ROLE OF PHYTO PLANKTON & ZOOPLANKTON IN FRESHWATER AQUACULTURE
Post no-1415 Dt 24/12/2019
Compiled & shared by-DR RAJESH KUMAR SINGH ,JAMSHEDPUR,JHARKHAND, INDIA, 9431309542,rajeshsinghvet@gmail.com
Plankton:
The plankton are minute, microscopic organisms of plants & animal origin. They spend their whole life in the water. For their movement, they depend on the mercy of physical conditions of the water body and environment e.g. depth, water current, wind, light, air etc.
Classification of plankton
1. On the basis of quality:
a.) Phytoplankton – Plant plankton
b.) Zooplankton – Animal plankton
2. On the basis of size:
a.) Macroplankton – The larger units of plankton, visible to the unaided eye.
b.) Netplankton (mesoplankton) – Plankton secured by the plankton net equipped with
No. 25 silk bolting cloth.
c.) Nanoplankton (microplankton) – Very minute plankton not secured by the
plankton net with No.25 silk bolting cloth.
3. On the basis of local environmental distribution:
a.) Limnoplankton – lake plankton.
b.) Rheoplankton (potamoplankton) – running – water plankton.
c.) Heleoplankton – pond plankton.
d.) Halioplankton – salt – water plankton.
e.) Hypalymyroplankton – brackish -water plankton.
4.On the basis of origin:
a.) Autogenetic plankton – plankton produce locally.
b.) Allogenetic plankton – plankton introduced from other localities.
5.On the basis of content:
a.) Euplankton – true plankton.
b.) Psedoplankton – debris mingled in plankton.
6. On the basis of life history:
a.) Holoplankton – organisms free-floating
throughout their life.
b.) Metoplankton – organisms free-floating
only at certain times or stage of lifecycle.
What Are zooplankton?
Zooplankton are small animals that float freely in the water column of lakes and oceans and whose distribution is primarily determined by water currents and mixing. The zooplankton community of most lakes ranges in size from a few tens of microns (Protozoa) to >2 mm (macrozooplankton). In terms of biomass and productivity, the dominant groups of zooplankton in most lakes are Crustacea and Rotifera and these protocols emphasize those groups. Zooplankton play a pivotal role in aquatic food webs because they are important food for fish and invertebrate predators and they graze heavily on algae, bacteria, protozoa, and other invertebrates. Zooplankton communities are typically diverse (>20 species) and occur in almost all lakes and ponds. Zooplankton are rarely important in rivers and streams because they cannot
maintain positive net growth rates in the face of downstream losses.
• zooplankton are small floating or weakly swimming organisms.
• animal origin that drift with water currents for their movement.
o Very important as primary consumers.
o Important food base for secondary consumers, including fish.
Major groups of freshwater zooplankton
Rotifers
1.The rotifers (Rotifera, commonly called wheel animals).
2. Most rotifers are around 0.1 – 0.5 mm long (although their size can range from 50 μm to over two millimeters).
3. Rotifers are an important part of the freshwater zooplankton, being a major food source.
4.Filter-feeding with corona.
5.Mostly littoral, sessile, but some are completely planktonic.
Rotifers as food for fish
1.Too small to be important as food for most fish.
2.May be important in diets of some larval fish.
Cladocerans
1.Small crustaceans (0.2-3.0 mm) with head, and body. covered by bivalve carapace.
2.Swim by using large 2nd antennae
3.Size of phytoplankton ingested proportional to body size.
4.Rate of filter feeding increases with size and temperature.
5.Filter phytoplankton, detritus for food (some are predators).
6.Selective filtering by cladocerans can remove big “chunks” of the phytoplankton, and alter phytoplankton succession.
Cladocerans as food for Fish
1. Large species favored by many fish (visual and filter-feeders).
2. More energy return from bigger species.
3. Eliminates large forms, small ones flourish (big forms often predatory).
Copepods
1. Microcrustaceans in same size range as Cladocerans.
2. Several different groups based on differences in body structure.
3. 2 major groups: cyclopoids and calanoids.
Proximate composition of zooplankton
The proximate composition of mixed zooplankton.
As percentage of dry weight are:
• Protein range from – 40.27 to 63.24%
• Chorbohydrate range from – 1.68 to 4.55%
• Carbon range from – 21.34 to 35.17%
• Lipid range from – 16.23 to32.09%
Role in aquaculture
1. Zooplankton are preferred natural food for larval stage of fish and prawn.
2. The rotifer Brachionus species can be mass cultivated in large quantities and is an important live feed in aquaculture.
3. Live usual as dried Cladocerans for excellent food for the aquarium fishes or ornamental fishes.
4. Aquatic organisms in the ecosystem depends on the area and volume of the water body and the level of plankton primary production.
5. Improve growth and survival rate of Heterobranchus longisfilis larval when fed on enriched zooplankton, compared to un-enriched.
6. Balance diet for fish and prawn.
7. Rotifers and Cladocerans are important component of most freshwater communities.
8. The population of zooplankton is a function of availability of suitable food for aquatic organisms.
9. The importance of long chain omega-3 Polyunsaturated fatty acids in rotifers as food for Sea bream larvae.
ENVIRONMENTAL FACTORS
Zooplankton are susceptible to variations in a wide number of environmental factors including water temperature, light, chemistry (particularly pH, oxygen, salinity, toxic contaminants), food availability (algae, bacteria), and predation by fish and invertebrates. It is generally desirable to have as much information on these variables as possible. Clearly, this will frequently be practical. Some variables are relatively easy to measure (e.g. temperature), but others are more difficult (e.g. fish-predation intensity, toxic contaminants). Many environmental factors affect zooplankton only at extreme levels (e.g. toxic contaminants, salinity, oxygen) and will not be important in all lakes. Ideally, most sample collections should be accompanied by measures of water temperature, pH, and algal biomass (chlorophyll-a concentration or phytoplankton biomass). Temperature and pH can be measured using portable field instruments whereas the estimation of phytoplankton biomass requires more involved techniques (see Findlay and Kling, EMAN protocols for phytoplankton).
PHYTOPLANKTON
Ponds typically contain an abundance of phytoplankton. These organisms play an important role in pond ecology and influence water quality. Although phytoplankton usually are beneficial, under some conditions, they can be quite problematic in fish and shrimp production.
Phytoplanktonic algae consist of thousands of species distributed primarily among the phyla Pyrrhophyta, Euglenophyta, Chlorophyta, Heterokontophyta and Cyanophyta. The Pyrrhophyta are mainly marine and include the dinoflagellates. The Euglenophyta — like the Pyrrhophyta — are motile, flagellate organisms, but many are freshwater organisms such as species of Euglena. The Chlorophyta are mostly freshwater organisms, and this phylum includes the common green algae. The Heterokontophyta contains many freshwater and marine species and includes the yellow-green, golden and brown algae as well as diatoms. The Cyanophyta are prokaryotic (the cell nucleus is not bound in a membrane) like bacteria, and many authorities consider them to be bacteria (cyanobacteria). But most aquaculturists refer to Cyanophyta as blue-green algae, as I will do. There are many freshwater, but fewer marine species of blue-green algae.
Phytoplankton propagules are literally everywhere — in the soil, water and air. Any water body is naturally seeded with phytoplankton just as it is with bacteria. Ponds do not need to be inoculated with phytoplankton. The requirements for phytoplankton growth are few: water, light, favorable temperature and inorganic nutrients. Because there is such a wide variety of phytoplankton, there are species that can grow in almost any kind of water. Nevertheless, different species have different environmental requirements, and the dominant species will vary among water bodies.
Phytoplankton as a friend
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. 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.
For example, a Schizochytrium sp. has been cultured to harvest its high omega-3 fatty acid for human health and specialty ingredient. The Chlorophyte microalgae Haematococcus pluvialis has high levels of the valuable antioxidant astaxanthin, which is highly beneficial in all stages of shrimp farming as well as a health food for people.
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 bottoms. 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 aquacultured animals.
Shading is important because when the young shrimp postlarvae or fish fry are stocked into unshaded ponds, they will not be overtly stressed under the bright sun. Nowadays, when desirable and appropriate pond water color cannot be established early by the growth of natural phytoplankton populations, as is often the case with HDPE-lined or concrete ponds, an artificial commercial colorant can be added to the pond water to provide shading.
Phytoplankton species are primary autrotrophic producers that are able to produce food from their photosynthetic activity, and are the 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 postlarvae and fish fry stocked into the ponds.
Phytoplankton as a foe
Phytoplankton blooms, however, can have a dark side too, and can cause a number of problems if not properly managed. For example, 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 ponds, there is a diel pH shift. As the sun becomes brighter, photosynthesis and the resultant uptake of carbon dioxide accelerates and pH levels increase. As the sun sets, phytoplankton respiration releases carbon dioxide, resulting in the formation of carbonic acid and the lowering of the pond water pH. This 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 systems.
Undesirable phytoplankton species and groups like Anaebaena sp. (a filamentous blue-green algae or cyanobacteria), and dinoflagellates like Gymnodinium sp. and Ceratium sp. will thrive in nutrient-rich waters. But when massive die-offs occur because of dissolved oxygen depletion in ponds, these species can release biotoxins that are harmful to shrimp and many other aquacultured species.
Management in aquaculture ponds
Aquaculture ponds often have 50 or more species of phytoplankton at any given time, but only a few species — usually less than four or five — will make up most of the phytoplankton community. Moreover, 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 usually is controlled by concentrations of nutrients — especially those of inorganic nitrogen and phosphate. Waters with elevated inorganic nitrogen and phosphate concentrations typically contain a large amount of phytoplankton. These organisms impart a color to the water — this situation is called a phytoplankton bloom. Of course, some waters are turbid from a large concentration of suspended clay particles or humic 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 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 additions of fertilizer 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 supplement manufactured feed — this is particularly important for small postlarval 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 into the pond water. Phytoplankton rapidly removes ammonia nitrogen from the water lessening the concentration of this potentially toxic substance. Finally, turbidity created by phytoplankton restricts light penetration to the pond bottom, and a phytoplankton bloom is a good control measure for aquatic macrophytes that grow below the water surface.
Potential negative impacts
In spite of phytoplankton being beneficial in aquaculture ponds, their blooms may become excessive and cause negative impacts. During the nighttime in ponds with dense phytoplankton blooms, respiration by phytoplankton and other organisms may cause excessively low dissolved oxygen concentrations that may stress or kill the culture species. In un-aerated ponds, a phytoplankton bloom that reduces the depth of underwater visibility to less than 20-30 centimeters — as measured with a Secchi disk — is likely to cause excessively low nighttime dissolved oxygen concentration.
During daytime, high rates of photosynthesis may deplete water of free carbon dioxide leading to excessively high pH. Also, high levels of dissolved oxygen supersaturation may occur when photosynthesis is rapid. Oxygen super-saturation usually does not cause gas bubble trauma in aquatic animals in ponds, because culture animals usually can move to deeper water where the degree of oxygen saturation is lower. However, in ponds used to supply water to hatcheries, gas supersaturation in source water can have adverse effects on eggs or fry.
