Diel variations in limnological characteristics of Omkareshwar reservoir of Narmada river , India

The present study was carried out to ascertain the magnitude of seasonal diel variations in physicochemical and biological variants with reference to phytoplankton, zooplankton and primary productivity at newly formed Omkareshwar reservoir on the river Narmada for the year 2012. Diel variations in physicochemical parameters such as temperatures, pH, transparency, total alkalinity, total hardness, chloride content, dissolved oxygen, biochemical oxygen demand (BOD) and chemical oxygen demand (COD) were observed between 1100 and 0700 h. No phytoplanktonic diel variations have been observed, but zooplanktons were well known for their diel variations as they can actively swim towards most suitable environmental niche in the river. The result shows the oligotrophic nature of this reservoir. Maximum value of total phytoplankton (271.97 Org/L) was observed around 1100 h in summer but the highest value of total zooplankton (930.78 Org/L) was recorded at 2300 h in summer season. Maximum value of net primary productivity (600 g C/m 3 /3 h) was noticed at 1100 h in summer season. The suitable correlation coefficients were calculated for 15 pairs of variables and correlation matrices were then established seasonally. Net primary productivity (NPP) depicted a positive correlation with dissolved oxygen and phytoplanktonic productivity (r = + 0.577 and r = + 0.459), respectively.


INTRODUCTION
River plays a major role in assimilating or carrying Industrial and municipal waste water, manure discharge and run off which are responsible for river pollution (Choudhary et al., 1991;Toman, 2009).In India, it is important to study the status of pollution of the rivers in relation to various anthropogenic activities as river water has been used as drinking water, for mankind (Suther et al., 2010).River is vital and vulnerable freshwater ecosystems that are critical for the sustenance of all life.The physicochemical properties affect the nutrient status of water body hence trophic status depends on rate of energy flow which may be assessed by estimating primary productivity (Dudani et al., 1987;Zambave et al., 2004;Iqbal et al., 2004).The study of diel cycle is important for a number of obvious reasons.Diurnal fluctuations in these parameters adversely affect the population of an aquatic ecosystem.Practically the entire wheel of life, represented by a vast array of the number of organisms is powered exclusively by the solar energy that is incorporated by the primary producers during day time.Correlated with this rhythm in light intensity are also the rhythms of temperature and humidity and of the tidal cycles due to the effect of the gravitational pull and its modalities due to planetary movements (Schevt et al., 1997).Environmental conditions may change rather dramatically within aquatic ecosystem during a 24 h period.Solar radiation varies from high intensities at mid-day to darkness.Surface temperature and concentrations of dissolved gases may also fluctuate between extremes of day and night, especially in shallow, littoral areas (Wetzel, 1979;Abowei (2010); Singh et al (2010); and Sipkoska-Gastarova et al (2010).
The planktons form the basic link of food chain for all aquatic animals and thus play a pivotal role in fisheries.The direct and indirect alterations in physicochemical parameters of water affect the survival of primary productivity and growth of planktons (Shastri et al., 2000;Gang et al., 2006).Factors which are known to influence the vertical migrations of planktons on diel basis are cellular periodicity that is independent of light; that is, circadian rhythm and the nutrient depletion, presence of anoxic water, under water light regime and temperature or density gradients.The present investigations revealed that by and large the phytoplankton did not show any clear diurnal movement and remain concentrated at the surface or near surface zone.The observed irregular diel migration may be the result of passive movement of phytoplankton by turbulence and water movements due to wind action.Temperature of the water also has been considered as determining factor for the seasonal distribution of the fauna and flora.The phytoplankton was found to play the main role in determining the pattern of fluctuations in the yield of total plankton (Pant et al., 1983;Kamat and Sima, 2000).In this investigation an attempt has been made to corroborate the interrelationships of some physicochemical features of reservoir with the primary productivity and health of phyto-planktons.

MATERIALS AND METHODS
Narmada river is the fifth largest river of India, which is known as the 'Life Line' of Madhya Pradesh (India).It originates from Amarkantak in Madhya Pradesh.The Narmada Valley Development project is the single largest river development project in India, which involves the construction of more than 3000 large and small dams.Omkareshwar is one of the largest dam which has been developed on the river Narmada and it is situated at a distance of about 4 km from famous Mandhata Island (Figures 1 and 2).
Water samples were taken after every four hours at 1100, 1500, 1900, 2300, 0300 and 0700 h in one whole day for diel study.The water samples were analysed, and several physicochemical parameters such as temperature, pH, alkalinity, transparency, total hardness, total dissolved solids, chloride content, dissolved oxygen, biochemical oxygen demand (BOD), chemical oxygen demand (COD) were analyzed (NEERI, 1986;APHA et al, 2001).The chemicals used for analysis were all of analytical R. grade.Primary productivity in terms of net primary productivity (NPP), Gross primary productivity (GPP) and respiration was estimated by using light and dark bottle method (Sharma and Savang, 2004).Quantitative studies of the planktonic population (phytoplankton and zooplankton) were made as per the standard methodology ( Sardana et al., 1980;Sharma and Bhardwaj, 1999;Patil and Patil, 2010).Correlation matrices of different physicochemical and biological parameters for three seasons were incorporated with the values of significance (Tables 1, 2 and 3).

RESULTS AND DISCUSSION
Temperature, being a key abiotic factor in aquatic ecosystem has wide effect on biodiversity of planktons with respect to their growth and other physiological processes.
In the present study diurnal variation in the air temperature ranged between 28.50 to 33.50, 26.50 to 30.70 and 22.41 to 24.00°C during summer, monsoon and winter periods, respectively and the diurnal variation in the water temperature ranged between 24.60 to 27.63, 25.20 to 28.70 and 21.23 to 22.10°C during summer, monsoon and winter periods, respectively.The values of temperature gradually increased from 0700 to 1500 h and thereafter their values slightly decreased till 0300 h.Maximum value of temperature was recorded in summer at 1500 h while minimum temperature was found to be at 0700 h.Air and water temperature revealed a definite diel trend over the study period and changes appear to have been brought about by several factors in addition to diel and seasonal changes in solar irradiance.Although, there is little variation in water than in air, temperature is, nevertheless, a major and critical limiting factor for biotic components because most of the aquatic organisms often have a narrow thermal tolerance (Shankar and Sangu, 1986;Mudgal et al., 2009) (Figures 3 and 4).A significant positive correlation was found between air and water temperature (r = + 0.956) at significance level of 0.01.
Water transparency is an important limiting factor in the growth and distribution of flora and fauna.Diurnal variation in the transparency ranged between 0.00 to 98.00, 0.00 to 64.00 and 0.00 to 116.00 cm.during summer, monsoon and winter periods, respectively.The Secchi transparency gradually increased from 0700 to 1500 h thereafter it was not observed from 1900 to 0300 h.Maximum transparency (116.00 cm) was observed at 1500 h during winter.Although, in the present study the steady rise in visibility was noticed from 0600 h onwards but decline was reported due to the change in the water surface conditions or occasionally even by planktonic blooms.The transparency increasing from morning to evening has an indirect relationship with nutrients and was chiefly affected by the algal growth.Secchi transparency showed an inverse correlation with a number of physicochemical parameters in diel variations (Figure 5).
pH is numerical expression that indicates the degree to which water is acidic or alkaline.All values were within the permissible limit.pH is known for exerting its influence on the occurrence, distribution and growth of planktons.The high pH value of water was associated with greater photosynthetic activity which resulted in utili-zation of CO 2 from bicarbonates and formation of carbo-nates (Baijot et al., 1994).Diel variation in pH fluctuated between 8.10 to 8.40, 8.13 to 8.35 and 8.24 to 8.30 during summer, monsoon and winter periods, respec-tively.The values of pH gradually increased from 0700 to 1500 h, thereafter slightly decreased till 0300 h.Its maxi-mum value (8.40) was observed at 1100 h while mini-mum (8.10) was noticed at 0300 h in summer season.In the present study the pH values showed a definite diurnal trend of daytime increase in reservoir.The rise in pH during daytime indicated high rate of photosynthesis, although the range of fluctuation was usually small (Figure 6).
Total alkalinity of water is due to the presence of mineral salts present in it.It was primarily caused by the carbonate and bicarbonate ions.High alkalinity can be attributed to the rise in temperature which also coincides with the concentration of nutrients and bicarbonates in particular.It has also been observed that relatively higher values of alkalinity were conducive for the growth of planktons.High value of total hardness indicated the greater productivity and surge in eutrophication of water body.Hard and alkaline water may be attributed to high planktonic growth to some extent (Joshi et al., 1991;Sujitha et al., 2011) (Figures 7 and 8).
Total dissolved solids (TDS) are suitable parameter in describing the density of chemicals and as a general measure of edaphic relationship that contributes to the productivity of water (Raja et al., 2008).TDS mainly consists of inorganic salts such as carbonates, chlorides, sulphates, phosphates and nitrates of minerals and small amount of organic matter.TDS was found to be the highest 562.00 mgL -1 at 1900 h in monsoon season, its highest value was found to be due to loss of water as a result of rising temperature and concentration of salts present in water (Figure 9).
The content of chloride gave a concrete idea of organic matter and presence of nitrates in reservoir.Its content has been reported between 20.87 to 27.99, 22.51 to 26.99 and 18.11 to 21.99 mgL -1 during summer, monsoon and winter periods, respectively.Its contents were gradually increased from 1100 to 2300 h; thereafter its values started declining till 0700 h.A maximum of chloride (27.99 mgL -1 ) was recorded at 2300 h in summer.An increase in chloride from 0300 to 1500 h may be ascribed due to the discharge of oils and other harmful substances (Prasad and Patil, 2008) (Figure 10).Dissolved oxygen (DO) is of paramount importance in the study of aquatic status of a reservoir.Oxygen content is indispensible for many organisms and also affects the solubility and availability of many nutrients which has direct influence on primary productivity.The diel varia- tion in dissolved oxygen ranged between 7.50 to 8.40, 6.90 to 8.40 and 8.00 to 8.90 mgL -1 during summer, monsoon and winter periods, respectively.Diel fluctuations in DO showed a gradual increase from 0700 to 1500 h there after it was found to be declining till 0300 h.In the present study maximum DO (8.90 mgL -1) was observed at 1500 h during winter season, while minimum (6.90 mgL -1) was recorded at 0700 h in monsoon .High DO at 1500 h may be due to the high photosynthetic activity during day time.DO finds its way into the aquatic system either by passive nonbiological diffusion at water surface or is liberated by autotrophs as a byproduct of photosynthesis and gets dissolved in water depending on its partial pressure.A marked diurnal pulse of dissolved oxygen content is evident in water depending on the extent of autotrophic production and its modalities under influence of various environmental factors (Figure 11).Dissolved oxygen content and the range of its diel variation had an inverse relationship with total carbon dioxide.The water was under saturated with oxygen during most of the observations and the percent dissolved oxygen concentration behaved diurnally in the same way as the dissolved oxygen values expecting slight modalities during winter probably due to increased solubility.DO was positively correlated with pH (r = +0.837and r = +0.481)during monsoon and winter seasons respectively and negatively correlated with temperature (r = -0.560).Such correlation supports the role of photosynthesis in governing above parameters.Biochemical oxygen demand is defined as the amount of oxygen required by bacteria while stabilizing decomposable organic matter under aerobic condition.The range of BOD was found to be from 12.52 to 17.50 mgL -1 in monsoon and winter season, respectively.The comparison of BOD and DO in the present study indicated that there was an inverse relationship between both parameters (Figure 12).BOD was negatively correlated with DO (r = -0.708,r = -0.516) in winter and summer season, respectively.It showed positive correlation with temperature at 0.01 significance level.
COD determines the oxygen required for chemical oxidation of most organic matter and oxidisable inorganic substances with the help of strong chemical oxidant.In conjunction with the BOD, the COD test is helpful in indicating the presence of biologically resistant organic substances.The range of COD was found to be from 34.30 to 36.50 mgL -1 in winter and monsoon season, respectively.(Figure 13).COD was positively correlated with BOD (r = + 0.968).It showed positive correlation with temperature (r = + 0.804) at significance level of 0.01.Net primary productivity is the best criterion for assessing the trophic structure of a water reservoir.It is equal to organic matter synthesized by photosynthesis minus the rate of respiration.It depends upon light, temperature, water, nutrients and photosynthetic efficiency of producers.In Omkareshwar reservoir, the seasonal values of diel variations in net primary productivity were estimated to be 0.600 gC /m 3 / 3 h in summer which is the highest value.This clearly proved the role of light in net primary productivity.The increase in trophic production is highly significant and   showed rapid rate of eutrophication process in the reservoir.On the basis of primary productivity, the reservoir was found to be oligotrophic (Figures 14,15 and 16).NPP showed negative correlation (r = -0.309)with total zooplanktonic productivity, while it was positively correlated with total phytoplanktonic productivity (r = + 0.185) at the significance level of 0.05.
The present investigations revealed that by and large the phytoplankton did not show any clear diurnal movement and remain concentrated at the surface or near surface zone.The zooplanktons are well known for their diurnal seasonal migration as they can actively swim towards most suitable environmental niche in the river (Pant et al., 1983;Dutta and Malhotra, 1987).The maxi-mum diel variation in total zooplankton population at surface was found during winter and summer whereas the minimal fluctuation occurred during the monsoon season.It is interesting to note that their number was found to be minimum on the surface around 1100 to 1500 h.It seems that the low oxygen conditions at the subsurface levels   forced them to come out at the surface as is evident from almost even distribution of phytoplankton at all the depths (Bhattacharya et al., 1988).During monsoon, the maximum number of total zooplanktons on surface was encountered around 2300 h.
In the present study, the maximum diel variations in total zooplankton population were found at surface during winter and summer season whereas the minimal fluctuation occurred during the monsoon season.The changes in the physicochemical quality of the medium, space and time have a vital influence on the diel, seasonal migration of the planktonic organisms.The diel seasonal migration of zooplanktonic communities is known since long.The interaction of organisms to any one environmental factor is modified by the variations in another making it useless to seek any one factor being most important.Various factors are known to influence the diel seasonal movement in zooplanktons such as light, food, water current, dissolved oxygen (Jakhar et al., 1981).The maxima of total zooplanktonic population were observed at surface during night time.The present investigations revealed that phytoplankton did not show any clear diurnal movement when compared with the zooplankton population at Omkareshwar reservoir of Narmada River (Figures 17  and 18).Now it was concluded that a striking diel variations were observed in selected physicochemical characteristics during 24 h.The physicochemical and biological characteristics have been variously employed in the classification of water bodies and the level of their eutrophication.(Sindhu and Sharma, 2008).The present study will be immensely helpful for further ecological assessment and monitoring of water quality, productivity, fisheries development and reservoir management policy in future.

Figure 1 .
Figure 1.The map showing the Omkareshwar reservoir of Narmada river (India).

Figure 3 .
Figure 3. Diel variations in air temperature of different seasons.

Figure 4 .
Figure 4. Diel variations in water temperature of different seasons.

Table 1 .
Correlation matrix of different physicochemical and biological parameters of winter season.
*Value above 0.51 showing significant at 0.01 level.Value between 0.51 and 0.4 showing significant at 0.05 level.

Table 2 .
Correlation matrix of different physicochemical and biological parameters of summer seasons.

Table 3 .
Correlation matrix of different physicochemical and biological parameters of monsoon season.
*Value above 0.51 showing significant at 0.01 level.*Value between 0.51 and 0.4 showing significant at 0.05 level