Phytoplankton Management in Aquaculture
Phytoplanktons are indispensable in the commercial rearing of various species of marine, brackish water and freshwater animals as a food source for all growth stages of bivalve molluscs, larval stages of some crustacean species and very early growth stages of some fish species. Some phytoplanktons are furthermore used to produce mass quantities of zooplankton (rotifer, copepods and brine shrimp which serve in turn as food for larval and early juvenile stages of crustacean and fish. Besides, for rearing fish larvae, microalgae are used directly in the larval tanks referred as “Green water technique” where they are believed to play a role in stabilizing the water quality, nutrition of the larvae, and microbial control. The role of phytoplankton as the primary trophic level of the aquatic food chain for coastal commercial pond production of marine shrimp has been well established (De Pauw & Persoone 1988; Paerl & Tucker 1995). These microalgae also significantly affect water quality, especially dissolved oxygen and pH, of shrimp pond or raceway systems (McIntosh, Fitzimmons, Collins & Stephens 2006; Velasquez, Cabrera, Rosas & Troccoli 2007). Phytoplankton plays a crucial role in extensive, semi-intensive and intensive commercial shrimp ponds (Castille & Lawrence 1989; Lawrence & Houston 1993; Tacon 1996). In extensive and semi-intensive pond-based grow-out culture systems, water quality and food supply are more affected by the presence of microalgae than in intensive culture systems, This is largely due to the fact that in these systems the higher biomass requires greater amounts of supplemental feed and, as a result, pond management is substantially different (e.g., aerators to maintain required dissolved oxygen levels). However, in intensive systems, if there is a lack of phytoplankton, benthic algae will grow, having also a negative effect on the quality of the culture water. This is largely due to the fact that the oxygen, carbon dioxide and nitrogen balance in the water column cannot be maintained. Furthermore, aeration demand increases and large rations of nutritionally complete feed are required, causing potential economic losses, as production costs increase.
Phytoplankton can be a dilemma in aquaculture. Properly managed populations can be beneficial to aquaculture production system, but if inadequately managed, they can proliferate out of control and can have significant negative effects Many species of phytoplankton that are beneficial in shrimp and fish farming in terms of nutrition and removal of excessive nutrients (such as ammonium, nitrate and phosphate), are also responsible for a diel pH shift which influences the dynamics of ammonia and hydrogen sulfide, both of which can be highly toxic to aquaculture species . Pond oxygen depletion at night is likely when excessive phytoplankton blooms occur, which can affect and even kill cultured fish and shrimp. Phytoplanktons groups like dinoflagellates can release toxins when they die and affect the health of shrimp and fish. There are many phytoplankton species that are highly nutritious to many aquacultured species such as Chaetoceros sp, Tetraselmis sp, Isochrysis sp, Skeletonema sp, Spirulina sp, Nanochloropsis and Chlorella sp. These species are nourishing and vital to shrimp larval nutrition during the early larval stages . Many phytoplankton species also produce omega-3 fatty acids that have multiple health benefits. Ponds dominated by Chlorella sp and other phytoplankton species (Green water) or ponds where diatoms predominate (Brown water) have enhanced water quality. Green water was particularly important in the early days of shrimp farming because it can be maintained for a longer time compared to diatoms. Phytoplankton can utilize ammonium, nitrate and phosphate and hence reduce their concentrations in pond waters. Ammonium and nitrate are by-products of the breakdown of protein, while phosphate is present in the feed disseminated to the shrimp in the pond. If present in high concentration, they reduce water quality and can limit shrimp and fish growth. Phytoplankton also provides shading and can limit or prevent the establishment of undesirable benthic algae species on pond bottom. When there is significant benthic algae growth on pond bottoms, they can float in mats buoyed by the formation of gases during sunny days, and can accumulated in stagnant corners of the pond. When they sink again, they decompose and produce hydrogen sulfide, very toxic to Aquaculture animals. Shading is important because when the young shrimp post larvae or fish fry are stocked into unshaded ponds, they will not be overtly stressed under the bright sun. Phytoplankton species are primary autotrophic producers that are able to produce food from their photosynthetic activity, and are starting point of natural productivity in pond ecosystems and food chains in nature. After their populations are established, others follow, including zooplankton species which graze on phytoplankton. Both phytoplankton and zooplankton in turn are important natural food sources for the young shrimp post larvae and fish fry stocked in the ponds or raceways. Phytoplankton community also play an important role in the biofloc system. Microalgae assimilate mainly ammonia and nitrate to produce biomass, additionally consume carbon dioxide and produce oxygen.
Phytoplankton blooms, however can have a dark side too, and can cause a number of problems if not properly managed. Excessive blooms can cause oxygen depletion at night and result in massive plankton and aquatic life die offs. The excessive organic load resulting from these mass mortalities can cause significant water quality deterioration (Particularly increasing dissolved oxygen demand), and strong growth of pathogenic bacterial and fungal populations that can result in a variety of diseases in aquacultured animals. In phytoplankton dominated pond there is a diel pH shift. As the sun becomes brighter, photosynthesis and the resultant uptake of carbon dioxide, resulting in the formation of carbonic acid and the lowering of the pond water pH. The diel pH shift can affect the pond water quality, as pH affects the dissociation of ammonium and hydrogen sulfide . At pH values above 8.5 there is a higher percentage of toxic ammonia and pH levels below 6.5 ,toxic hydrogen formation will occur. The degree of diurnal shift in pH values is directly affected by the phytoplankton density and concentration, and is more severe the higher these are. Phytoplankton growth is proportional to the light intensity, and is faster the brighter, stronger the sunlight intensity is. At higher sunlight intensity, biofloc systems tend to shift towards and become phytoplankton – dominated systems, so it can be beneficial to shade ponds or tanks operating biofloc production system . Undesirable phytoplankton species and groups like Anaebaena sp (A filamentous blue green algae) and dinoflagellates like Gymnodinium sp and Ceratium sp will thrive in nutrient-rich waters. But when massive die-offs occur because dissolved oxygen depletion in ponds, these species can release biotoxins like saxitoxin, brevitoxins etc, that are harmful to shrimp and many other aquacultured species.
Phytoplankton abundance is related to availability of light and nutrients. In water bodies of low nutrient status and clear water, under water weeds will flourish. Waters with a greater availability of nutrients will develop phytoplankton blooms unless they are light limited because of humic substances or clay turbidity. Plants produce organic matter and release oxygen through photosynthesis. Organic matter from photosynthesis is the base of the food web in water bodies. All organisms respire and much of the oxygen produced in photosynthesis is used in respiration. Respiration, as an ecological process, is essentially the reverse of photosynthesis. Phytoplankton communities in water bodies typically undergo rapid succession, and the species composition of phytoplankton communities frequently changes over periods of a few weeks. The abundance of phytoplankton is usually controlled by concentrations of nutrients especially those inorganic nitrogen and phosphate. Waters with elevated inorganic nitrogen and phosphate concentrations typically contain a large amount of phytoplankton. These organisms impart a colour to the water this situation is called phytoplankton blooms. Of course, some waters are turbid from a large concentration of suspended clay particles or humid substances and there is insufficient light for appreciable phytoplankton growth. Highly acidic waters also may not develop dense phytoplankton blooms even if there are plenty of nutrients. Aquaculture ponds are the ideal habitats for phytoplankton. These systems are managed to avoid excessive turbidity from suspended clay particles. If they are acidic, they are limed; nutrients are abundant because of addition of fertlilizer and feeds. Phytoplankton are necessary in ponds for several reasons. They are the base of the natural food web that culminates in biomass of the culture species. Even in ponds with feeding, phytoplankton usually are important because natural food organisms supplemented manufactured feed – this is particularly important for small post larval crustaceans and fingerling fish soon after stocking. Phytoplankton is an important source of dissolved oxygen. In the daytime, these plants produce oxygen by photosynthesis at a much faster rate than oxygen can diffuse from the atmosphere in to the pond water. Phytoplankton rapidly removes ammonia nitrogen from the water lessening the concentration of this potentially toxic substances. Finally, turbidity created by phytoplankton restricts light penetration to the pond bottom.
The further expansion of aquaculture needs to take integration in account. This means that nutrients should be recycled at the maximum. It appears that biofloc technology might make a substantial contribution to this. This however implies that much more attention will have to be given to the management of the phytoplankton community composition and activity in addition to the management of the target aquaculture species.
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