CHAPTER- V TROPHIC STATUS THE LAKE

Transcription

1 CHAPTER- V TROPHIC STATUS OF THE LAKE

2 TROPHIC STATUS OF THE LAKE The trophic status of lakes can be determined through physiographical, chemical and biological parameters. Fresh water lakes have been classified according to their biological productivity. The physiographical parameter include mean depth (Rawson, 1956) volume development (Zafar, 1959) and the ratio of epilimnion to hypolimnion (Pennak, 1955). Naumann (1927), Strom (1924), Rao (1953) and Zafar (1964) have adopted the above factors for the classification of lakes. According to them, there is no differentiation in lakes from ponds except only in dimensions. Welch (1952) proposed a common name 'Lake' for all the stagnant fresh water bodies irrespective of their dimensions. Strom (1924) was the first to adopt chemical criteria like the richness (+) or paucity(-) of water in calcium, nitrogen, phosphorus and humus for lakes, which are accepted universally. But the expression rich and poor still remain relative having no mention of the qualifying concentrations in literature. Probably they can be substituted by the ranges and averages proposed by Wetzel (1975). In this regard, average concentration of these elements occurring in fresh waters of the world over given by Bowen (1966) and Wetzel (1975) could also be made use of. Thus it is attempted to fit the present fresh water bodies in to the general oligoeutrophic series which are being used in the present study. Miralam Lake found to be high in bicarbonate, calcium, phosphorus, and nitrate concentrations as given by Wetzel (1975), to include in oligo-eutrophic series. The present lake comes under euotrophic due to high concentration of various ions. The 158

3 accepted limits of inorganic nitrogen and orthophosphate are 0.3 mg/1 and 0.02 mg/1respectively (Clark et al., 1977). Another classification which is based on the basic ratio of N/P and humus content overcomes these difficulties to some extent and appears to be more accurate (Zafar, 1959). Higher values of calcium in fertile lakes were reported by Allanson (1961), Palmer (1967), Gorhan et. al; (1974). According to Sahai and Sinha (1969), high Ca 2+ content is an indicator of the eutrophic waters. The present water bodies are high in calcium concentration which indicates their eutrophic nature. Therefore from the chemical view point, the water bodies under investigation can be placed under eutrophic category. Oligotrophic surface waters are relatively unproductive and poor in nutrients, while eutrophic lakes exhibit abundant plant and animal life, owing to high nutrient loadings. Everywhere, the relative nutrient enrichment of aquatic bodies has been a direct consequence of social or cultural advances made by growing human populations. The present Mir alam lake was high productivity indicating its eutrophic nature. Increased nutrient loading in a water body usually increases its capacity to support greater production and maintain larger standing crops of phytoplankton, put at its simplest, the more the nutrients are the more the algae (Harries, 1980 b and Reynolds, 1978 C). Vollenweider (1968) related nutrient supply to mean depth in several lakes in the 159

4 world and identified nuisance levels associated with induced eutrophication. From these relationships shallow lakes clearly seem to be more sensitive to nutrient income than deeper lakes, because the concentration effect increases as the lake depth decreases, creating higher concentrations as well as increased nutrient reuse. As the present water body are shallow, nutrient supply is high, organic matter is high and dissolved oxygen is low indicating their eutrophic nature. Normally lakes that show depletion of dissolved oxygen in hypolimnion during thermal stratification are classified as eutrophic, while those that maintain high oxygen levels throughout the year are called oligotrophic. Eutrophication diminishes water quality, more algal blooms prevail, and the water at Miralam Lake has shown low concentration of dissolved oxygen throughout the period of investigation. So they can safely be placed under eutrophic category. The presence of different phytoplankton is a parameter to know the eutrophication level of water bodies (Devaraju et al., 2005). Generally low productivity indicates oligotrophic nature of the lakes and high productivity indicates eutrophic nature (Wetzel 1983). Algal dynamics in relation to some factors causing eutrophication was investigated by Tiwari and Kumar (1985). The blooms of Cyanophyceae characterize eutrophic waters, especially in late summer represented by the species of Microcystis, Oscillatoria, Lyngbya etc. (Patrick, 1965). Eutrophic lakes also often have large summer growths of Chlorococcales such as Pediastrum, Scenedesmus, Dictyosphaerium, Crucigenia, Tetrahedron, Chlorella etc. in small lakes and ponds (Round, 1957). 160

5 Cyclotella occurs both in oligotrophic and eutrophic waters. Wetzel (1975) and Sandgren (1988) recorded it in oligotrophic waters. In contrast Sudhakar et al; (1994) and Sudha Rani (2004) recorded it in eutrophic waters. The present study in agreement with their observation. In the present study Chlorophycean members in general and Chlorococcales In particular like Pediastrum, Scenedesmus, Dictyosphaerium, Crucigenia, Tetrahedron, Chlorella species were found in large numbers. At times Pediastrum and Oocystis fall under oligotrophic category due to lower quantities of nutrients in the water body. This is in contrast to the observations made by Rawson (1956) and Sandgren (1988) then placed these algae in mesotrophic waters. Prescott (1948) reported that Chlorophycean flora dominates in the oligotrophic waters with conspicuous desmid members. Strom (1924) and Pearsall (1930 and 1932) reported that the waters favoring Chlorophycean blooms are distinct from waters which support Cyanophycean members. The latter dominates eutrophic waters (Rawson, 1956) where as the former fails to tolerate high nutrient levels and both are the best indicators of oligotrophic waters (Rawson, 1956; Patrick, 1965; Brook, 1965; Palmer, 1969; Jyothi, 1990; Garg et al., 2006 and Ashesh Tiwari et al; 2006). Round (1957), Duthie (1965) and Brook (1965) point out that desmids are generally believed to favor oligotrophic waters. This is supported by the present investigation at station I as diverse Cosmarium, Closterium species were recorded, and this is in agreement with the findings of above authors. Micro-green algae form the dominant populations of phytoplankton in 161

6 oligotrophic waters. This is true both in terms of biomass and numbers and also in relation to the amount of organic carbon fixed by photosynthesis. Diatoms form an important part of phytoplankton of many water bodies. INDICATOR SPECIES The representation of certain species of fresh water plankton is often a sensitive indicator of trophic level. Rawson (1956) has suggested an approximate distribution of limnetic algae in lakes of Western Canada over the range of trophic state. According to him the oligotrophic species include Pediastrum, Coelosphaerium, Microcystis and Anabaena. Although Rawson's list of species is not accepted worldwide, it nevertheless offers an example for the type of approach that can be useful in establishing trophic state and change. Mir alam lake supports luxuriant growth of macrophyte vegetation. Eichhornia and Typha occupied most parts of the lake. On the basis of foregoing account, the Miralam lake can be brought under the category of eutrophic with high primary production, high plankton density, less species diversity and high nutrient supply. From the account, it appears that the biological indicators are more reliable to the trophic status of a lake rather than chemical factors (Round, 1981). 162