S T U D I E S O N T H E D I U R N A L V A R I A T I O N S I N P H Y S I C O - C H E M I C A L A N D B I O L O G I C A L F A C T O R S

I N T E P P A K U L A M TANK*

BY SUMITRA VIJAYARAGHAVAN

[Department of Biological Sciences, Maducai University, Alw-:~'~-2 (S. India)]

Received May 18, 1970

(Communicated by Prof. S. Kris~q~.sw~ani, r.,.x.~c.)

ABSTRACT

l. Diurnal variations in physico-chemical and biological factors were studied in Teppakulam tank. Samples from the surface and near the bottom were co!tooted at 3-hour intervals for 24 hours. Results of an average of five series in winter and three series in summer are presented.

2. Water temperature increased during daytime and decreased during night hours. Diurnal variation in temperature an~ounted to 6.8 ° C at the surface and 5-l: C at the bottom.

3. There was no marked diurnal variation in pH values both in summer and winter. Differences between the maximum and minimum levels of oxygen over 24 hours amounted to 2.4 ml/l at the surface and 2.1 ml/l at the bottom.

4. Carbonate and bicarbonate atkalinities revealed an inverse cor- relation. The variations in the above-mentioned factors during the diurnal cycle was attributed to the biological activities taking pI:~ce in the medium.

5. The phytop!ankton exhibited peak of abundance at daytime and reduction at night hours at the surface. Fluctuations in chlorophyll values was found to be closely associated with Chloropby-ceac which formed the dominant group among the phytoplankters.

6. The zooplankton population was mainly composed of Copepoda and Cladocera. They revealed a characteristic diurnal pattern of day- time decrease at the surface and increase during nigh*, hours both in summer and winter.

7. An inverse correlation betweea phyto- and zooplankton popula- tion recorded in the present study was attributed to the grazing activity of the zooplankton.

*Formed part of Ph.D. thesis submitted to Madurai University. 63

64 SUMITRA VIJAYARAGHAVAN

8. The numerical abundance of the phytoplankton during daytime and surface concentrations of the zooplankton at night hours indicated the overall effect of light causing quantitative variations in the planktonic organisms at different hours of the day.

INTRODUCTION

STUDIES on diurnal variations in physico-chemical and biological factors are extensive x/zith reference to temperate waters. Similar worl<s with refer. enee to the tropics are comparatively few. "l-he publications of Carter and Beadle (1930), Ganapati (1949, 1955), Vaas and Sachlan (1953), Schutte and Elsworth (1954), George (t961), Michael (I964), Krislmamoorthi and Visweswara (1965) and Verma (t967) are noteworthy. The present study was carried out in order to understand the inter-relationship between the physico-chemical and biological factors in a tank over a period of 24-hour on a seasonal basis. Although in the tropics the different seasons of the year are not clearly demarcated as in the temperate region, yet a period of higher temperature, longer hours of bright sunshine and absence of rainfall ( 'Summer'--March to July) and a period of low temperature, relatively shorter hours of sunshine and heavy rains ( 'Winter '--August to December) can be readily recognised at the latitude at ~hich the present studies were carried out. These two sets of conditions are taken into account for lhe present studies and in future for the sake of convenience, reference will be made as sunnner and winter.

DESCRIPTION OF THE TANK

Teppakulam tank located within the municipal limits of Madurai City (long: 78°9 'E ; fat. 9 ° 54' 30"N) is an artificial temple ta~.tk built around 17th century. It is ahnost perfect square measuring 303-0 meters from North to South and 290.0 meters from East to West. In the middle of the tank is an island (46-3 × 46-3 meters) with parapet walls all around and steps presenting a vertical face to ~he water. The water from neigh- bouring river vaigai fills up the tank every year during January and February. The tank when full has a maximum depth of 2.5 meters and minimum depth recorded during the course of the study was 0-6 meter. There are no submerged or rooted vegetation.

MATERIAL AND METHODS

Data pertaining to the diurnal variations in the water temperature,, pH, dissolved oxygen, alkalinity, transparency and plankton were collected

Diurnal Variations in Physico. Chemical amt Biglogical Factors 65

at three hours interval frcm 9 A.M. on one day to 9 A.M. the next day. Water samples were collected near the central island to avoid any distur- bance due to washing of cloths or bathing.

Pump and Hose technique (Welch, 1948) was adopted to obtain water samples from the surface and near the bottom of the tank as it was found to be the most convenient and quickest method available. Temperature was recorded with an ordinary thermometer graduated frem 0~50 ° C. pH was determined with B.D.tt. narrow range pH papers in the field and later the values were corrected with a pH meter in the lab-oratory. Un- modified Winkler's method was adopted for the estimation of dissolved oxygen (Ellis etal., 1948). Alkalinity was determined by using N/50 sulphuric acid with phenolphthaleirl and methyl orange indicators (Welch, 1948). Transparency was measured with a seccki disc.

The plankton samples were collected along with water samples. Hun- dred litres of water was pumped out from the surface and bottom layers of the tank, passed through a fine lawn net (92-6 squares,linear inch). The plankton concentrate was preserved with 5% formalin. Enumera- tion of the phyto-and zooplankton was done by taking 1.0 ml of sub- sample and counting the different groups of algae and the zooplankters in a counting slide under 12 × 3 magnification. The phytoplankton are expressed as units3itre and the zooplankton as organisms/litre.

As a coarse net was used to assess the phytoplamkters the counts would be less than what would have been obtained with bolting silk No. 25, there- fore it was thought that expression of phytoptankters in terms of chlorc- phyll values would be more accurate. So a standard graph was drawn correlating the chlorophyll values (X-axis)with phyteplax~kten count (Y- axis) using the seasonal data of this tank, collected at monthly intervals. More or less a linear relationship was found between the phytoplankten count and chlorophyll values (chlorophyll estimation of the monthly samples were made according to Strickland and Parsons, 1965). From the standard graph (Fig. I), the chlorophyll values were derived for the phytoplanktorl count obtained during the diurnal cycle.

RESULTS

The results of the diurnal variations in physico-chemical and biological factors during summer and winter are given in Tables I and II. The results r~'present the mean of a total of eight, five series taken during winter ar, d three series taken during summer.

66 SUMITRA VIJAYARAGHAVAN

DISCUSSION

The water temperature showed a definite diurnal trend of increase in daytime and decrease during night hours as ret;orted also by Kato (1941), Michael ( t964)and Verma (1967). The diurnal variation between the maximum and' minimum temperature over the whole year was about 6 .8°C at the surface and 5"1 ° C at the bottom. No definite correlatioh could be established between temperature and oxygen content in the present studies, which is in contrast Kato (loc. cir.), Itazawa (1957) and Michael (loc. cit.) who have reported a direct correlation between the two factors.

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George (196I), Michael (1964) and Verma (loc. cit.) reported a gradual increase in pH during daytime and decrease at night hm'r3. Lauff (1953) and George (lot. cir.) have shown that there exists an apparent correlation between pH and bicarbonates. No suck co~relations were observed in the present studies and there was no marked difference in pH values during the daytime and night hours.

The difference in the dissolved oxygen content over 24-hour amounted to 2-4rnl/l at the surface and 2.1 ml/1 at the bottom. Although the phyto- plankton peak occurred at 12 noon (Tables I and II, Column 10) tke oxygen values recorded a gradual increase from 3 P.M. to 9 P.M. (Tables I and II, Column 3). Tiffs might be probably due to the cumulative effect of photo-

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Diurnal Variations in Physico-Chemical and Biological Factors 69

synthesis and subsequent accumulation of oxygen with time lag. This is in accordance with Goldman (1968) who suggested that some of the oxygen produced by the aquatic plants may not be immediately released, thus causing a lag period in the evolution of oxygen.

The diurnal change in carbonate alkalinity was more pronotinced than that of the bicarbonate alkalinity. In general, the carbonate and bicarbo- nate alkalinities revealed an inverse relationship (Tables I and II, Colunms 4 and 5). Thus, the carbonate alkalinity revealed an increase during daytime and decrease at night; while the reverse was true with reference to bicarbonate alkalinity. This can probably be attributed to the photo- synthetic activity taking place during daytime, resulting in the removal of half- bound carbon dioxide from bicarbonates leaving certain amounts of carbonates in the water which rises carbonate values. At night the respiratory activity of both phyto- and zooplankters occurs together with the absence of photosynthetic release of oxygen. Tkis leads to carbon dioxide accumulation and increase in bicarbonate levels at night.

The total phytoplankton population represented by chlorophyll values exhibited maximum concentration at noon both in summer and winter thereby indicating the photosynthetic activity during the daytime and sub- sequent reduction in cklorophyll values at night hours. However, it must be pointed out here that the chlorophyll values recorded in the present study cannot be interpreted in terms of net phytoplankton a!one, as there is also the possible contribution from nanoplankton. Nevertheless, among the three groups of algae, the fluctuations in ch[orophyte population and ckloro- phyll values show more or less parallel trends (Tables I and II, Columns 9 and 10).

The total zooplankton composed mainly of Cladocera and Copepoda showed a characteristic diurnal pattern of increase at night hours and decrease during daytime. This is in agreement with the findings of Vans and Sachlan (t953), George (1961), Michael (1964), Jolly (1965) and Verma (1967). ttow- ever, Krishnamoorthi and Visweswara (1965) reported a daytime rise of the Daphnids and copepods to the surface and descent to the bottom during night hours.

It is interesting to note that the total count for zooplar&ton both at the surface and bottom layers show an increase during dark hours. Usually increase in zooplankton concentration at the surface during night hours is attributed to their migration from the bottom layer to the surface. Reduction in the zooplankton numbers during daytime at the surface is due

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Diurnal Variations hz Phy~ico-Chemieal and Biological Factors 71

to their descent to the bottom layer. Such a situation is met with in the present study only during summer. In winter, the surface and bottom layers show quantitative increase at night while, during daytime, the count for zooplank- ton in both the layers appears to be somewhat equal. This suggests the possibility of the organisms taking shelter at the soil-water interface during daytime and their subsequent ascent to the surface at night.

Although in the present studies no attempt has been made to study the diurnal pattern of different phyto- and zooplankters at the species leve!, nevertheless the groupwise study has revealed a typical diurnal pattern of rise in chlorophyll values (as an index of phytoplankton population) during the daytime and upward migration of zooplankton at night hours. This illustrates the overall effect of light causing quantitative variations in plank- ton at different hours of the day. Similar conclusion has been drawn by Welch (]935), Pennak (1944), Lagarigue (1949), Ha.a-is (1953), Baylor and Smith (1957) and Wells (1966). Tash and Armitage (t960) reported that light, o~,erturn, size, thermocline and ~avity were important factors that affect the vertical distribution of zooplankton. Michael (loc. c#.) stated that in addition to light, probably dissolved oxygen and free COs also affect the pl~ktonic movements. Jolly (loc. tit.) reported that the depth and transparency of the lake influences the rhythm of migration. No such correlations are evident in the present study.

Throughout the period of investigation an inverse relationship was evident between the phyto- (represented by chlorophyll values) and zoo- plankton population (Fig. 2 A, and B). In the absence of any evidence in the present work in support of tile theory of animal exclusion by Hardy (cited by Anderson et al., 1955), the inverse correlation could probably be attributed to the grazing effect of the zooplankton upon the phytoplank- ton. This is in agreement with the findings of Riley and Bumpus (1946), Bainbridge (t953) and Raghu Prasad (1956) who all have attributed varia- tions and inverse relationship between phytoplankton and zooplankton to the grazing effect.

ACKNO~EDGEMEN'rs

The author wishes to acknowledge Dr. S. V. Job for unfailing guidance and help throughout the course of tbSs investigation. Tha_uks are due to Prof. S. Krishnaswarni for his keen interest and valuable suggestions and to Dr. R. George Michael for his constant help in preparation of this paper.

72 S UM'ITRA VIJAYARAGHAVAN

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Dmrmd Variations in Physico-Chemical and Biological Factors 73

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