Introduction • Different types of plants and animals which live in water, form the complex biotic community and exhibit inter-relationship among themselves. • Fish ponds treated with organic manures and chemical fertilizers are occasionally infested with various types of aquatic plants which disturb physiological activities of fish and other aquatic animals and, as a result, fish culture operations become impossible.
Harmful effect of aquatic plants • Aquatic plants limit the living space for fish and their reduction in survival, growth and reproduction • upset the equilibrium of physico-chemical factors of pond soil and water, • cause imbalance of biochemical oxygen demand • misbalanced of balanced relationship between plants and animals, • promotes siltation of ponds and lakes and • ultimately these ecosystems become unfit for human use.
But one cannot ignore beneficial effects of aquatic plants in fish ponds such as • production of biogas, paper pulp, animal feed and organic manures • pollution control, • absorption of hevy metals such as chromium, mercury, nickle, zinc, iron, and lead • absorption of essential nutrients such as nitrogen and phosphorus from water that has already been treated with chemical fertilizers.
Units of Aquatic Vegetation • Aquatic plants are not uniform in any pond or lake. • Depending on climatic and edaphic factors aquatic plants of a pond or lake may be divided into a number of natural units whose composition is different and distinct. • A large unit of aquatic vegetation in a pond under identical climatic conditions is called a plant formation. • A formation is said to be a climax when it is dominated by one or more species which are abundant in it. • For example, in floating vegetation there may be certain dominant marginal plants.
A major sub-division of a plant formation is termed as association. • An association is similar in general outward appearance floristic composition, and ecological structure such as marsh association or hydrophytic. • A plant association may have one or more dominant species. • Within the association there may be communities of plants closely associated.
Types of Aquatic Plants • Aquatic plants are defined as those vegtation which grow and reproduce in small and large bodies of water. • From the edge of pond or lake to the bottom, different types of aquatic plants or hydrophytes grow in different depths of water. • According to their distribution in a freshwater pond/lake ecosystem, hydrophytes have been classified as follow: • Floating Plants : These plants float above the surface of water with roots hanging from underneath. They remain in contact with air and water but not soild such as Eichornia sp. (water hyacinth), Wolffia sp., Lemna major and Lemna minor (duck-weeds), Azolia spp. (water fern), Pistia sp. etc. (Figure 4.1).
Rooted Emergent Plants: These plants are partly or completely exposed to air. The root system is completely under water and fixed with soil such as Nelumbo sp. (lotus), Nymphaea ap., Sparganium sp., Sagittariasp. etc. • Rooted Submerged Plants: These plants remain submerged in water but rooted in soil such as Hydrilla sp., Vallisneriasp., Ottelia sp., Lagarosiphon sp., Myriophyllum sp., etc. • Marginal Plants: They are mostly rooted and infest the shallow water body such as Typhasp., Cyperus sp., Panicum sp., Ipomoea sp. etc.
It is important to note that except a few species of rooted emergent plants (such as Cham sp.), varieties of rooted emergent species are developed in different depths of water (Figure 4.2). • However, some 'authors claim that in fish ponds and lakes, aquatic vegetation (both algae and phanerogams is very important because of the following reasons: • Different species of algae form the food of minute creatures which in turn, are the food of small animals, and again these small animals become food for still larger animals followed by large-sized fish until the cycle is completed. • Phanarogams (Seed plants) are the higher plants generate oxygen they bind the bottom soil by their roots and hence keep the water transparency, some fish species obtain food organisms from roots, leaves and stems; also shade the fish from bright sun and provide sanctuary to young fish from their enemies.
Combination of Aquatic Plants • For different groups of fish, it is important to the fish farm to have the exact combination of plants for the ponds. • While garden ponds need emergent plants, fish culture ponds require submergedplants.This is due to the fact that the submerged plants are the most useful because they not only provide an assembly place for fish food organisms but good oxygenators. • Marginal and floating plants can be used in the rearing ponds for shade purposes. • Though aquatic plants are important for real use in fish ponds and lakes to greater or lesser degree, some water weeds suddenly develop in a pond from seeds or fragments dropped by any means or carried out by the wind. These weeds, as soon as recognized, should be destroyed.
It has been observed that about three hundred species of aquatic macrophytes are distributed all over the world. • Some species grow anywhere and under most conditions. Some thrive only in hard water, others in soft water. • Some species are exclusively indigenous to a particular country or region. • Generally, aquatic plants are flourished in different lentic and lotic environments in tropical and sub-tropical fish ponds. • Therefore, their abundance, variations, and distribution in different soil types and freshwater ecosystems are extremely variable.
What is the difference between a lentic and lotic water system and how do biotic conditions vary from one system to the other • Both lentic and lotic refer to continental freshwater systems or aquatic ecology versus marine or estuarine waters. • Lentic means still water systems whereas lotic refers to flowing water systems. • Lentic systems can range from small, temporary rainwater pools a few inches deep to ponds of various sizes and depth to the largest and deepest lakes any where on earth. • Lotic systems can range from a spring that is only a few centimeters wide to a major river that is miles in width and thousands of miles long.
Many of the biota and food webs are quite similar from lentic to lotic systems while others are very different. • Lotic systems can have sections that are similar to lentic systems, especially where the flowing water is deep and moves slowly. • In general, there is much more variability within lotic systems because biota, especially micro scale organisms, must be specialized to live with flowing water conditions. • While light, temperature, chemistry and substrate materials play important roles in determining the type of bacteria, primary producers, invertebrates and vertebrates that thrive in lentic and lotic systems, flow is a key determinant of both abiotic and biotic influences on the ecology of lotic systems.
Nutrient Assimilation • Several species of floating plants (such as Azalia pinnata) assimilate oxygen in association with algae. • Moreover, they contribute abut 20-30 kg N /hectare per season through nitrogen fixation and addition of phosphorus fertilizers at the 250 kg/hectare rate increased nitrogen level upto 69 kg N per hectare per season in many ponds of temperate, tropical and sub-tropical zones. • It has been reported that combined effects of nitrogen and phosphorus fertilizers stimulate plant growth and nutrient assimilation rate also increased. • In presence of some chemical fertilizers like diammonium phosphate and nitro-phosphate, blue-green algae secrete considerable amount growth promoting substances into the surrounding water.
Therefore, blue-green algae may exhibit synergistic influence on the potentiality of nutrient assimilation by aquatic plants from water. • Phytoplankton and algae have capacity of fixation phosphorus . They can able to store ten times as much phosphorus as found in pond water. • Therefore, occasional growth of aquatic plants of particular species for several weeks lower the levels of metabolites of fertilizers and manures below the toxic concentration in fish culture ponds where manuring and fertilization porgrams are undertaken for improving physical properties of soil and constant fertility of water.
Nutrient Removal • Removal of toxic compounds by different species of floating plants such as Eichoinia crassipes, Azalia pinnaia, Lemna minor, and Pistiastratiotesdepends upon the days of retention of plants and nutrient levels in ponds. • For example, water hyacinth removes upto 200 kg nitrogen per' hectare per year and 322 kg phosphorus per hectare per year from water. • This plant also removes about 10-95 percent nitrogen and 25-90 per cent phosphorus within 2 to 50 days. • Marsh (a swamp- loving plant) successfully removes phosphorus at the 0.949 g per squre metre from effluent-receiving water.
Hydrillaverticillatais able to eliminate about 1.329 g phosphorus per square metre from eutrophicated water. • Nutrient content of aquatic plants was found to be significantly positive correlation with nutrient concentrations of water. • Generally, the intensity of nutrient removal by aquatic plants depends upon the plant density, bacterial activity, amount of fertilizers and manures used in ponds and their interactions with various abiotic-biotic factors of ecosystem such as temperature, season, wind, and rainfall.
Macrophytes, mixed algal forms in ponds also have many advantages in removing ammonia-nitrogen, nitrate-nitrogen, and nitrite-nitrogen within 30 days. • Algal forms also exhibited dramatic reduction in the concentration of hardness, potassium, mercury, iron, zinc, and copper from water. • It has been suggested that different species of algae such as Anabaena variabilis, A. cylindrica, Aulosira sp.,Calothrix elenkenii, Tylopothrix ienuis, and Nostoc muscorum imparted a notable reduction in pollution load from eutrophicated ponds rather than a single species of algae. • Scientists have advocated about the usefulness of macrophytes and algae and their use in fish culture ponds to control pollution.
Quantity of Pollutants Absorbed by Macrophytes • A number of field and laboratory experiments have been made to treat wastewaters using different types of macrophytes. • Macrophytes are so efficient that water hyacinth, for example, covering one hectare water area is sufficient to recycle the waste produced by 2,500 persons. • When intensive and semi-intensive fish culture systems become eutrophicated, immediate removal of pollutants (particularly ammonia and nitrate) should be undertaken on emergency basis. • The efficiency of water hyacinth, for example, in reducing excess quantities of ammonia, nitrate, and phosphate from pond water is quite dramatic. • In sewage-fed fisheries where diluted sewage (sewage: freshwater = 1 : 1 or 1 : 2) is used, may increase the level of nutrients beyond threshold concentrations. • Therefore, three-forth of the water area is covered with single species of young macrophyte for short duration. Experimental studies have shown that water hyacinth absorbed highest quantity of nitrate-nitrogen followed by total phosphorus and ammonia-nitrogen
Mechanism of Pollutant Removal • When macrophytes grow over ponds profusely, concentration of dissolved oxygen in water is depleted that create anaerobic conditions, This condition favours the process of denitrification and nitrate removal is initiated. • Some species of macrophytes are able to transport oxygen from the foliage to the root that results in the formation of oxidized environment in water. • Such environment is principally due to the diffusion of oxygen which is consumed by bacteria. • A large number of anaerobic bacteria are associated with the roots of plants.
Mechanism of Pollutant Removal • Organic compounds are degraded by aerobic bacteria. • The degraded products remain in suspended condition in water which strike the root surface. • Consequently, their electrical charges are lost. • The presence of enzyme dehydrogenase in the root system of plants facilitate the removal of suspended particles from waters.
Factors Affecting Removal of Pollutants The removal of pollutants by aquatic plants depends on a number of factors such as • temperature, • pH • depth of water • nutrient composition, • nature and quantity of plants and their stages of development. • For example, young plants are able to absorb pollutants more rapidly than the older 'ones. • While increase in water temperature favors the rate of absorption of pollutants, alkaline ponds resist it.
Accumulation of Nutrient Elements • Floating aquatic plants such as water hyacinth and duckweed are also able to accumulate calcium, magnesium, potassium, and sodium from different types of water bodies • Such accumulation is significant for plant growth because these elements are utilized by plants for various physiological processes.
Utilization of Aquatic Plants • Absorption of nitrogen and phosphorus by aquatic plants is significant to fish farmers. Because these plants are of floating nature, their collection from water bodies is not difficult. • However, there is an ample scope for controlling fertilizer pollution by triggering the dynamics of nutrient pool through soil-water-plant relationship and different feeding status of plant-animal-man. • Such relationship makes the innovative way to recycle the nutrients to greater extent. • Nutrient-enriched plants are used for biogass production and preparation of animal feed as well as compost.
The compost is a natural product of breakdown of dead plant material. It is used in fish culture to provide nutrients to soil and water. • The compost is particularly used in fish ponds where there is severe loss of water through soil. • It has been reported that the annual biogas production was highest in water hyacinth (44.381 litres) and somewhat lower in water fern (17.186 litres). • On the other hand, single layer of water fern covering a hectare of water area weighing about 150 quintal of green matter will ensure about 30 kg of nitrogen after the breakdown of dead plants and animals by bacteria and fungi. • The nitrogen is slowly released and utilized by aquatic ecosystem.
Dehydrated and chopped water hyacinth contains 10-20% crude protein, 20-30% carbohydrate, 20-30% ash free crude, and 12% ash. • Similarly, dehydrated azolla also contains 5-12% crude protein, 7% ash and 10-12% carbohydrate. • Therefore, water hyacinth and azolla are very palatable food for pigs, ducks, sheep, cattle and some species of fish. • Such utilization of aquatic floating plants can be realized only when integrated approach would be undertaken as a village industry thereby increasing the resource management, employment generation and improving the quality of polluted waters for adoption of fish culture management techniques (4.5).
Removal of Decaying Vegetation • It is important to note that some aquatic plants die away in a particular season and again appear after several months. • The dead and decaying plants remain settle into the bottom of the pond and decomposition process starts. • Therefore, it is necessary to remove the dead plant and avoid toxicity. • Aquatic vegetation that has been treated with chemicals should not be allowed to remain in the pond, otherwise it will be rotted, thus de-oxygenating the water. • Dead weeds should be properly shifted from the edges of ponds and lakes and if it is not done or just dragged out on the margins of ponds and lakes, small broken pieces of seeds of weeds might be back into the water resulting in further occurrence of weeds all over the water surface.
Algal Blooms • Optimum level of algal blooms is beneficial for fish production. • They are the food for fish food organisms and therefore, considered as an important link in the food chain. • With high rainfall and temperature the algal blooms rapidly cover the ponds. • The diatoms appear at first and they feed on dissolved substances present in the water. • After death and decay of diatoms, blue-green algae (such as Anabaena sp. and Oscillaioria sp.) appear. • However, in freshwater ecosystems about nineteen hundred species have been reported so far. • They are either attached to other plants and substratum in the pond bed or free-floating.
They require nutrients for normal growth and development. • The nutrients are derived from the decomposition of dead and decaying plants and animals present in the mud. • Heavy growth of these algal species indicates that the ponds and lakes are productive. • Algal mats which are formed over pond surface by the growth of some species of blue-green algae such as Spirogyra sp., Oedogoniumsp., Cladopherasp., etc. may cause the fish to seek the pond bottom because fish gills are clogged by these filamentous algae. • After several days fish obtain food from the tiny animal life to be found on the algal mats.
Control of Aquatic Plants • Generally older and fertilized ponds and lakes succumb to aquatic vegetation which is very important for aquatic ecology. But it becomes necessary to remove aquatic vegetation if they grow rapidly. • Therefore, in order to keep fish in good health and to maintain fish farms an economically viable, adequate control of several types of aquatic plants should be adopted as noted below. Manual Control • This is done in small water area. • simply either by hand picking or uprooting emergent and marginal plants or cutting them with sickles. • Of course, these practices are executed in countries where labour is cheap and easily available. • The shallow submerge plants are removed mechanically by (1) raking with weeders made of wood which is fitted with iron spikes and wire (2) rotating bamboo poles fitted with basal cross attachment (3) cutting the weeds with long-handled sickles.
mechanical lifts or power winches can be used to eradicate dense submerged weeds. • Motor boats equipped with cutting knives are also used. • Sometimes a weed cutting launch with its bow connected with V-shaped sickles that have reciprocal action, is used for the purpose. • In order to check the migration of floating plants into ponds and also to control marginal plants, creation of barriers and grazing of harbivorous animals, respectively, are necessary. • For controling algal blooms, ponds and lakes are shadded by some means. • Such work should be done in monsoon or winter seasons. If this is done in summer, deoxigenation of water may result and thus fish will suffer.
Chemical Control • Now-a-days, a variety of herbicides are used to control aquatic plants. • The basic consideration of the use of herbicides in fish culture is that (1) herbicides should be used as sublethal levels for weed control strategies and will not adversely affect the aquatic ecosystem as a whole (2) the rate of application should not be exceeding 10 mg/l and 10 kg/ha for those applied in water and on surface area basis, respectively (3) herbicides should be non-toxic to warm-blooded animals (4) easily available and should not require costly equipment to spray herbicides.
For controlling aquatic plants in large ponds and lakes it is necessary to use chemicals, but use of chemicals in small water areas should be avoided. • The chemicals are lethal to fish and other aquatic life and if they are not used at safety limits, the fish will be killed. • Action of chemicals differs in hard and soft water, structure and texture of bottom soil. • However, the pond may be emptied half-way before use of chemicals to reduce the cost of treatment. • The pond is filled again with water to its desired level. Of course, it is better to use the chemical before the onset of monsoon season.
Use of Arsenic: Sodium arsenite is practiced in many Asian and some western countries in weed control management strategies . the safest effective concentration is 1.0 to 2 mg/l. • Use of Copper Sulfate: Copper sulfate is highly effective in controlling algal blooms, Chara sp. and Ottelia sp. at the rates of 2.0%, 18 kg/ha, and 250 mg/l, respectively. • Use of 2,4-0 and Simazine : Simazine and 2, 4-0 are extensively used in India to control water hyacinth from large ponds and lakes. In such cases, the spreading of the chemical should be continued for several days. • The surface water should be roughly divided into several equal sections to ensure proper distribution and safeguard water from being treated twice or even left untreated.
Biological Control • Some species of aquatic plants can be controlled by culturing some herbivorous fishes such as Oreochromis mossambicus, Cienopharqngodon idella, Puntiusjavanicus,Osphronemusgouramietc. • These fishes are hardy, does no interfere with other fishes and easily consume weeds. • In different parts of the world, grass carp, C. idella, has been found to be effective in controlling some species of aquatic weeds such as Azolla sp., Salvinia sp., Hydrilla sp., Wolffia sp., Ottelia sp., Lemna sp., Spirodella sp., Myriophyllum sp., Trapa sp., Limnophila sp., Guinea grass, leaves of cabbage and Ipomoea sp.These plants are highly palatable and hence accepted by the fish. • Some other species such as Eichornia sp., Pistia sp., Nymphoides sp., etc. are also consumed by the fish but the fish does not appear to feed voraciously.
The common carp, C. carpio, is also equally important in the uprooting of aquatic plants at the time of digging and burrowing in search of food. • This species eliminates filamentous algae and Myriophyllumsp. from fish ponds successfully and did not interfere with other species. • The culture of different species of tilapia has also been found suitable for weed control management strategies. • This species consumes filamentous algae and submerged vegetation such as Najassp., Cham sp. etc.
It is important to note that the feeding rates of different species of herbivorous fish depends on the • environmental temperature, • fish species to be reared, • availability of natural food organisms, • various life stages of fish (such as juveniles, fingerlings and adults), • concentration of aquatic vegetation, • stocking density of fish and • geographical as well as climatic conditions of a particular area. • For example, stocking of grass carp in ponds under Indian conditions at the rate of 79-173 kg/ha and 175-225 kg/ha consume hydrilla and lemna, respectively within a month. • In China, this fish when stocked in ponds at the rate of 100 per hectare, consumes a wide variety of aquatic plants.
In Israel and USA, stocking of Cyprinuscarpioin ponds at the rate of 8,000 and 400 per hectare, respectively, successfully controlled aquatic plants of the species Ceratophyllum sp., Myriophyllumsp., and filamentous algae. • Stocking of tilapia at the rate of 2,470-4,940 per hectare in large Indian impoundments controlled a variety of aquatic plants. • In many Asian and European countries, geese and ducks are used in fish culture ponds for dual purpose
First, they effectively destroy some species of floating plants such as azolla and lernna and second, manuring the fish ponds. • About 2,500 ducks are stocked per hectare water area having 6,000 fish fingerlings of indigenous and exotic species. • In this case, manuring and aeration of pond is dispensed with, because of duck faecal washing and paddling in the pond. • However, once aquatic vegetations have been completely removed from ponds and lakes, periodical maintenance should be undertaken to prevent regeneration of aquatic plants.
Control of some aquatic plants through biological means have been briefly summarized in Table 4.6
Conclusion • Different types of aquatic plants grow in ponds and lakes and in association with aquatic organisms, they form a complex biotic community. • Fertilizers and manures trigger the development of aquatic plants (particularly macrophytes and algae) and as a result, physiological activities and fish culture strategies are jeopardized. • Though aquatic plants upset the equilibrium conditions of physico-chemical qualities of water, their beneficial effects in fish culture (such as pollution control,production of animal feed and compost) should not be ignored. • Efficiency of floating plants in nutrient assimilation, removal of toxic elements, accumulation of excess nutrients, and their utilization for myriad purposes must be emphasized.
In spite of various beneficial impact of aquatic plants, there are some harmful effects and therefore, these plants should be controlled when fish culture management strategies are contemplated. • Treatment of eutrophicated ponds with macrophytes is a cheapest and common method through which pollutants can easily be removed from water within a very short period. • In cases where organic manures are widely used for pond productivity, occasional spreading of macrophytes can be regarded as a more common solution for drastic reduction of pollutants.