M. N. KUTTY
African Regional Aquaculture Centre
Port Harcourt, Nigeria
Lectures presented at ARAC for the Senior Aquaculturists course
UNITED NATIONS DEVELOPMENT PROGRAMME
FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS
NIGERIAN INSTITUTE FOR OCEANOGRAPHY AND MARINE RESEARCH
PROJECT RAF/82/009
JUNE, 1987
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2.1. Structural Adaptations of Plankton
2.2. Plankton Sampling and Analysis
2.3. Major Representatives of Phytoplankton and Zooplankton
2.4. Plankton Development in Fertilized Fish Ponds
CHAPTER 11
PLANKTON AND BENTHOS
The organisms in the aquatic environment can be devided into three large groups - the plankton, nekton and benthos. In the benthos are included sessile, creeping or burrowing organisms found in the bottom of water bodies. The nekton is composed of swimming animals such as the fish and in the plankton is included all of the floating or drifting organisms. The term plankton was proposed by Victor Hesner in 1887 to designate that “heterogeneous assemblage of organisms which float and move at the will of the waves and other water movements”.
Much of the available information on plankton and benthos existing refer to those in natural water bodies, often large lakes and seas, which are described well in text books of limnology and marine biology - oceanography. These are also certainly of aquaculture importance in special cases, especially coastal aquaculture (“Pen and cage culture”) and also in extensive aquaculture, beginning with “stocking of open waters”, both natural and man-made.
Our interest here is to increase the richness of the water bodies by water quality assessment (physical and chemical feature of water, we already referred to in chapter 8 and 9 and also biological productivity, referred to in chapter 10), to judge their suitability for aquaculture. While biological productivity of a water body can be obtained by measurement of primary productivity, a good index of biological productivity is the measure of abundance of plankton and benthos.
Indeed another objective of our study, once the culture set up is ready is to find out how one would increase productivity of the water by methods such as fertilization. In routine management of a water body under culture, typified by the fish pond, the development of plankton and benthos as fish food organisms, is a most important activity. In rural aquaculture this has added significance, for fish feed is often given here as supplementary feed. And in low input management fish production could be limited to natural production of fish food organisms and also that induced by application of organic and/or inorganic fertilizers (see “Fertilization” under “Pond culture”).
An assessement of biological productivity or production potential of a water body could be made by the quantitative and qualitative analysis of plankton and benthos. Let us first deal with plankton and then benthos.
The plankton is divisible into two main groups, the phytoplankton and the zooplankton. The primary productivity which we discussed in chapter 10 is primarily the functional aspect of phytoplankton - the other chlorophyll bearing organisms are also to be included, but in most water bodies such as the culture pond an index of primary productivity could be obtained by the mass or number of phytoplankton in a unit volume of water. It is also possible to obtain an index of productivity from the concentration of chlorophyll ‘a’ in the water (Fig. 11.1). We can also obtain the mass of particulate organic matter (most often corresponding to total plankton) (chap. 10) which is also well correlated with productivity of fish ponds, as also to secchi disc visibility (see chapter 8), since light penetration is cut down increasingly by the increase in particulate matter, the plankton. Thus most of these parameters can be leading to the same solution, in judging the productivity of the water, if the methods needed are judiciously chosen and tested knowing the limitations of each. We have presented sufficient information earlier (chap. 10) to indicate that the chlorophyll bearing organisms, the autotrophs, mainly represented by the phytoplankton, form the base of pyramid of biomass. Zooplankton graze on the phytoplankton, we have referred also to the other members of the trophic chain.
Before we go into the descriptions of phytoplankton, and zooplankton, let us look at the main divisions of plankton. Most of the plakton organisms are microscopic in size but there are some which are larger. Based upon their sizes, the plankton are divided into several groups:
Macroplankton: Those which are taken with a coarse net, i.e. organisms of about 1mm or more in length that would be normally caught by a net of No. 00 or 000 bolting cloth - these can be readily seen by an unaided eye. Table 11.1 gives the mesh sizes of various Nos. of bolting sick.
Mesoplankton: Forms between 1mm and 1cm, (the largest of the plankton forms are called at times as megaloplankton).
Microplankton: Forms below 1mm, large enough to be retained by a No. 20 net (with a mesh aperture of 0.076 mm)
Nannoplankton: 5 to 60 mm - smaller diatoms, dinoflgellates, protozoans and bacteria which will pass through a No. 20 net, and must therefore be collected by centrifuging the water sample.
Ultraplankton: Those below 5
The plants and phytoplankton in water can survive only in the upper layer of water in deeper water bodies, owing to restriction of light (see under “light” in chapter 8). The density of protoplasm (1.02 – 1.06) being higher, normally life forms would sink in water. To remain floating the planktonic organisms have several adaptations - the major one is that of reduction in size, by which the organisms can have larger surface area in relation to their mass, hence the particles will have more frictional resistance and would avoid sinking (see also “suspended particles” -turbidity, in chapter 8). But this reduction in size alone is not enough. Several plankton organisms carry globules of fat/oil (of. floating eggs of fishes), so that their specific gravity will be less. Some shelled plankton such as diatoms are even heavier - often these have flattened disc shapes, so their sinking takes longer time (Coscinodiscus) - some have also bladders or vacuoles, filled with light fluid or sap mentioned. Needle and hair type of adaptation also enable floating e.g. Rhizosolenia. Some are ribbon shaped e.g. Fragillaria. Another diatom, Chaetoceros has many spines and projections to avoid sinking.
Fig. 11.1. Relationship of net production of Tilapia aurea with phytoplankton, secchi disc visibility, gross productivity and chlorophyll ‘a’ in fish ponds (Redrawn from Boyd, 1979).
The shelled forms (diatoms) which are in surface waters always have a lighter (thinner) shell than those live in the bottom. Similarly, the summer forms have lighter shells in keeping with reduced viscosity of warmer waters.
The dinoflagellates have two whip like flagella and are not strictly passive - some have low horn like structures the length of which change with ambient temperature.
Most of the zooplankton forms have elongated appendages, spines or bristles and are at times dorso-ventrally flattened.
This aspect is described in practical handout on plankton. Standard plankton nets are used (Table 11.I). In large water bodies the net is towed horizontally for a specific period of time; vertical tows/hauls are also taken in deep waters. In either case the volume of water filtered is to be estimated and the plankton accumulated in the collection bucket/bottle is removed. The plankton is now washed into a suitable container and preserved if required with formalin.
Table 11.I
Average mesh (aparture) size of standard grade Dufour bolting silk (Source: Sverdrup et al, 1961)
| No. | Mesh size mm |
| 0000 | 1.364 |
| 000 | 1.024 |
| 00 | 0.752 |
| 0 | 0.569 |
| 1 | 0.417 |
| 2 | 0.366 |
| 3 | 0.333 |
| 4 | 0.318 |
| 5 | 0.282 |
| 6 | 0.239 |
| 7 | 0.244 |
| 8 | 0.203 |
| 9 | 0.168 |
| 10 | 0.158 |
| 11 | 0.145 |
| 12 | 0.119 |
| 13 | 0.112 |
| 14 | 0.099 |
| 15 | 0.094 |
| 16 | 0.086 |
| 17 | 0.081 |
| 18 | 0.079 |
| 19 | 0.077 |
| 20 | 0.076 |
| 21 | 0.069 |
| 22 | 0.064 |
Usually plankton from fish ponds are collected by sampling water from different parts of the pond. About 50 litres so collected are filtered through a plankton hand net and the sample of plankton collected in a tube. Measures of plankton settled in a 1" diameter tube can give a rough index of productivity of the water (see ‘Fry production’ in “Pond culture”). For quick measurement a pinch of salt can be added to the water in the tube, so as to allow death and faster settlement of the plankton. A productive nursery pond, well fertilized, would give ¼ - ½" depth of settled plankton when 50 litres of water is filtered. Alikunhi (in Jhingran, 1982) considers that carp rearing ponds should have about 1 – 2 ml of sedimented plankton for 50 litres of pond water filtered.
The plankton collected can be more precisely measured using a centrifuge. The graduated centrifuge tube will give clearly the packed volume of the sedimented plankton. Plankton collected can also be filtered through a tared filter paper and dry weight of plankton obtained.
Quantitative analysis of plankton is done using a sedgwick rafter
counting cell. A fixed volume of plankton mixed with water - say one ml
taken from a 10 ml mixed suspension of plankton is transferred to the
counting cell which has a volume of 1 ml and its bottom is marked in squares.
The size of the squares as well as the size of the plankton could be known,
if the microscope is used in combination with a micrometer eye piece.
The sketches of plankton organisms can be made using
Stomach contents of plankton eating fish also analyed similarly, so as to study the grazing efficiency and feeding of the fish.
These will be dealt with in detail in practical hand out. Standard books on the subject should be referred. Durand and Lavesque (1981). in his “Flore at Faune aquatique de l'Afrique Sahelo Soudanienne” gives detailed descriptions and drawings of local importance. In addition those beginning the study of plankton could refer to some elementary descriptions with figures, such as those available in Bard et al (1974), APHA (1975 or any other edition). G.M. Smith's Freshwater Algae and publications such as Van Meel's (1954) Exploration hydrobiologique de lac Tanganika (B. Atlas, Vol. IV Fasc., Brussels) would also be very useful in studying plankton samples. Other useful publications are cited under ‘References’.
The phytoplanktons as exemplified by chlorophytes, are either single celled (eg. Chlorella) colonial (eg. Volvox) or filamentous (eg. Ulothrix).
Some of the common phytoplankton and zooplankton are:
Phytoplankton: Green algae (Chlorophytes) Chlorella, Scenedesmus, Closterium, Coleastrum, Micrasterias, Crucigenia, Ankistrodesmus, Characium, Pleurococous, Euastrum, Volvox, Zygnema, Spirogyra, Ulothrix.
Blue green algae: (Cyanophyta): Anacystis, Aphanocapsa, Merismopedia, Spirulina, Lyngbya, Anabaena, Nostoc, Gleocapsa.
Diatoms: Navicular, Synedra Scenedesmus, Dinoflagellates: Ceratium, Cryptomonas.
Euglenophyles: Euglena, Phacus.
Zooplankton: Besides the crustaceans (cladoceraus, copepods mainly) and rotifers as larvae of crustaceans and several other aquatic animals including fishes, crustaceans and molluscs occur in zooplankton. In fish ponds the following are common:
Cladocerans: Daphnia, Moina, Bosmina.
Copepods: Cyclops, Diaptomus
Rotifers: Brachionus, Koratella, Filinia, Manostyla, Lecana,
The effect of fertilization in fish ponds in increasing plankton density is well known. The present status of the studies in this area is well documented by Boyd (1982). The plankton density increases with addition of organic and inorganic fertilizers up to a point. Selective use of fertilizers should cause increase in densities of selected plankton organisms. As the primary tropic level of the food pyramid in the pond, and also as source of food of plankton-eating fish it is important to know how we can manipulate the plankton density, qualitatively and quantitatively, in the fish pond. The quantitative manipulation of plankton in carp nursery for hatchlings is clearly shown by Alikunhi's studies in India (see also, Jhingran, 1982).
It is of interest to find that manipulation of the qualitative nature of plankton for increasing the content of preferred plankton food of fish is not that direct. Boyd (1982) cites the studies and observations of Swingle on Alabama, ponds and states that under identical treatment different ponds in same site had different dominant species at the same time - it is not known what causes this difference. How could the microenvironment of the ponds in the same area under same treatment be different is not known. This is an important area for further elaborate studies.
There is sufficient evidence as at present to indicate that fish production, (especially plankton consuming) are directly related to the density of plankton. The data obtained by Boyd (1979) for Tilapia aurea are presented in Fig. 11.1. The correlation of Chlorophyll ‘a’, secchi disc visibility and gross productivity tith Tilapia production is also presented in the figure.
Oglesby (1977) observed that in small water bodies such as fish ponds the primary productivity was not well correlated with fish production but actual standing crop of phytoplankton correlated well, according to his equation:
log Yf = -1.92 + 1.17 log CHLs
where, Yf is the annual yield of fish expressed in dry weight and
CHLs is the average summer phytoplankton standing crop.
However, as shown under “Biological Productivity” (Chap. 10) the predicted fish yields from the fertilized fish pond in Israel was much lower than the actual yields obtained, suggesting that the fish have used some other external source of organic matter in addition to the phytoplankton (Noreiga - Curtis, 1979). This again highlights the complexity of energy-flow through the fish pond and points out need for further detailed studies.
As explained these include sessile, creeping and burrowing organisms found in the bottom of water bodies. The sessile organisms include mussels, some worms, weeds and several diatoms. The creeping forms include crabs, amphipods, tendepeds, some bivelves and certain fishes. The burrowing forms include several worms and some crustaceans.
There are several description of benthos of the oceans and large lakes. We may not have direct interest in the major divisions of benthos as at present. The littoral area in the shoo has been used for aquaculture in some contries but deeper water are as yet unused. In the sea there are several divisions of the benthos the mod or divisions being littoral and the deep or benthic. The littoral area is subdivided into the (down 40 – 60m deep) and the sublittoral (60 – 200m deep), but as we pointed out these deeper areas are not of much aquacultural interest. Aquaculture is presently restricted to this shallow eulittoral area in the neritic zone. The major classifications of benthos was made by Ekman in 1935 and those interested should refer to be text books in marine biology/oceanography for more information. (eg. Sverdrup et al, 1961).
The productivity of the benthos in the littoral area and lakes and man-made reservoirs are important for extensive aquaculture and also for caostal aquaculture as referred to.
How does the benthos of the fish pond look like - this is important to us, becauses many benthic organisms form food of cultured fishes and
Benthic organisms can be collected by traditional equipment such as grabs and dredges. Different types of trawls (beam, other) can be used in the case of large water bodies. In the case of fish ponds the sampling methods will have to be suitably modified to sample smaller area/ squares to obtain appropriate estimations.
The number of benthic organisms estimated thus increase markedly in fertilized ponds. McIntire and Bond (1962) observed that fertilization with nitrogen and phosphorus increased the biomass of benthic food organisms.
Some of the common benthic macrofauna are listed below:
Oligochaeta: Tubifex, Lumbriculus, Branchiura, Dro, Aulophorus, Chaetogaster
Diptera: Chironomus, Pentaneura, Culicodes
Odonata: Dragon fly nymphs
Ephemeroptera: May fly nymphs
Coleoptera: Cybister, Dytiscus, Gyrinus and their larvae
Crustacea:
• Ostracada: Cypris, Eucypris.
Mollusca:
• Gastropoda: Vivipara, Planorbis, Lymnaea, Pila, Melanoides
• Bivalvia: Unio, Lamellidens, Pisidium.
Bard, J. et al. 1974. Manuel de Pisciculture tropicale. Nogent-sur-Marne, Centre Technique Forestier Tropical, 209 pp.
Boyd, C.E. 1979. Water quality in warm water fish ponds. Auburn Univ., Agriculture Experiment Station, Alabama. 359 pp.
Boyd, C. E. 1982. Water quality management in pond fish culture Developments in Aquaculture and Fisheries Science, 9. Elsevier Scientific Publishing Co., Amsterdam. 318 pp.
Barbour, M.G., B.A. Bonner and G.J. Breckon. 1975. Botany - a laboratory manuel. John Wiley & Sons Inc., 263 pp.
Burgis, M. I. 1974. Revised estimate for the biomass and production of zooplankton in Lake George, Uganda. Freshwater Biol. 4:535 – 541.
Durand, J. R. et Levesque, C. 1981. Flore et Faune Aquatique de l'Afrique Sahelo-Soudanienne, Tome 1, ORSTOM, Document tech. 44.
Dutta, A. C. 1951. Botany for degree students, 5th ed. Oxford Univ. Press, Calcutta. 909 pp.
Hepher, B., 1962. Primary production in fish ponds and its application to fertilization experiments. Limnol. Oceanogr., 7(2):131 – 136.
Imevbore, A. M. A. 1967. Hydrology and plankton of Eleiyele reservoir, Ibadan, Nigeria. Hydrobiologia, 30(1):154 – 176
Jhingran, V. G., 1982. Fish and Fisheries in India, 2nd ed., Hindustan Publishing Co., New Delhi: 666 pp.
McIntire, C.D. and C. E. Boyd. 1980. Effects of artificial fertilization on plankton and benthos abundance in four experimental ponds. Trans. Amer. Fish. Soc., 91:303 – 312.
Noreiga - Curtis, P. 1979. Primary productivity and related fish yield in intensely manured ponds. Aquaculture, 17:335 – 344.
Nwadiaro, C. S. and Ezefili, E. O. 1985. A preliminary check list of the phytoplankton of New Calabar River, Lower Niger Delta, Nigeria. Hydrobiol. Bull. 19(2).
Odunlami, Funlola. 1987. Effects of fertilizer application on plankton production with special reference to succession. M. Tech. (Aquaculture) thesis, African Regional Aquaculture Centre/Rivers State University of Science and Technology, Port Harcourt, Nigeria.
Ogunsola, F. A. 1984. Production of plankton in fresh water and brackish water by application of fertilizers. M. Tech. (Aquaculture) thesis. African Regional Aquaculture Centre/Rivers State University of Science and Technology, Port Harcourt, Nigeria.
Oglesby, R. T., 1977. Relationships of fish yield to lake phytoplankton standing crop, production and morpho-adaphic factors. J. Fish. Res. Board Can., 34 (12): 2271 – 2279.
Robinson, A. A. and P. K. Robinson, 1971. Seasonal distribution of zooplankton in northern basin of Lake Chad. Zool. London. 163:25 – 61
Sverdrup, H. U., M.W. Johnson and R. H. Fleming, 1961. The Oceans, their physics, chemistry and general biology. First indian ed. Asia Publi House, Bombay, 1087 pp.
Szarowska, M. 1985. Studies on the intensification of carp farming & number and biomass of the main components of benthos. Acta. Hydrobiol. 27:197 – 203.
Van Meel, 1954. Exploration hydrobiologiques de lac Tangayika, B. Atlas, Vol. IV. Fasc., Brussels.
