BIOLOGICAL PRODUCTIVITY OF WATER BODIES

Jitendra Kumar¹*, Ramesh Kumar Tripathi³, Neeraj Pathak², Archit Shukla⁴, Saurabh Dubey¹

¹ College of Fisheries, Mangalore, ² College of Fisheries, Veraval, ³Central Institute of Fisheries Education, Mumbai, ⁴ College of Fisheries, Ludhiana, Punjab

Introduction

Biological productivity as an index of water quality and production potential of cultured organism needs prime consideration for site selection. Productivity in terms of qualitative and quantitative aspects of plankton and benthos are treated separately, in this manual. In the present discussion we shall look first at primary production and then show this influences secondary production in water bodies. We shall explain first certain concepts to elucidate aspects of productivity and then the energy flow in an ecosystem and the importance of this information in aquaculture. Finally we shall briefly refer to the methods of measuring productivity.

Before we enter into a discussion on productivity, it would be helpful to look at the concepts of the ecosystem, habitat and ecological niche, and food cycle in water bodies, including food chain and trophic structure. These terms are of specific interest to aquaculture and productivity in general and therefore deserve some consideration.

Ecosystem

The environment in which the man and other organisms live is called the biosphere. The biosphere is made up of different regions that have different types of flora (plants) and fauna (animals). The types of organisms in an area are determined by various factors such as the climate, temperature, rainfall, etc.

An ecosystem is a complete community of living organisms and the non-living materials of their surroundings. Thus, its components include plants, animals, and microorganisms; soil, rocks, and minerals; as well as surrounding water sources and the local atmosphere. The size of ecosystems varies tremendously. An ecosystem could be an entire rain forest, covering a geographical area larger than many nations, or it could be a puddle or a backyard garden. Even the body of an animal could be considered an ecosystem, since it is home to numerous microorganisms.

The organisms, in addition to being dependent on the environment for their needs, are also dependent on each other. This dependency is especially for food. This results in the presence of food chains and food webs.

Habitat and ecological niche

The habitat of an organism is "the place where it lives", whereas the ecological niche, a term coined by Elton in 1927, can br termed as "the position or status of an organism within its community and ecosystem resulting from the organism's structural and functional adaptations". By analogy, it is often said that habitat is the organism's "address" and the niche is its "profession".

The concept of ecological niche is of much interest to the aquaculturist, for it is around it that the practice of "polyculture" of fishes is centered. The well known examples are the polyculture of Chinese and Indian carps including in some cases a predator species also. Here it is basically the complete exploitation of the feeding niches in the water body as exemplified by the mixing of surface feeders, some depending predominantly on phytoplankton (silver carp), some on zooplankton (Catla), column feeders (rohu), bottom feeders (common carp and mrigal), feeders of aquatic (grass carp) and predators (snakeheads). While the practice of polyculture of fish in Asia is old and based mainly on empirical information, it is necessary to critically examine this practice and adapt or improve the presently practiced systems. In Africa polyculture of compatible species of tilapias in combination with predator fishes (Clarias gariepinus/ Ophiocephalus obscurus, for e.g.) have been practiced in some countries. We shall be looking into these aspects more closely, in the section on "Pond Culture", in the present series.

Food cycle in water bodies

The transfer of food energy from the plants through a series of organisms is referred to as the "food chain". Since it is not a single chain, but often interlinking of energy transfer through several chains as a net or web, the food chain should be more precisely referred to as food web. At each transfer a large proportion of the potential energy in the food is lost as heat. The number of steps or links in the transfer sequence is usually limited to 4 or 5. The shorter the food chain (or the nearer the organism to the original food source) the greater the availability of energy which can be converted into "biomass". For this reason aquaculturists can produce fish more efficiently if they culture herbivorous fish.

Three types of food chains are often recognized in water bodies as on land (i) the predator chain start from a plant base through small animals to larger animals; (ii) the parasite chain, from larger to smaller organisms and (iii) the saprophytic chain, from dead matter into micro-organisms.

Weight Relationship of Components of Biota and Dissolved Organic Matter in a Lake

(After Welsh, 1957)

Concept of productivity

The biological productivity of aquatic systems, as of land, has been at several levels, the basic or primary productivity which is again divisible into gross and net primary productivity, and secondary productivities at the various trophic levels (discussion preceding). Basic or primary productivity is defined as the rate at which energy is stored by photosynthetic activity of producer organisms (chlorophyll bearing organisms, mainly plants and phytoplankton) in the form of organic substances which can be used as food substances.

Gross primary productivity is the total rate of photosynthesis including organic matter used up in respiration during the measurement period (also known as total photosynthesis or total assimilation). Net primary productivity is the rate of storage of energy as food matter i.e. excluding the energy dissipated as respiration by plants (also referred to as "apparent" photosynthesis or net assimilation). Usually a value obtained for the rate of respiration of plants is added to apparent photosynthesis to obtain estimates of gross primary productivity.

The rates of energy storage at trophic levels of consumers and decomposers are referred to as "secondary productivities"; these rates as already indicated are less and less at each succeeding trophic level. To be correct, at secondary productivity level, there is only 'assimilation of food already produced by the autotrophs (plants) at each succeeding trophic levels (heterotrophs) and as such the term 'productivity' should not be associated with these trophic levels.

The term "productivity" is always used in this context to mean 'rate of production' or 'rate of energy flow'. Gross production (P_G) and net production (P_N) are secondary productivities (P₂ - P₅) are indicated in the figure (energy flow).

Primary production in fish ponds

Several studies have been made in estimating the primary productivity of fish ponds. One of the more important studies in this respect is that of Hepher (1962) who compared the primary production in ponds; at different levels of fertilization. Hepher obtained rates of primary production at different depths in the fish pond. The results obtained by him are included in Fig.10.4. Hepher found that a very high initial fertilization did not result in high primary productivity, but medium level of fertilization, where addition of fertilizers to pond was done every two weeks in smaller quantities were more effective. The productivity depth curves for standard level of fertilization (60 kg superphosphate + 70 kg ammonium sulphate per ha every two weeks) and double this level indicate this (Fig.10.4). Even though the double fertilization resulted in higher gross production at the surface total production, considering the whole depth of the pond was higher in pond fertilized with standard fertilization. This has important bearing for fertilization regimes in fish pond. These tests have to be done under existing pond conditions and recommendation made.

Relationship of primary production and fish production

From the above discussion it would be obvious that fish production (FY) (secondary production) in natural and tended waters should be related to primary production (P_G) of water bodies concerned. As indicated already several attempts have been made to correlate these FY with P_G.

We also indicated that the dissolved solids (nutrient load) measurable as conductivity of the water concerned, have been well correlated with the production of fish (fish yield). The Mospho-Edaphic Indix (MEI), referred to above, (the productivity of the water bodies is directly correlated with conductivity and inversely correlated with depth of the water body), has been found to be correlating well with fish yield in African lakes (Henderson and Wellcome, 1974; Melack, 1976). Melack compared fish yields of 8 African lakes with corresponding P_G and obtained a relation:

Log. FY = 0.113 P_G + 0.91; r = 0.57

where, FY = fish yield in kg ha^-1/yr^-1

P_G = Gross photosynthesis in g O₂ m^-2 D^-1

He found even a better correlation between FY and P_G for a group of tropical lakes in Madras, India.

FY = 0.122 P_G + 0.95; r = 0.82.

It would appear that this relationship could be more obvious when production of lakes stocked with herbivores fishes are considered, as in the case of Madras lakes (Srinivasan, 1972).

(The MEI was poorly correlated (r = 0.004) for African lakes, but the correlation was better for the Madras lakes:

FY = 4.1 (k/z) 0.80; r = 0.60,

where 'k' is conductivity and 'z' is depth)

Fish yields in fish ponds have also been correlated with primary productivity in manured fish ponds in Israel by Noreiga-Curtis (1979). His observations are presented below:

P_G in relation to depth in fish ponds. Values obtained by Ali (Aluu) are given with reference to different times prior to and after fertilisation. Hephers (1962) values for fertilized ponds at Israel

Measurement of productivity

Several methods are used in measurement of productivity or rate of production.

Harvest method:

This is the simplest and measuring the productivity of a water body such as fish pond by harvest at the end of the season. The productivity given is secondary productivity and indicates net productivity and also quite often fish production given is in net weight giving productivity value.

Oxygen measurement method

Primary productivity can be measured from the amount of oxygen consumed by a volume of water in a fixed period of time; water for which productivity is to be determined is enclosed in sealed white and dark bottles (bottle painted dark so light would not enter). Do (dissolved oxygen) measurement of water is made at the beginning of the immersion period. The two bottles are then immersed in the water body concerned at the level from which the water is taken. The phytoplankton and other elements in the water produce oxygen in the water bottle, but some oxygen disappears due to respiration. The latter is measured from the readings of dark bottle, where only respiration takes place. Thus from the oxygen produced by photosynthesis of enclosed organism (representing a sample of the water body) can be known. However this oxygen production indicates net primary productivity only. From the DO difference in dark bottle oxygen consumed by the enclosed organisms can be obtained and when this respiration value is added to the oxygen production in the white bottle, a value for gross primary productivity is obtained.

Diel method

Estimates of primary productivity can also be made from diel changes in oxygen, considering the day as the light bottle and night as the dark bottle. The increase in DO in the day time is net primary production and the decrease in the night is half the diel respiration. This can be added on to the day-time gain to obtain daily gross photosynthesis. This volume should normally be corrected for the loss or gain in oxygen due to concentration gradient over the day.

C₁₄ method

The most accurate method for determining productivity is the method of using radioactive carbon (C₁₄) added as carbonate. Labelled carbonate is added into a bottle containing water with the phytoplankton and other organisms and after a short period of time the plankton is separated, dried and planchetted and the radioactive carbon fixed can be measured from the radioactive counts made. The productivity measured thus is net primary productivity as the carbon fixed in the tissues only are measured here.

In selecting a water body for aquaculture measurement of primary productivity and estimation of potential yield would be of great assistance in planning the culture activity. This would be specially done while evaluating water bodies (natural or man-made) for stocking(in extensive culture) and also for cage and enclosure culture.

References

http://www.scribd.com/doc/14179924/13-Structure-and-Function-of-Ecosystem

www.fao.org/docrep/field/003/AC176E/AC176E05.htm

biological-productivity

www.wikipedia.org/wiki/Ecosystem