Estelar. Trophic Structure of the Reservoir. Chapter IX

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1 Chapter IX Trophic Structure of the Reservoir Any reservoir after formation passes through three distinct phases-initial high fertility, trohpic depression and final fertility. The newly formed reservoir inundates vast areas of forest and agricultural land and consequently decay of submerged vegetation releasing nutrients causes initial fertility leading to intense development of fish food organisms-plankton, bottom micro flora and fauna. This stage lasts for 2-3 years and is followed by trophic depression stage, caused by rapid utilization of nutrients by flora and subdued release of nutrients from reservoir-bed due to sedimentation. This phase is marked by low production of fish-food organisms and lower fish growth hence lesser production. In Indian reservoirs, trophic depression is expected for shorter duration and varies from reservoir to reservoir. After depression period, the reservoir recovers with accumulation of nutrients. The final fertility of reservoir on stabilization is somewhere near half the magnitude of the initial phase, which gets adjusted to the basic productivity of food webs are well documented, notably in wetlands (Brinson et al., 1981; Mitsch & Gosselink, 1993). The Florida Everglades is a large wetland characterized by unusually dense mats of periphyton that have been implicated as the primary energy source for aquatic foodwebs (Browder et al., 1994; Radar & Richardson, 1994). Though dual detritus and photoautotrophic contributions to energy flow are widespread (Moore & Hunt, 1988), the detritus contributors may equal or exceed photoautotrophic ones (Hairston & Hairston, 1993; Wetzel, 2001); details of the relative contribution of -244-

2 detritus in supporting higher trophic levels are poorly understood (Moore et al., 2004; Sobczak et al., 2005). Maximum chain length of a food web is determined both by addition and subtraction of top consumer species and changing diets of intermediate consumers (Post, 2002). Food-chain length may be affected by myriad abiotic and biotic factors in the environment, including resource availability, environment stability, ecosystem size, colonization history, and predator-prey body size ratio (Pimm, 1982; Briand and Cohen, 1987; Wootton and Power, 1993; Post, 2002 and Jennings and Warr, 2003). The economically significant, but biologically susceptible groups with short food chain and better conversion efficiency always become easy victim of exploitation (Lorenzen, 1995). Jhingran (1992) opined that forage fishes are invariable component of food chain and any imbalance may have deleterious effect over production capabilities of that water body. In the present case also different categories showed dependency over other groups. Abundance of weed fishes sustained more carnivores, but less major carps. Productivity of a reservoir is dependent on biogenic capacity to transform solar energy into chemical energy. The energy fixed at primary producer level passes through trophic chain and fraction of it ends up as fish flesh. Therefore, the structure of different food biotic communities (trophic dynamics) assumes great significance to reservoir fishery management. Shortening food-chain will lead to higher rates of fish production, but in reservoir management there is little scope for changing community structure of plankton to increase primary productivity. However, alterations in species spectrum of fish may be done, and for this only stocking of fish is a successful -245-

3 tool in management (Ayyappan et al., 2006). Observations and Discussion The basic knowledge of aquatic ecology is important in management production aspects of reservoirs. The aquatic ecosystem consists of biotic communities of producers (autotrophs), consumers (heterotrophs) and decomposers (saprophytes). The phytoplankton is the largest group, involved in autotrophic production processes, called primary producers. The studies conducted in Bhavanisagar, Nagarjunsagar, Rihand and Gobindsagar reservoirs have showed that photosynthetic efficiency was high in productive reservoirs like Bhavanisagar (0.412%), Nagarjunasagar (0.290%) and Gobindsagar (0.682%) but low in unproductive Rihand (0.202%). It is interesting to note that energy harvest as fish was much low in Nagarjunasagar (0.055%) as compared to Bhavanisagar (0.290%), which showed that management failed in harvesting fish from Nagarjunasagar though it was productive (Ayyappan et al., 2006). The photosynthetic efficiency was moderate in the Baigul reservoir (0.382%). The Baigul reservoir has two distinct ecological zones Viz. lotic and lentic. The lotic zone has sparse phytoplankton and zooplankton populations. Only macrophytic vegetation is present in the lotic zone, which provides good substrate for the production of periphyton. Lentic zone on the other hand has higher population of phyto and zooplankton; macrophytic vegetation chiefly consists of surface floating weeds such as, Eichhornia. The highest population density of phytoplankton and zooplankton in lentic zone was found to be cells/l and ind/l, respectively. In the lentic zone, as such, four interdependent trophic levels can be recognized: -246-

4 The first trophic level consists of phytoplankton, represented chiefly by green algae, diatoms, blue-greens and dinoflagellates. The second trophic level consists of zooplankton, represented mainly by rotifers, cladocerans and diaptomids (copepods). The organisms of the second trophic level feed on phytoplankton. As such, their density is controlled by grazing rate and other biotic factors. The reservoir also harbours a good population of herbivorous and omnivorous fish like Labeo gonius, L. calbasu, Puntius sarana, etc. Of them, some are euryphagus and feed on phytoplankton as well as zooplankton. The organisms of the third trophic level are the large fish, which are both omnivorous and carnivorous, such as Notopterus spp, Gudusia chapra, Channa spp., etc., which subsist on organisms of the above trophic levels. The fourth trophic level consists of carnivorous birds and predatory fishes, such as Heteropneustes fossilis, Clarias batrachus, Wallago attu, Channa marulius, etc. A large number of migratory birds visit the reservoir every year in winter, which feed on the organisms of the above trophic levels including fish larvae, small fish and other organisms like molluscs, etc. A consideration of the numbers of the organisms belonging to the above trophic levels indicates that the numbers go on decreasing as we proceed on from lower to the higher trophic levels. However, the biomass of the organisms belonging to the higher trophic levels is always more than the preceding trophic level. As such, the biomass of large predatory fish and predatory birds is always more. The complex food webs indicating the interrelationships of -247-

5 organisms of different trophic levels are shown diagrammatically in Fig Trophic structure is largely determined by species richness in the ecosystem, which regulates the flow of energy through the organisms belonging to different trophic levels. The flow of energy among the organisms of different trophic levels indicates that the organisms of higher trophic levels subsist on a great variety of lower organisms. During energy transformation operating on the above trophic pyramid, the energy fixed by primary producers passes through different trophic strata and efficiency of the conversion is reduced to a tenth from one level to the next higher level (Ayyappan et al., 2006). Therefore, the total amount of energy trapped by the organisms of the successive trophic levels goes on decreasing. In other words, the amount of total energy passed from one level to the next is decreased, because of heat loss. The reservoir has large pool of decaying organic matter particularly in the lotic zone. This is utilized by a variety of organisms including microbes, zooplankton, benthic organisms and fish. The full potential of decaying organic matter, however, remains underutilized and may be fully utilized by introducing organisms, which feed on this type of matter, specially the fish such as Cirrhinus mrigala and other like species

Boxes of varying sizes indicate percentage biomass of different components.

6 E E D 1 D 2 B 1 C B 2 B 3 A 1 Figure 9.1 : Basic diagram of plankton food-web in Baigul reservoir. Arrows of different sizes indicate relative proportions of food taken by heterotrophic organisms. Boxes of varying sizes indicate percentage biomass of different components. Abbreviations used: A1 = Blue-greens A2 = Phytoplankton (except blue-greens) B1 = Rotifers B2 = Copepods (except Mesocyclops leuckerti) B3 = Cladocerans C = Mesocyclops leuckarti (adult = juveniles) D1 = Omnivorous fish D2 = Herbivorous fish E = Predatory fish and birds A

Unit 3 Lecture 3 Food chain, food web, ecological pyramid

Unit 3 Lecture 3 Food chain, food web, ecological pyramid Food chain, food web, ecological pyramid Definition of food chain The transfer of food energy from the source in plants through a series of organisms with repeated eating and being eaten is referred to