of Prymnesium parvum:

Immediately inject relatively fertile river water or fertile water from nearby ponds into the pond to temporarily relieve the poisoning symptoms. Broadcast ammonium sulfate over the whole pond at 20 g/m3 to make the ammonia ions in the water reach 0.06-0.10 g/m3, so as to inhibit or kill Prymnesium parvum.

4. Pyrrophyta

Gymnodinium

Ceratium hirundinella

Peridinium

Occurrence of Pyrrophyta

Pyrrophyta likes to grow in water bodies with much organic matter, high hardness, and slight alkalinity. When they reproduce massively, the water turns red-brown, commonly known as "red water."

Harm of Pyrrophyta

Pyrrophyta is a general term for a large group of flagellated algae, with oval bodies. When they reproduce massively, they can often change the color of the water body in which they live. Some Pyrrophyta can serve as feed for certain aquatic larvae, but some groups among them can produce multiple toxins during reproduction, causing numbness of fish nerves, metabolic disorders, and respiratory disorders, ultimately leading to death.

Oodinium can also attach to fish and live parasitically, absorbing nutrients from the fish body and stimulating the fish body to proliferate mucus, making the fish body look as if coated with a layer of rice flour, hence called "powder disease." The prevention and control of Pyrrophyta are the same as for cyanobacteria.

5. Cryptophyta

(1) Advantages of Cryptophyta

Cryptophyta is one of the most important flagellated algae in natural waters, and ponds with abundant Cryptophyta have very high productivity. Cryptophyta has a relatively high protein content (50%-70% ), and some species, such as Cryptomonas ovata, also contain relatively abundant unsaturated fatty acids, especially GLA (gamma-linolenic acid) and EPA (eicosapentaenoic acid) with high nutritional value, which account for 20%, 16%, and 4% of total fatty acid content respectively. Cryptophyta has high nutritional value, and in addition its cell wall without cellulose is extremely easy to digest and absorb, so it is one of the high-quality feeds for fish and certain precious aquaculture animals.

(2) Disadvantages of Cryptophyta

Cryptophyta has a relatively short growth cycle and is prone to death, with a gray-brown water color.

6. Bacillariophyta

Cyclotella, Navicula, Melosira, Pinnularia

Advantages of Bacillariophyta

Small individuals and good palatability. Movement speed and distribution are consistent with the activity habits of larvae, making feeding easier, that is, there are more opportunities for ingestion. Nutrient-rich and easy to digest and absorb. It and its metabolites are non-toxic and do not affect the normal growth of larvae and fry.

It is beneficial to the reproduction of excellent planktonic unicellular algae. It ensures sufficient nutrition for fish and shrimp, ensures that fish and shrimp fry eat enough and grow quickly, enhances disease resistance, and improves the survival rate. It is efficient, leaves no residue, protects the aquaculture environment, stabilizes water quality, and does not pollute the pond. It reduces the ammonia nitrogen content in water, with high dissolved oxygen and good water color.

7. Chlorophyta

Pediastrum

Closterium

Scenedesmus

Chlamydomonas

Eudorina

Coelastrum

Pandorina

Hydrodictyon (harmful to fish fry)

Spirogyra (filamentous algae in lean and clear water bodies)

Micractinium

(1) Application of Chlorophyta

Because green algae have the characteristics of being easy to culture, having thin cell walls, and being nutrient-rich, they are currently mainly used as feed in the aquaculture industry.

(2) Disadvantages of Chlorophyta

In our pond aquaculture, what grows in large amounts is mostly Chlorella. Because of its small size and rich nutrition, it is an excellent feed for rotifers.

8. Xanthophyta

Goniochloris

It appears in fertile fish ponds and other water bodies in warm seasons. When it reproduces massively, it forms cloud-like blooms, and the water color is yellow-green. It is a good feed for silver carp and bighead carp.

Winter primary productivity

In temperate or eutrophic lakes, in winter under low light, short daylight, and low temperature, phytoplankton productivity and biomass are generally relatively low. When the water surface freezes, if the ice layer is not thick and there is no snow cover, the illumination under the ice is usually far higher than the compensation point of algae, and photosynthesis can still proceed to varying degrees. When the ice layer consists of thick dark ice or snow covers the ice for a long time, net production turns negative. Because there is no turbulence under the ice, algae easily sink, and biomass drops to the lowest point. Under-ice phytoplankton mainly consists of flagellated algae such as Cryptophyta, Pyrrophyta, and Chrysophyta.

Spring

First is the massive growth of diatom populations. The diatom peak generally does not exceed 3 months. After that, due to silicate depletion (＜0.5 mg/L) or other reasons (animal filter feeding, fungal parasitism, etc.), the population begins to decline and is replaced by green algae or certain Pyrrophyta. During this period, if the nutrients in the production layer can be replenished in time, productivity remains very high.

Summer and autumn seasons

With the development of green algae, the nitrogen content in the water drops to an extremely low point, so after midsummer, nitrogen-fixing cyanobacteria (Anabaena, Aphanizomenon, etc.) replace green algae and increase sharply. The reason why cyanobacteria are dominant at this time is also related to high temperature (above 25 °C), strong light, high pH, and being less fed upon. During the cyanobacterial bloom period, biomass is very high, but productivity usually declines.

After autumn, due to weakening light and falling temperature and other reasons, cyanobacterial populations suddenly decline. After that, with the vertical mixing of the water layer in autumn, environmental conditions become similar to spring again, thus producing a second diatom peak.

Article source: Aquatic Products Portal

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