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International Journal of Environmental Research
University of Tehran
ISSN: 1735-6865 EISSN: 2008-2304
Vol. 3, Num. 2, 2009, pp. 223-228

International Journal of Environmental Research, Vol. 3, No. 2, Spring 2009, pp.223-228

Effect of the Euphotic Depth and Mixing Depth on Phytoplanktonic Growth Mechanism

Khanna, D.R., Bhutiani, R.* and Chandra, K.S.

Department of Zoology and Environmental Science, Gurukul Kangri University, Haridwar (U.A.), India

*Corresponding author E-mail:rbhutiani@gmail.com

Received 14 March 2008;
Revised 25 Dec 2008;
Accepted 5 Jan. 2009

Code Number: er09024

ABSTRACT

Light is never being distributed homogeneously and it forms a gradient over biomass. The unidirectional nature of light gives rise to a vertical gradient of light intensity as a function of depth. The maximum depth of the light zone suitable for phytoplankton photosynthesis is designated as the euphotic depth. This study was designed to test the hypothesis of mixing depth of phytoplankton and its compensation depth. In a water column undergoing turbulent mixing, where phytoplankton cells are around by the fluid motion, the irradiance encountered by cells will be a function of the ratio between the euphotic depth (zy) and the depth of mixing (zm). During the course of study the zy/zm ratio value were observed in the range of 0.4427 to 3.242. However, identification of Compensation point values for zy/zm indicates that light conditions are suitable for net phytoplankton production, because the mean value (0.3413) was good for the phytoplanktonic growth, which was greater than the minimum 0.20 limit.

Key words: Mixing depth, Phytoplankton, Compensation depth, Euphotic depth, Biovolume

INTRODUCTION

Planktons are primary producers responsible for a large part of the Earth’s global primary photosynthetic proχ duction. These organisms are thus the objects of intensive multidisciplinary studies at different levels of organization, from molecular genetics and physiology to population dynamics and community ecology. The success of these photosynthetic organisms lies in their ability to use solar energy and nutrients and to cope with a fluctuating environment. Thus, light, nutrients, and water mixing plays a key role in the evolution of their life history traits, their physiology and their ecology. The present study deals with the test of the hypothesis of mixing depth of phytoplankton and its compensation depth. Moreover, in recent decades, ecologists have considered to an increasing extent their interactions with other biological communities, as herbivores or decomposers. Biotic factors appear indeed to play a determinant role in the physiological characteristics, adaptive strategies, and dynamics of primary producers. Estimation of phytoplankton assemblage structure can provide a number of useful indications, on the functioning of an aquatic ecosystem, because the growth is related to the light intensity and either background particles (Living or non living) in the water ecosystem. According toGordan (1989), Lambert-Beer’slaw provides a fairly good approximation in aquatic environments. Cullen and Lewis (1988), investigate that the photosynthesis parameters will vary when algae adapt to changes in light availability during vertical mixing. Light limited growth models use mathematical and numerical techniques to simulate the physical and biological processes that affect the growth dynamics of phytoplankton (Khanna et al., 2005). The importance of light in water column is very much studied by different workers around the World (Hansen and Hubbell, 1980, Rhee and Gotham, 1981, Tilman and Sterner, 1984). Ecological interactions including photosynthesis and primary productivity have been the subjects of some work on deterministic modeling of streams (Kelly, 1975, Stehfest, 1977). Reynold (1999) reviewed the conceptual models and its problem with special reference to phytoplankton. Phytoplankton succession is a wellinvestigated phenomenon in aquatic ecology and several studies have described the patterns and underlying mechanisms of the dynamics (Sommer et al., 1986; Marshal and Peters, 1989; Vanni and Temte, 1990; Hansson et al., 1998; Rothhaupt, 2000; Huisman, et al.,1999). To have a better understanding of the factors responsible for changes in the distributive pattern of phytoplankton, it is important to understand the link between changes in environmental parameters and phytoplankton dynamics.

MATERIALS & METHODS

The study was performed on a monthly basis for one year at two different sampling stations of river Suswa. In the present study photosynthetically active radiation profiles (PAR), was measured by Lux meter (Model ESCORP [TES-1332]) and was then converted into unit µ moles m-2 s-1 using conversion factor µmolm2 s−1 =Lux×0.14. A 20 cm diameter secchi disc was used to measure the secchi depth (Ez). While the PAR light attenuation coefficient k (m-1) was calculated according to Sverdrup (1953) using Secchi disc depth (Ez) by applying the formula

Since the water column was permanently mixed, subsurface water samples were taken with a Van Dorn type sampler suspended solids (SS) were performed according to APHA (1998). Phytoplankton samples were collected at both stations for taxonomic identification and were preserved in acid iodine solution (Lugol’s) and stored in glass vials. Sedgwick-Rafter counting chambers (50mm x 20mm) were approached for estimating phytoplankton numbers. Fixed sub samples were analyzed under inverted microscope equipped with contrast illumination: 10X, 40X, and 100X objectives; 10X eyepieces and micrometer eyepieces (graduated in µm).

The cell biovolume and surface area were calculated based on geometric shapes and the average of the microscopic dimensions of 10–30 organisms per taxon (Hillebrand et al., 1999). The classification system of Van Den Hoeck et al. (1995) was followed for Classes and Divisions. Krammer and Lange-Bertalot (1986, 1988, 1991), Tomas (1997), and Round et al. (1992) were used as main taxonomic references for genera. Total biovolume (V) was computed from measurements of abundance (n) and estimates of cell volume (v) (Jun and Liu, 2003):

V = ∑ni vi

Where ni =abundance of genus

I= vi volume for genus i.

The chlorophyll biomass (Chl’) was estimated using the formula sited in Li and Dickie (2001).

The maximum depth of the light zone suitable for phytoplankton photosynthesis is designated as the euphotic depth (zy), and is estimated as the depth where light energy is reduced to 1% of the intensity immediately below the water surface. The depth of the euphotic zone can be calculated from the equation:

In a water column undergoing turbulent mixing, where phytoplankton cells are around by the fluid motion, the irradiance encountered by cells will be a function of the ratio between the euphotic depth (zy) and the depth of mixing (zm). If zm is equals to zy i.e. zy/ zm=1 then the cells are constantly illuminated and photosynthesis is continuous during the daylight period. If the mixing depth is greater than the depth of the light zone then the phytoplankton spend a proportion of their daylight period in the dark where photosynthesis cannot occur but respiration continues. As the zy/zm ratio further decreases, the proportion of time that the cells spend in the light is reduced until eventually net growth cannot occur because respiratory carbon use equals or exceeds the carbon supply from photosynthesis.A number of models have been developed to calculate the daily rate of photosynthesis integrated over the depth of light penetration ( ∑ P), but model proposed by Talling (1957) is used here.

WhereΣP is the carbon fixed under unit surface area per day, and b is the Chlorophyll concentration (mg m-3), PM is the maximum Chlorophyll specific rate of photosynthesis (mg C mg Chl-1 h-1), k is the vertical light attenuation coefficient (m-1), T is the length of the daylight period (hours), Iio is the mean daily irradiance just below the water surface (μ moles m2 s-1), and Ik is the light intensity below which photosynthesis becomes increasingly light limited (μ moles m2 s- 1).The daily respiration rate integrated over the mixing depth and expressed as a rate per unit of surface area is given by the equation:

Σ R= 24bRzm

Where ΣR is the carbon respired under unit surface area per day. R is the Chlorophyll specific rate of respiration (mg C mg chl-1 h-1).The column compensation point is defined as the mixing depth that result in the daily integral photosynthesis balancing the daily integral respiration:

Σ P = Σ R

Using this equation and replacing k with the equivalent In 100/zy gives the ratio of the euphotic depth (zy ) to mixing depth (zm ) that results in daily photosynthesis balancing daily respiration:

This equation can be simplified by assuming that respiration rate R which is constant and equivalent to 0.1PM (Reynolds, 1984) and that the function In( Iio / 0.5Ik ) is equal to a mean value of 2.75 (Reynolds, 1984) to given by:

RESULTS & DISCUSSION

The dependency of phytoplankton population changes on the zy/zm ratio was used to assess the likelihood that heights turbidity in the river Suswa resulted in light limitation of phytoplankton production, and to indicate the increased growth (Fig. 1). Direct measurements of the mixing depth were not made and so it was estimated as the maximum depth of the sampling sites (Fig. 2). The depths of the two sampling sites of river Suswa were 1.5m at Sampling station A (Satyanarayan Area) and 1.2m at Sampling station B (Downstream Vedic Nagar). The calculated zy/ zm ratio changed considerably over time (Figs. 3 & 4), but for 100% of the 12 month study period it was much higher than 0.2, the minimum value of the critical range set for the zy/zm ratio, indicating that light limitation was not considered. The exceeded ratio of z y/zm (0.35) indicates that the light was sufficient for phytoplanktonic growth during sampling period. In the months of March to June the maximum z y/zm ratio were observed which indicate the maximum growth of the phytoplankton.During the course of study the zy/ zm ratio value were observed in the range of 0.4427 to 3.242.

The column compensation point is defined as the mixing depth that results in the daily integral photosynthesis balancing the daily integral respiration. The day length meanly affects it. If the length T is 12 hours, then at the column compensation point zy/zm has a value of 0.33. A respiration rate of 0.1PM is relatively high and an average value of 0.07 may be more appropriate (Reynolds 1984), reducing the critical zy/zm ratio for column compensatio point to 0.23. The result show the variation in compensation point of 0.281 to 0.434 with mean value of 0.3413 (n= 18).

In assessing the effect of reduced light penetration on phytoplankton growth the depth of the euphotic Zone was compared with the maximum depth of the river rather than the mixing depth because we already taken this consideration. This approximation was made to calculate compensation depth for which estimation of mixing depth is essential. The assumption that the mixing depth is equivalent to the maximum depts fophytoplankton with the ratio of the water column may be reasonable for much of the time as water fo the river Suswa was well mixed. FIrstly we compare the biovolune, which was taken in place of biomass of the phytoplankton with the ratio of zy /zm (Fig. 1) with increase in the value of zy /zm ration there was considerable increase in phytoplankton biovolume (DF1,4 = 5, S= 0.230, r = 0.911, T=0.164.) indicating the growth of phytoplankton (Fig.2). The zy/zm ratio also provides a reasonable estimation of the relative proportion of the population (in form of biovolume) that is within the euphotic zone, in case the vertical movement of the cells is minimized. Despite of many assumption made in setting the value of compensation point there was significant increase in concentrations of phytoplankton, which was always associated to the value of the ratio equal to or greater than the 0.281 to 0.434 (Fig. 5). The availability of light is significant throughout the sampling period i.e. light availability is more than standard ratio (0.33). There were major changes in the value of zy /zm ratio, which indicates the decrease in phytoplanktonic growth. But the value of z y/zm ratio was low (0.4427) in the months of November to February (winter).However, identification of Compensation point values for zy/ zm indicates that light conditions are suitable for net phytoplankton production, because the mean value (0.3413) was good for the phytoplanktonic growth, which was greater than the minimum 0.20 limits. Because the latitudinal and altitudinal variation effects the day length and both the two sampling station are situated approximately at same latitude and altitude there were no such differences in compensation point at both the sampling station.

CONCLUSION

From the above study it may be concluded that light is the sole energy source for nearly all plant species, ranging from the tiny phytoplankton to giant sequoia tree. As a consequence, the availability of light has a major impact on the dynamics and structure of the most aquatic and terrestrial communities. Light differs from all other resources because it cannot be mixed. Certain dissolved and particulate substances create the light gradient. The optically important water quality parameters are dissolved solid and suspended solid.

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