plankton

The so-called marine plankton refers to marine organisms that lack developed moving organs and have no or only weak swimming ability, but are suspended in the water layer and often move with the current. Among them, those who can convert inorganic compounds in seawater into organic compounds needed for biological metabolism through their own photosynthesis are called phytoplankton, and those who do not have this ability, that is, those who must feed on phytoplankton, are called zooplankton. According to preliminary statistics, there are more than 180 species of plankton that can form red tides in the world's oceans, among which 63 species are published in the List of Planktons in China.

The plankton that can form red tide has an alias, which is usually called "red tide organism". Among the 63 species of plankton known as red tide organisms, there are 24 species of diatom, 32 species of dinoflagellate, 3 species of cyanobacteria, 1 species of Chrysophyte, 2 species of cryptophyta, and protozoa 1 species. In China, there are 25 species of red tide organisms recorded by red tide data. The other 38 species are distributed in China waters, but no red tide has yet formed. Therefore, red tide does not necessarily occur in the sea areas where red tide organisms are distributed, depending on whether the density can reach enough to discolor some sea areas.

Human activities

With the rapid development of modern chemical industry and agricultural production and the increase of population in coastal areas, a large number of industrial and agricultural wastewater and domestic sewage are discharged into the sea, and a considerable part of them are directly discharged into the sea without treatment, which leads to the increasingly serious eutrophication in offshore and harbor areas. At the same time, due to the increase of coastal development and the expansion of marine aquaculture, it has also brought pollution to the marine ecological environment and aquaculture itself; The development of shipping industry leads to the introduction of harmful alien red tide species; The change of global climate has also led to the frequent occurrence of red tides. The pollution of mariculture itself is also one of the factors that induce red tide.

Eutrophication of seawater

Eutrophication of seawater is the material basis and primary condition for the occurrence of red tide.

Because a large number of urban industrial wastewater and domestic sewage are discharged into the sea, nutrients are enriched in the water body, resulting in eutrophication of the sea area. At this time, nutrients such as nitrogen and phosphorus in the water body; The contents of trace elements and organic matter such as iron and manganese are greatly increased, which promotes the reproduction of red tide organisms. The results of red tide detection show that the water body in the red tide occurrence area has been seriously polluted and eutrophic. Nitrogen, phosphorus and other nutrients greatly exceeded the standard. According to research, some metals in industrial wastewater can stimulate the reproduction of red tide organisms. Adding iron chelating agent less than 3mg/dm3 and manganese chelating agent less than 2 mg/dm3 to seawater can make red tide organisms Chrysophyta and Euglena achieve the highest proliferation rate. On the contrary, in seawater without iron and manganese, the population density will not increase even under the most suitable conditions of temperature, salinity, PH and basic nutrition.

ocean temperature

Hydrometeorology and the change of physical and chemical factors of seawater are the important reasons for the occurrence of red tide.

Seawater temperature is an important environmental factor for the occurrence of red tide, and 20-30℃ is the suitable temperature range for the occurrence of red tide. Scientists have found that the sudden rise of water temperature above 2℃ in a week is a precursor of red tide. Chemical factors of seawater, such as salinity changes, are also one of the reasons for the proliferation of biological factors-red tide organisms. Red tide may occur in the salinity range of 26-37, but thermocline and halocline are easily formed when the seawater salinity is 15-2 1.6. The existence of thermocline and halocline provides conditions for the aggregation of red tide organisms, which is easy to induce red tide. Due to the interaction of runoff, upwelling, water mass or ocean current, nutrients on the seabed rise to the surface, resulting in high eutrophication in coastal waters. The content of nutrients rose sharply, which led to the proliferation of diatoms. Too many of these diatoms, especially the dense diatoms in bones, often cause red tides. These diatoms provide rich bait for noctiluca scintillans, which promotes the rapid propagation of noctiluca scintillans, thus forming a pink noctiluca scintillans red tide. Monitoring data show that when the red tide occurs, the water area is dry and rainy, the weather is sultry, the water temperature is high, the wind is weak, or the tide is slow.

Marine biological culture

The pollution of mariculture itself is also one of the factors that induce red tide.

Natural factors are also an important cause of red tide.

The frequent occurrence of red tides is not only caused by man-made, but also related to natural factors such as latitude, season, ocean current and the degree of sea area closure.

Red tide defense:

1. Control the eutrophication of the sea area

① Pay attention to the treatment of urban sewage and industrial sewage, and improve the sewage purification rate.

② Rational development of marine aquaculture.

In order to reduce eutrophication caused by mariculture, the following measures should be taken:

A, according to the environmental conditions of the water area, select some breeding varieties that can purify the water quality, reasonably determine the breeding density and control the breeding area.

B. Multi-species polyculture, rotation culture and stereoscopic culture, especially fish, shrimp, shellfish and algae polyculture, and establish an ecological culture system.

C, improve the culture technology, improve the bait composition and feeding technology, so as to facilitate the feeding of cultured organisms, reduce the bait residue and reduce the pollution of water quality and sediment.

D. Sewage and wastes from pond culture cannot be directly discharged into seawater, and should be filtered step by step.

2. Artificial improvement of water and sediment environment.

For example, in the eutrophic inland sea or shallow sea, selectively cultivating large economic seaweeds such as kelp, Undaria pinnatifida, Sargassum fusiforme, kohlrabi, laver, hedgerow, etc., can not only purify the water body, but also have high economic benefits; Using the energy of natural tides to improve the exchange capacity of water bodies; Dredger and suction dredger can be used to remove polluted sediments, or plow the seabed, or cover polluted sediments with clay minerals, lime homogenate and sand to improve the water body and sediment environment.

3. Control the introduction of exotic species of toxic red tide organisms.

It is necessary to formulate perfect laws and measures to prevent toxic red tide organisms from being brought into aquaculture areas through the transplantation of ships and aquaculture varieties.

Formation of water bloom:

Water bloom refers to a natural ecological phenomenon in which algae multiply in fresh water, which is a feature of water eutrophication. It is mainly due to the phenomenon that wastewater containing a large amount of nitrogen and phosphorus enters the water body in life and industrial and agricultural production, and cyanobacteria (also known as cyanobacteria, including Nostoc commune, Nostoc flagelliforme, etc.). ), green algae, diatoms and so on multiply, making the water appear blue or green. Some water blooms are also caused by zooplankton flagellates.

The defense of water bloom:

In the method of water bloom control, how to control algae in water through various ways is a subject that needs long-term research. Relatively speaking, the innate advantages of biotechnology, especially bio-ecological engineering technology, may show a good development trend, and broad-spectrum and efficient algae-eating bacteria and viruses are still the research direction. Cyanobacteria is a protozoan between algae and bacteria. It has some characteristics of bacteria and chlorophyll, so algae experts still classify cyanobacteria as algae. However, cyanobacteria have no nucleus and are easily attacked by viruses and fungi, which makes it possible to develop algae-eating fungi and viruses. Cyanobacteria have bubble structure and can float to avoid light. In view of the habit that most cyanobacteria sink in the morning and float at night, effective measures can be taken to control the growth of cyanobacteria. Cultivating dominant fungal populations and adjusting the ratio of nitrogen to phosphorus will also be an effective way to solve the water bloom phenomenon.

The difference between water bloom and red tide;

Bloom fresh water, red tide seawater

The first condition for the occurrence of red tide is that the proliferation of red tide organisms should reach a certain density. Water bloom is a natural ecological phenomenon in which algae multiply in fresh water, and it is a feature of water eutrophication.

Some flagellates (or dinoflagellates) are food for some fish and shrimp. However, due to special environmental conditions, some red tide organisms overproduce and form red tide. Bloom is mainly due to cyanobacteria (strictly called cyanobacteria), green algae, diatoms and other algae becoming the dominant population in the water body, which makes the water body appear blue or green after a large number of reproduction.