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Effects of fish bioturbation on the vertical distribution of water temperature and dissolved oxygen in a fish culture-integrated waste stabilization pond system in Vietnam
Aquaculture 281 (2008) 28–33
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Aquaculture j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / a q u a - o n l i n e
Effects of fish bioturbation on the vertical distribution of water temperature and dissolved oxygen in a fish culture-integrated waste stabilization pond system in Vietnam Minh Phan-Van a,⁎, Diederik Rousseau b,1, Niels De Pauw c,2 a b c
Nong Lam University, Research Institute of Biotechnology and Environment, KP 6, Linh Trung, Thu Duc District, Ho Chi Minh City, Viet Nam UNESCO-IHE Institute for Water Education, Department of Environmental Resources, P.O. Box 3015, 2601 DA Delft, The Netherlands Ghent University, Department of Applied Ecology and Environmental Biology, J. Plateaustraat 22, B-9000 Gent, Belgium
A R T I C L E
I N F O
Article history: Received 2 July 2007 Received in revised form 20 April 2008 Accepted 20 April 2008 Keywords: Bioturbation Stabilization pond Tilapia Stratification
A B S T R A C T The integration of wastewater stabilization with fish culture in a single-pond system is a double-benefit both due to the reclamation of the wastewater and fish production. In conventional fish ponds, it is known that fish activities (bioturbation) result in ecological benefits as they stir the sediment layer, improving aerobic conditions and, enhancing the oxidation of detritus in this layer. However, the effects of the fish bioturbation in integrated wastewater stabilization fish production systems are little known, especially the enhancement of aerobic conditions in the lower layers of the water column where anaerobic conditions dominate, constraining oxidation of organic matter. The present study was designed to evaluate the effects of three factors (fish bioturbation, season and water depth), as well as their interaction, on the vertical distribution of two factors influencing wastewater treatment efficiency in stabilization ponds—water temperature and dissolved oxygen (DO). In contrast to the pond without fish, there was no significant difference in temperature between the water depth (20, 50 and 80 cm) in the pond with fish in the dry season (P N 0.05). As indicated by DO monitoring in early morning and on a diel basis, in both dry and rainy seasons the DO saturation percentage of the surface layer of water (20 cm) was not different between the ponds with and without fish. However, with an increase in water depth, the DO saturation percentage of the pond with fish was significantly higher than that of the pond without fish (P b 0.05). Quiescence of DO variation (invariable linear pattern) in the pond without fish at depths of 50 and 80 cm was recorded. Only fish production and water depth factors had significant effects (P b 0.001), and the interaction between them was highly significant (P b 0.01). © 2008 Published by Elsevier B.V.
1. Introduction The rapid urbanization without parallel evolvement of sufficient environmental infrastructure in some Asian countries has pushed their peri-urban areas into the pollution by urban waste discharges, leading farming systems to facing the threats of failing crops due to the water pollution (Liang et al., 1998; Phan-Van and De Pauw, 2005). It is most likely that the current wastewater treatment technologies conventionally based on highly optimized physical, chemical and microbial processes (Brix, 1999) are not affordable for farmers to tackle the water pollution. Within such a context, the ecological technology, especially fishculture-integrated waste stabilization pond, has been demonstrated as an appropriate alternative for its low-cost effectiveness and
⁎ Corresponding author. Tel.: +84 8 8982078. E-mail addresses: [email protected] (M. Phan-Van), [email protected] (D. Rousseau), [email protected] (N. De Pauw). 1 Tel.: +31 15 2151783. 2 Tel.: +32 9 2643768. 0044-8486/$ – see front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.aquaculture.2008.04.033
nutrient recycling into fish biomass (Bartone and Khouri, 1990; De Pauw and Salomoni, 1991; Edwards, 1992; Oswald, 1995; Liang et al., 1998; Saha and Jana, 2003; Phan-Van and De Pauw, 2005). There have been many studies on waste stabilization ponds integrated with fish culture, as reviewed by Edwards (1992, 2005). However, they have focused mainly on the treatment efficiency of pollutants in the system. Ecological aspects of the stabilization pond which can be affected by the co-existing fish community are poorly known. In the last decade, research on fish culture showed that perturbation activities of fish bioturbation on the pond bottom cause effects on the pond ecosystem (Scheffer, 1997; Riise and Roos, 1997; Avnimelech et al., 1999; Saha and Jana, 2003; Ritvo et al., 2004). Ritvo et al. (2004) demonstrated indirectly that the major effect of fish bioturbation is the improvement of aerobic conditions at the pond bottom soil–water interface as the concentration of reducing compounds (total dissolved sulphide, soluble manganese, easily oxidized matter, and exchangeable NH4-N) at this layer were decreased. Water temperature and dissolved oxygen were determined to be two factors significantly influencing the wastewater treatment
M. Phan-Van et al. / Aquaculture 281 (2008) 28–33
process in stabilization ponds (Mara, 1997; Hosetti and Frost, 1998). Thus, the bioturbation activities of fish would also improve the vertical distribution of temperature and dissolved oxygen in the water column, especially in the lower water layers of stabilization ponds, which lead to better treatment efficiencies of pollutants. This fact would more significantly demonstrate the double benefit of integrating fish culture with wastewater stabilization: not only the fish biomass produced but also the enhancement of waste treatment efficiency. The present study was designed to evaluate the effects of three factors: fish bioturbation, season, and water depth, including their interaction, on the vertical distribution of water temperature and dissolved oxygen. 2. Materials and methods 2.1. Experimental design The experiment involved a factorial design with three factors: water depth (samples were taken at three levels: 20, 50 and 80 cm), season (two levels: dry and rainy season) and fish production (two levels: with and without fish). All treatments were triplicated. Each replication lasted for 5 weeks which corresponded to a local cycle of tilapia fingerling production. 2.2. Experimental set-up The study was carried out on a private farm located in Subdistrict 16, District 8, Ho Chi Minh City (HCMC), the catchment of the Ruot Ngua River. It is low-lying land polluted severely by wastewater discharges of Districts 1, 5, 6, 8, 11, and Binh Chanh and Tan Binh. The experimental system was composed of two earthen ponds, each about 800 m2 (32.5 × 25 m), supplied with polluted water from the Ruot Ngua River by pumping from an adjacent 5 m wide canal. Before each experimental replication, pond bottoms were cleaned to remove old sediment and leveled to keep the same water depth of 1.0 m throughout the ponds. More details of the system construction were described by Phan-Van and De Pauw (2005). Polluted water with a biological oxygen demand (BOD5) concentrations between 111 and 130 mg O2 l− 1 was pumped into the pond to a depth of 100 cm, and then left for natural purification (Week 0). One week later, when the water BOD5 concentration was in a range of 21 to 24 mg O2 l− 1, tilapia brood fish were stocked in the pond during the early morning of the next day (Day 8). They were then kept in the pond for natural spawning for the next 2 weeks (Week 3/Day 21) and then seined out of the pond with a 5 cm mesh-size net (Day 22). Owing to the large mesh-size, tilapia fries/fingerlings remained in the pond for natural nursing/rearing until the end of Week 5. All tilapia fingerlings were then harvested by seining, the ponds were drained and the experimental replication ended. No external feed was supplied to either tilapia breeders or fingerlings in the ponds throughout the experimental period. The same polluted water was also pumped in parallel into the control pond without tilapia brood fish being stocked in the pond, i.e. without tilapia fingerling production. Polluted river water was also kept in the control pond for 5 weeks. The experimental set-up was triplicated in each dry and rainy season. After the ponds (with and without fish) were filled with polluted water, both of them did not receive any extra water supplement from that moment on till the end of the experimental period. The water evaporation and rainfall effect were considered as one of effects of the season factor. 2.3. Water temperature and dissolved oxygen monitoring Water temperature and dissolved oxygen (DO) monitoring were carried out from 8:30 to 10:00 am on a weekly basis at two sampling
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bridges installed at a distance of 3 m from pond dikes at each pond to avoid pond dike effects. To elaborate the 24-h difference of DO between experimental treatments, a diel monitoring of DO was undertaken every 2 h at Day 21 (Week 3) of all experimental treatments. Pond water temperature (with Horiba digital meter-U10, Japan) and DO (with Oxi digital meter-320, and membrane electrode-CellOx 325, Japan), were recorded on-site and simultaneously at three water depths of 20, 50 and 80 cm. The recording was also done at the same time for the ponds with and without fish. 2.4. Tilapia broodstock Tilapia brood fish (Oreochromis niloticus) of approximately equal size, averaging 166 ± 9.3 g and 201 ± 8.1 mm in total length, were selected for the experiments. The stocking density of brood fish was 1000 fish per pond (about 1 fish per m2) with a sex ratio (male: female) of 1: 5. 2.5. Statistical analyses Experimental data were analyzed statistically with the software package STATGRAPHICS Plus version 3.0, (StatPoint. Ltd. USA, www. statgraphics.com). Multifactor-ANOVA was used to determine the main effects of the studied factors and the interaction between them. Oneway ANOVA with Duncan Multiple Range Test (DMRT) was used for analyses of the differences among means of the treatments at P b 0.05. Transformation of data by logarithm for DO saturation and by square root of (data + 0.05) (Steel and Torrie, 1960) for water temperature were employed. 3. Results 3.1. Variations of water temperature In both ponds with fish (treatment) and without fish (control), the mean water temperature varied from 29.4 ± 0.6 °C to 30.8 ± 0.9 °C during the dry season and 28.7 ± 0.4 °C to 29.1 ± 0.5 °C during the rainy season (Table 1). Differences in water temperature at the three depths in the dry and the rainy seasons were significant (DMRT, P b 0.05); however, the differences between the treatment and the control depended on the season. It was notable that in the rainy season there was no significant difference in water temperature among the experimental ponds despite the presence of the fish production and water depth factors, i.e. no thermal stratification in the ponds. Contrarily, in the dry season, a thermal stratification was clear between the surface and the bottom layers but only in the pond without fish. No significant differences in temperature between water depths were detected in the pond with fish (P N 0.05). ANOVA showed that all three factors (season, fish production and water depth) had highly significant effects on the water temperatures in the experimental ponds (P b 0.001 for season, P b 0.01 for water
Table 1 Water temperature (given as total means of the weekly means of triplicates; n = 15) in the single-pond system at three depths (20, 50 and 80 cm) and in the rainy and dry seasons; the same common letters in the columns indicate non-significant difference between means at P N 0.05 Water temperature (°C) Season
Rainy season
Depth
20 cm
50 cm
80 cm
20 cm
Dry season 50 cm
80 cm
With fish Without fish
29.1 abc 28.9 abc
28.9 abc 28.8 ab
28.7 a 28.7 a
30.8 f 30.2 ef
30.3 ef 29.6 cde
30.0 def 29.4 bcd
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depth, and P b 0.05 for fish production). However, there was no interaction between the three factors (P N 0.05). 3.2. Variation of dissolved oxygen saturation 3.2.1. Morning variations of dissolved oxygen saturation throughout the experimental period There were large variations in mean DO saturation level, at the three different water depths of the ponds (20, 50 and 80 cm), and in ponds with fish and without fish (Fig. 1a,b). The DO saturation values increased from 0% in the influent (Week 0) to values of an aerobic system after 1 week of water detention. In the dry season, between 08:30 and 10:00 am at depths of 20, 50, and 80 cm, the average DO saturation of the treatment pond with fish ranged between 31 and 62%, 27 and 52%, and 8 and 14%, respectively, whereas those of the control pond without fish were between 21 and
57%, 10 and 17%, and 1 and 6%, respectively. The minimal values of these ranges were measured at 08:30 am. Under the climatic conditions of the present study, at a water temperature of 29 to 30 °C and salinity lower than 1‰, 100% DO saturation is equivalent to a dissolved oxygen concentration of 8.0 mg O2 l− 1, 50% to 3.8 mg O2 l− 1 and 25% to 2 mg O2 l− 1, respectively. Similarly, in the rainy season (Fig. 1a), the respective values of the average DO saturation at the above three depths ranged from 37 to 64%, 21 to 37% and 2 to 19% for the treatment, and 47 to 81%, 3 to 29% and 1 to 15% for the control. The DMRT gave an indication of the significant differences between the treatment and the control at three different water depths and in both seasons (Table 2). There was a clear DO stratification in both the treatment and the control (P b 0.05). The DO saturation percentages at 20 cm depth were always significantly higher than those at 50 cm depth, and these were higher than at
Fig. 1. Stratification of DO saturation (%) between 8:30 and 10:00 am in the ponds with and without fish (a) during the rainy season and (b) during the dry season, F-with fish; O-without fish; depth-20; 50; 80 cm.
M. Phan-Van et al. / Aquaculture 281 (2008) 28–33 Table 2 DO saturation percentages (given as total means of the weekly means of triplicates; n = 15) of ponds with and without fish production at three water depths in rainy and dry seasons; the same common letters in the columns indicate non-significant difference between means at P N 0.05 Mean DO saturation (%) Season
Rainy
Depth
20 cm
50 cm
80 cm
Dry 20 cm
50 cm
80 cm
With fish Without fish
50.55 de 62.29 e
27.93 c 15.12 b
10.81 b 1.35 a
53.58 e 36.48 cd
32.78 c 13.81 b
12.08 b 3.00 a
80 cm. The DO saturation percentages of the ponds with fish at the water depths of 50 and 80 cm were always significantly higher than those of the control ponds at the same depths. The DO saturation percentages of the ponds with fish at the 80 cm depth were comparable to those of the control at 50 cm one but higher than those of the control at 80 cm (P b 0.05). The DO saturation percentage of the surface layer of water (20 cm) of the ponds with fish and the control were not different. However when the water depth increased, the DO saturation percentage of the treatment became significantly higher than that of the control (P b 0.05). ANOVA indicated that fish production and water depth factors resulted in significant effects (P b 0.001) on DO saturation. On the contrary, the season factor (dry or rainy season) did not have any significant effect on DO saturation (P N 0.05). The highly significant
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interaction between water depth and fish production factors on the other hand was striking (P b 0.01). 3.2.2. Diel variations of dissolved oxygen saturation The diel variation of DO saturation in the single-pond system was an unbalanced cycle. The aerobic and anaerobic durations were not the same (Fig. 2a,b). For only about 4 to 6 h per day was the whole system mixed and in an anaerobic state with minimal DO saturation, normally from 2:00 till 8:00 am. However, during the rest of the day, about 18 to 20 h from 8:00 am till late midnight (after 12:00 pm), the system became aerobic but stratified. Its maximal peak normally occurred between 2:00 pm and 4:00 pm and could reach over-saturation levels of more than 200% during the dry season. The diel variations of DO saturation of the ponds with fish at three different water depths in both seasons fitted well to third-degree polynomial curves with high R2 coefficients (0.80 to 0.94) with two peaks, one maximum and one minimum as observed in reality (Fig. 2a, b). On the contrary, in the control, only the R2 coefficients of the curve standing for the DO saturation of the 20 cm depth were high (N0.90), whereas the others at depths of 50 and 80 cm were much lower (0.42 to 0.72), especially in the rainy season, with even a tendency of a linear regression (Fig. 3a,b). The ANOVA of the three experimental factors (season, fish production and water depth) in the case of the diel DO variation gave the same results as that of the DO morning variation mentioned above. Only fish production and the water depth factors had significant effects on DO saturation (P b 0.001). Furthermore, only a
Fig. 2. Diel stratification of DO saturation (%) (mean ± S.D) during the dry season at depths of 20, 50 and 80 cm (a) in the pond with fish and (b) in the pond without fish.
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Fig. 3. Diel stratification of DO saturation (%) (mean ± S.D) during the rainy season at depths of 20, 50 and 80 cm (a) in the pond with fish and (b) in the pond without fish.
highly significant interaction existed between water depth and fish production factors. At water depths of 50 and 80 cm, DO saturation percentages of the treatment were significantly different from those of the control (DMRT, P b 0.05) in both seasons. 4. Discussion 4.1. Variations of water temperature and effects of fish production The treatment efficiencies of the waste stabilization ponds could be strongly affected by environmental parameters, especially water temperature and DO due to their essential role in the bacteria-algae symbiotic treatment processes of ponds (Mara, 1997; Hosetti and Frost, 1998). However, knowledge about the effects of fish production on these abiotic conditions in stabilization ponds is still limited. In the present study, a range of water temperatures from 28.7 to 30.8 °C in all experimental ponds during both rainy and dry seasons was recorded, favorable for the growth of tilapia (Chervinski, 1982) and for the development of algae and aquatic invertebrates as observed in tropical sewage-fed ponds (Liang et al., 1998). When comparing the effect of the three factors (season, water depth and fish production) on water temperature, it was shown that all had significant effects on the variation of water temperature. The effects of season and water depth factors were highly significant (P b 0.001) in comparison to that of fish production (P b 0.01). As a result, the former two would be predominant over the fish production factor on the variation of water temperature.
In the rainy season there was no significant difference in water temperature between water depths and between the treatment (with fish) and control (without fish) (Table 1). This indicates that the whole system, a shallow aquatic ecosystem, was partially mixed by the impacts of wind and rain. Therefore, there was no thermal stratification in the rainy season in all experimental ponds. In the dry season, contrarily, the system exhibited a clear thermal stratification as seen in the ponds without fish. The temperature of the upper layer was significantly higher than the one near the bottom. However, in the pond with fish no thermal stratification was recorded (Table 1). The activities of tilapia (such as feed scavenging, mating and building nests on the pond bottom) appeared to agitate the whole water column and therefore disperse the high temperature of the upper layer down to the lower ones. Recently, Rasmussen et al. (2005), using Rhodamine tracer, demonstrated that irrespective of flow rate, the presence of fish enhanced the mixing process, with the mixing time in tanks with fish being one-third that for tanks without fish. The in-tank dispersion coefficients and dispersion numbers differed significantly in the presence of fish. 4.2. Variations of DO saturation and effects of fish production Low DO concentrations are normally the main cause of fish die-off in sewage-fed fishponds (Bartone and Khouri, 1990; Ghosh et al., 1999). Therefore, investigation of the role the DO regime in ponds plays is important in integrating polluted water reclamation with fish production (Ghosh et al., 1999).
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In the present study, 1 week after the retention of the polluted water in ponds, a broad range of DO saturation values was recorded in the diel DO regime, depending on water depth. Generally, DO oversaturation (N8 mg O2 l− 1) occurred in the early afternoon in the surface water layer whereas DO depletion (less than 12% or b1 mg O2 l− 1) occurred at dawn in the whole water column. In a temperate climate, Ghosh et al. (1999) observed that a DO concentration as low as 0.2 to 2 mg O2 l− 1 occurred at the bottom zone (70 cm) in the daytime (8:00 am–6:00 pm), causing the death of silver carp (Hypopthalmichthys molitrix) and common carp (Cyprinus carpio) during rainy days. In the present study such daytime DO concentration levels (below 20% DO saturation) were also recorded at about the same water depth in the rainy season but only in the pond without fish. Dawn DO (2:00 to 6:00 am) less than 20% saturation (b2 mg O2 l− 1) was common in the diel DO regime of the experimental ponds. However, no fish mortality was observed. Tilapia can survive at DO concentrations as low as 1 mg O2 l− 1 by utilizing atmospheric oxygen, and its mortality only occurred when the DO saturation was below 20% for more than 2 to 3 days (Chervinski, 1982). Statistical analyses showed that the effect of fish production factor on DO was highly significant (P b 0.001) in the single-pond system. When fish were present, the DO saturation of the pond water increased significantly in the two lower water depths (50 and 80 cm). The bioturbatory activity of fish supported the diffusion of DO throughout the water column of the ponds. The quiescence of DO variations (invariable linear patterns) in the pond without fish at depths of 50 and 80 cm in the rainy season demonstrated this effect (Fig. 3b). At the water surface layer, however, intense photosynthesis likely led to the establishment of a regime of high DO, over riding the significant effect of fish on the difference in DO between the ponds with and without fish (as shown by a highly significant interaction, P b 0.001, between the water depth and the fish production factor). Also, similar DO saturation levels of the water surface layer in both seasons showed the non-significant effect of season on DO. It appeared that solar radiation in the study area was in a range favourable for algae to develop equally in both dry and rainy seasons. This resulted in active photosynthesis in the upper layers of the ponds, thereby producing daily over-saturated levels of DO in the early afternoon in both seasons (Fig. 2a,b) and generating a sufficient source of DO for diffusion downward to the lower layers of pond water through fish bioturbation. It is clear that the presence of tilapia improved the aerobic conditions in the water column where intense photosynthesis co-existed without significantly adverse seasonal impacts (data about phytoplankton are available in Phan-Van, 2006). The results of the present study supported the work by Rasmussen et al. (2005) that fish presence and any active moving objects in the water, can generate mixing of the water column, thereby enhancing the dispersion/diffusion of dissolved substances throughout its environment. There is evidence that fish bioturbation can impact water layers as deep as the water–soil interface at the superficial layer of pond bottom. Fish bioturbation enables oxidation of sedimenting organic matter by resuspending it into aerobic upper water layers (Scheffer, 1997), enhances the release of phosphorus (Jana and Das, 1992; Jana and Sahu, 1993; Saha and Jana, 2003), changes the structure of soil particles and decreases the concentrations of total dissolved sulphide, soluble manganese, easily oxidized matter, and exchangeable NH4-N at the superficial layer (Ritvo et al., 2004). 5. Conclusion Based on this study and previous research discussed above, it can be concluded that fish bioturbation not only enhances the aerobic
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conditions inside the whole water column but also in the pond bottom soils. Furthermore, fish bioturbation probably also affects other ecological aspects of the fishculture-integrated waste stabilization system such as the water quality reclamation process, nutrient recycling and variations of food web structure but these need to be further studied. The bioturbutary activities of tilapia can over ride the vertical distribution of water temperature in stabilization ponds to a depth of 1 m during the dry season. At the same time it can bring some DO downwards to the lower layers of the water column, improving aerobic conditions on the pond bottom. Acknowledgements The authors would like to express sincere thanks to the Ho Chi Minh City Department of Science and Technology and the Belgium Technical Cooperation for granting this research. References Avnimelech, Y., Kochva, M., Hargreaves, J.A., 1999. Sedimentation and resuspension in earthern fish ponds. J.World Aquacult. Soc. 30, 401–409. Bartone, C.R., Khouri, N., 1990. Reuse of stabilization pond effluents for fish culture in Lima, Peru. Preliminary experiment. In: Edwards, P., Pullin, R.S.V. (Eds.), Wastewater-fed Aquaculture, Proceedings of the International Seminar on Wastewater Reclamation and Reuse for Aquaculture, Calcutta, India. Environmental Sanitation Information Center, Asian Institute of Technology, Bangkok, Thailand, pp. 135–146. 6–9 December 1988, xxix+296 pp. Brix, H., 1999. How ‘green’ are aquaculture, constructed wetlands and conventional wastewater treatment systems? Water Sci. Technol. 40, 45–50. Chervinski, J., 1982. Environmental physiology of tilapias. In: Pullin, R.S.V., LoweMcConnell, R.H. (Eds.), ICLARM Conference Proceedings 7. The Biology and Culture of Tilapias. International Center for Living Aquatic Resources Management, Manila, Philippines, pp. 119–128. 432pp. De Pauw, N., Salomoni, C., 1991. The use of microalgae in wastewater treatment: achievements and constraints. In: Madoni, P. (Ed.), Biological Approach to Sewage Treatment Process: Current Status and Perspectives, Perugia, pp. 329–352. Edwards, P., 1992. Reuse of human wastes in aquaculture: A technical review. UNDP-World Bank Water and Sanitation Program. The World Bank, Washington, D.C., U.S.A. 350 pp. Edwards, P., 2005. Development status of, and prospects for, wastewater-fed aquaculture in urban environment. In: Costa-Pierce, B., Desbonet, A., Edwards, P., Baker, D. (Eds.), Urban Aquaculture. CABI Publishing, Wallingford, Oxford, U.K., pp. 45–59. Ghosh, C., Frijns, J., Lettinga, G., 1999. Performance of silver carp (Hypopthalmichthys molitrix) dominated integrated post treatment system for purification of municipal waste water in a temperatate climate. Bioresource Technol., 69, 255–262. Hosetti, B., Frost, S., 1998. A review of the control of biological waste treatment in stabilization ponds. Crit. Rev. Environ. Sci. Technol. 28, 193–218. Jana, B.B., Das, S.K., 1992. Bioturbation induced changes of fertilizer value of phosphate rock in relation to alkaline phosphatase activity. Aquaculture 103, 321–330. Jana, B.B., Sahu, S.N., 1993. Relative performance of three bottom grazing fishes (Cyprinus carpio, Cirrhinus mrigala, Heteropneustes fossilis) in increasing the fertilizer value of phosphate rock. Aquaculture 115, 19–29. Liang, Y., Cheung, R.Y.H., Everitt, S., Wong, M.H., 1998. Reclamation of wastewater for polyculture of freshwater fish: wastewater treatment in ponds. Water Res. 32, 1864–1880. Mara, D.D., 1997. Design manual for waste stabilization ponds in India, Lagoon Technology International Ltd. University of Leeds, Leeds, U.K. 150 pp. Oswald, W.J., 1995. Ponds in the twenty-first century. Water Sci. Technol. 31, 1–8. Phan-Van, M. (2006). Single pond system for integration of polluted water reclamation with tilapia fingerling production. PhD thesis, Ghent University, 304pp. Phan-Van, M., De Pauw, N., 2005. Wastewater-based urban aquaculture systems in Ho Chi Minh City, South Vietnam. In: Costa-Pierce, B., Desbonet, A., Edwards, P., Baker, D. (Eds.), Urban Aquaculture. CABI Publishing, Wallingford, Oxford, U.K, pp. 77–102. Rasmussen, M.R., Laursen, J., Craig, S.R., McLean, E., 2005. Do fish enhance tank mixing? Aquaculture 250, 162–174. Riise, J.C., Roos, N., 1997. Benthic metabolism and the effects of bioturbation in a fertilized polyculture fish pond in northeast Thailand. Aquaculture 150, 45–62. Ritvo, G., Kochba, M., Avnimelec, Y., 2004. The effects of common carp bioturbation on fishpond bottom soil. Aquaculture 242, 345–356. Saha, S., Jana, B.B., 2003. Fish macrophyte association as a low-cost strategy for wastewater reclamation. Ecol. Eng. 21, 21–41. Scheffer, M., 1997. Ecology of Shallow Lakes. Kluwer Academic Publishing, Dordrecht, The Netherlands. 384pp. Steel, R.G.D., Torrie, J.H., 1960. Principles and Procedures of Statistics. McGraw-Hill Book Company, Inc, New York, U.S.A. 481 pp.
Contents lists available at ScienceDirect
Aquaculture j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / a q u a - o n l i n e
Effects of fish bioturbation on the vertical distribution of water temperature and dissolved oxygen in a fish culture-integrated waste stabilization pond system in Vietnam Minh Phan-Van a,⁎, Diederik Rousseau b,1, Niels De Pauw c,2 a b c
Nong Lam University, Research Institute of Biotechnology and Environment, KP 6, Linh Trung, Thu Duc District, Ho Chi Minh City, Viet Nam UNESCO-IHE Institute for Water Education, Department of Environmental Resources, P.O. Box 3015, 2601 DA Delft, The Netherlands Ghent University, Department of Applied Ecology and Environmental Biology, J. Plateaustraat 22, B-9000 Gent, Belgium
A R T I C L E
I N F O
Article history: Received 2 July 2007 Received in revised form 20 April 2008 Accepted 20 April 2008 Keywords: Bioturbation Stabilization pond Tilapia Stratification
A B S T R A C T The integration of wastewater stabilization with fish culture in a single-pond system is a double-benefit both due to the reclamation of the wastewater and fish production. In conventional fish ponds, it is known that fish activities (bioturbation) result in ecological benefits as they stir the sediment layer, improving aerobic conditions and, enhancing the oxidation of detritus in this layer. However, the effects of the fish bioturbation in integrated wastewater stabilization fish production systems are little known, especially the enhancement of aerobic conditions in the lower layers of the water column where anaerobic conditions dominate, constraining oxidation of organic matter. The present study was designed to evaluate the effects of three factors (fish bioturbation, season and water depth), as well as their interaction, on the vertical distribution of two factors influencing wastewater treatment efficiency in stabilization ponds—water temperature and dissolved oxygen (DO). In contrast to the pond without fish, there was no significant difference in temperature between the water depth (20, 50 and 80 cm) in the pond with fish in the dry season (P N 0.05). As indicated by DO monitoring in early morning and on a diel basis, in both dry and rainy seasons the DO saturation percentage of the surface layer of water (20 cm) was not different between the ponds with and without fish. However, with an increase in water depth, the DO saturation percentage of the pond with fish was significantly higher than that of the pond without fish (P b 0.05). Quiescence of DO variation (invariable linear pattern) in the pond without fish at depths of 50 and 80 cm was recorded. Only fish production and water depth factors had significant effects (P b 0.001), and the interaction between them was highly significant (P b 0.01). © 2008 Published by Elsevier B.V.
1. Introduction The rapid urbanization without parallel evolvement of sufficient environmental infrastructure in some Asian countries has pushed their peri-urban areas into the pollution by urban waste discharges, leading farming systems to facing the threats of failing crops due to the water pollution (Liang et al., 1998; Phan-Van and De Pauw, 2005). It is most likely that the current wastewater treatment technologies conventionally based on highly optimized physical, chemical and microbial processes (Brix, 1999) are not affordable for farmers to tackle the water pollution. Within such a context, the ecological technology, especially fishculture-integrated waste stabilization pond, has been demonstrated as an appropriate alternative for its low-cost effectiveness and
⁎ Corresponding author. Tel.: +84 8 8982078. E-mail addresses: [email protected] (M. Phan-Van), [email protected] (D. Rousseau), [email protected] (N. De Pauw). 1 Tel.: +31 15 2151783. 2 Tel.: +32 9 2643768. 0044-8486/$ – see front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.aquaculture.2008.04.033
nutrient recycling into fish biomass (Bartone and Khouri, 1990; De Pauw and Salomoni, 1991; Edwards, 1992; Oswald, 1995; Liang et al., 1998; Saha and Jana, 2003; Phan-Van and De Pauw, 2005). There have been many studies on waste stabilization ponds integrated with fish culture, as reviewed by Edwards (1992, 2005). However, they have focused mainly on the treatment efficiency of pollutants in the system. Ecological aspects of the stabilization pond which can be affected by the co-existing fish community are poorly known. In the last decade, research on fish culture showed that perturbation activities of fish bioturbation on the pond bottom cause effects on the pond ecosystem (Scheffer, 1997; Riise and Roos, 1997; Avnimelech et al., 1999; Saha and Jana, 2003; Ritvo et al., 2004). Ritvo et al. (2004) demonstrated indirectly that the major effect of fish bioturbation is the improvement of aerobic conditions at the pond bottom soil–water interface as the concentration of reducing compounds (total dissolved sulphide, soluble manganese, easily oxidized matter, and exchangeable NH4-N) at this layer were decreased. Water temperature and dissolved oxygen were determined to be two factors significantly influencing the wastewater treatment
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process in stabilization ponds (Mara, 1997; Hosetti and Frost, 1998). Thus, the bioturbation activities of fish would also improve the vertical distribution of temperature and dissolved oxygen in the water column, especially in the lower water layers of stabilization ponds, which lead to better treatment efficiencies of pollutants. This fact would more significantly demonstrate the double benefit of integrating fish culture with wastewater stabilization: not only the fish biomass produced but also the enhancement of waste treatment efficiency. The present study was designed to evaluate the effects of three factors: fish bioturbation, season, and water depth, including their interaction, on the vertical distribution of water temperature and dissolved oxygen. 2. Materials and methods 2.1. Experimental design The experiment involved a factorial design with three factors: water depth (samples were taken at three levels: 20, 50 and 80 cm), season (two levels: dry and rainy season) and fish production (two levels: with and without fish). All treatments were triplicated. Each replication lasted for 5 weeks which corresponded to a local cycle of tilapia fingerling production. 2.2. Experimental set-up The study was carried out on a private farm located in Subdistrict 16, District 8, Ho Chi Minh City (HCMC), the catchment of the Ruot Ngua River. It is low-lying land polluted severely by wastewater discharges of Districts 1, 5, 6, 8, 11, and Binh Chanh and Tan Binh. The experimental system was composed of two earthen ponds, each about 800 m2 (32.5 × 25 m), supplied with polluted water from the Ruot Ngua River by pumping from an adjacent 5 m wide canal. Before each experimental replication, pond bottoms were cleaned to remove old sediment and leveled to keep the same water depth of 1.0 m throughout the ponds. More details of the system construction were described by Phan-Van and De Pauw (2005). Polluted water with a biological oxygen demand (BOD5) concentrations between 111 and 130 mg O2 l− 1 was pumped into the pond to a depth of 100 cm, and then left for natural purification (Week 0). One week later, when the water BOD5 concentration was in a range of 21 to 24 mg O2 l− 1, tilapia brood fish were stocked in the pond during the early morning of the next day (Day 8). They were then kept in the pond for natural spawning for the next 2 weeks (Week 3/Day 21) and then seined out of the pond with a 5 cm mesh-size net (Day 22). Owing to the large mesh-size, tilapia fries/fingerlings remained in the pond for natural nursing/rearing until the end of Week 5. All tilapia fingerlings were then harvested by seining, the ponds were drained and the experimental replication ended. No external feed was supplied to either tilapia breeders or fingerlings in the ponds throughout the experimental period. The same polluted water was also pumped in parallel into the control pond without tilapia brood fish being stocked in the pond, i.e. without tilapia fingerling production. Polluted river water was also kept in the control pond for 5 weeks. The experimental set-up was triplicated in each dry and rainy season. After the ponds (with and without fish) were filled with polluted water, both of them did not receive any extra water supplement from that moment on till the end of the experimental period. The water evaporation and rainfall effect were considered as one of effects of the season factor. 2.3. Water temperature and dissolved oxygen monitoring Water temperature and dissolved oxygen (DO) monitoring were carried out from 8:30 to 10:00 am on a weekly basis at two sampling
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bridges installed at a distance of 3 m from pond dikes at each pond to avoid pond dike effects. To elaborate the 24-h difference of DO between experimental treatments, a diel monitoring of DO was undertaken every 2 h at Day 21 (Week 3) of all experimental treatments. Pond water temperature (with Horiba digital meter-U10, Japan) and DO (with Oxi digital meter-320, and membrane electrode-CellOx 325, Japan), were recorded on-site and simultaneously at three water depths of 20, 50 and 80 cm. The recording was also done at the same time for the ponds with and without fish. 2.4. Tilapia broodstock Tilapia brood fish (Oreochromis niloticus) of approximately equal size, averaging 166 ± 9.3 g and 201 ± 8.1 mm in total length, were selected for the experiments. The stocking density of brood fish was 1000 fish per pond (about 1 fish per m2) with a sex ratio (male: female) of 1: 5. 2.5. Statistical analyses Experimental data were analyzed statistically with the software package STATGRAPHICS Plus version 3.0, (StatPoint. Ltd. USA, www. statgraphics.com). Multifactor-ANOVA was used to determine the main effects of the studied factors and the interaction between them. Oneway ANOVA with Duncan Multiple Range Test (DMRT) was used for analyses of the differences among means of the treatments at P b 0.05. Transformation of data by logarithm for DO saturation and by square root of (data + 0.05) (Steel and Torrie, 1960) for water temperature were employed. 3. Results 3.1. Variations of water temperature In both ponds with fish (treatment) and without fish (control), the mean water temperature varied from 29.4 ± 0.6 °C to 30.8 ± 0.9 °C during the dry season and 28.7 ± 0.4 °C to 29.1 ± 0.5 °C during the rainy season (Table 1). Differences in water temperature at the three depths in the dry and the rainy seasons were significant (DMRT, P b 0.05); however, the differences between the treatment and the control depended on the season. It was notable that in the rainy season there was no significant difference in water temperature among the experimental ponds despite the presence of the fish production and water depth factors, i.e. no thermal stratification in the ponds. Contrarily, in the dry season, a thermal stratification was clear between the surface and the bottom layers but only in the pond without fish. No significant differences in temperature between water depths were detected in the pond with fish (P N 0.05). ANOVA showed that all three factors (season, fish production and water depth) had highly significant effects on the water temperatures in the experimental ponds (P b 0.001 for season, P b 0.01 for water
Table 1 Water temperature (given as total means of the weekly means of triplicates; n = 15) in the single-pond system at three depths (20, 50 and 80 cm) and in the rainy and dry seasons; the same common letters in the columns indicate non-significant difference between means at P N 0.05 Water temperature (°C) Season
Rainy season
Depth
20 cm
50 cm
80 cm
20 cm
Dry season 50 cm
80 cm
With fish Without fish
29.1 abc 28.9 abc
28.9 abc 28.8 ab
28.7 a 28.7 a
30.8 f 30.2 ef
30.3 ef 29.6 cde
30.0 def 29.4 bcd
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depth, and P b 0.05 for fish production). However, there was no interaction between the three factors (P N 0.05). 3.2. Variation of dissolved oxygen saturation 3.2.1. Morning variations of dissolved oxygen saturation throughout the experimental period There were large variations in mean DO saturation level, at the three different water depths of the ponds (20, 50 and 80 cm), and in ponds with fish and without fish (Fig. 1a,b). The DO saturation values increased from 0% in the influent (Week 0) to values of an aerobic system after 1 week of water detention. In the dry season, between 08:30 and 10:00 am at depths of 20, 50, and 80 cm, the average DO saturation of the treatment pond with fish ranged between 31 and 62%, 27 and 52%, and 8 and 14%, respectively, whereas those of the control pond without fish were between 21 and
57%, 10 and 17%, and 1 and 6%, respectively. The minimal values of these ranges were measured at 08:30 am. Under the climatic conditions of the present study, at a water temperature of 29 to 30 °C and salinity lower than 1‰, 100% DO saturation is equivalent to a dissolved oxygen concentration of 8.0 mg O2 l− 1, 50% to 3.8 mg O2 l− 1 and 25% to 2 mg O2 l− 1, respectively. Similarly, in the rainy season (Fig. 1a), the respective values of the average DO saturation at the above three depths ranged from 37 to 64%, 21 to 37% and 2 to 19% for the treatment, and 47 to 81%, 3 to 29% and 1 to 15% for the control. The DMRT gave an indication of the significant differences between the treatment and the control at three different water depths and in both seasons (Table 2). There was a clear DO stratification in both the treatment and the control (P b 0.05). The DO saturation percentages at 20 cm depth were always significantly higher than those at 50 cm depth, and these were higher than at
Fig. 1. Stratification of DO saturation (%) between 8:30 and 10:00 am in the ponds with and without fish (a) during the rainy season and (b) during the dry season, F-with fish; O-without fish; depth-20; 50; 80 cm.
M. Phan-Van et al. / Aquaculture 281 (2008) 28–33 Table 2 DO saturation percentages (given as total means of the weekly means of triplicates; n = 15) of ponds with and without fish production at three water depths in rainy and dry seasons; the same common letters in the columns indicate non-significant difference between means at P N 0.05 Mean DO saturation (%) Season
Rainy
Depth
20 cm
50 cm
80 cm
Dry 20 cm
50 cm
80 cm
With fish Without fish
50.55 de 62.29 e
27.93 c 15.12 b
10.81 b 1.35 a
53.58 e 36.48 cd
32.78 c 13.81 b
12.08 b 3.00 a
80 cm. The DO saturation percentages of the ponds with fish at the water depths of 50 and 80 cm were always significantly higher than those of the control ponds at the same depths. The DO saturation percentages of the ponds with fish at the 80 cm depth were comparable to those of the control at 50 cm one but higher than those of the control at 80 cm (P b 0.05). The DO saturation percentage of the surface layer of water (20 cm) of the ponds with fish and the control were not different. However when the water depth increased, the DO saturation percentage of the treatment became significantly higher than that of the control (P b 0.05). ANOVA indicated that fish production and water depth factors resulted in significant effects (P b 0.001) on DO saturation. On the contrary, the season factor (dry or rainy season) did not have any significant effect on DO saturation (P N 0.05). The highly significant
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interaction between water depth and fish production factors on the other hand was striking (P b 0.01). 3.2.2. Diel variations of dissolved oxygen saturation The diel variation of DO saturation in the single-pond system was an unbalanced cycle. The aerobic and anaerobic durations were not the same (Fig. 2a,b). For only about 4 to 6 h per day was the whole system mixed and in an anaerobic state with minimal DO saturation, normally from 2:00 till 8:00 am. However, during the rest of the day, about 18 to 20 h from 8:00 am till late midnight (after 12:00 pm), the system became aerobic but stratified. Its maximal peak normally occurred between 2:00 pm and 4:00 pm and could reach over-saturation levels of more than 200% during the dry season. The diel variations of DO saturation of the ponds with fish at three different water depths in both seasons fitted well to third-degree polynomial curves with high R2 coefficients (0.80 to 0.94) with two peaks, one maximum and one minimum as observed in reality (Fig. 2a, b). On the contrary, in the control, only the R2 coefficients of the curve standing for the DO saturation of the 20 cm depth were high (N0.90), whereas the others at depths of 50 and 80 cm were much lower (0.42 to 0.72), especially in the rainy season, with even a tendency of a linear regression (Fig. 3a,b). The ANOVA of the three experimental factors (season, fish production and water depth) in the case of the diel DO variation gave the same results as that of the DO morning variation mentioned above. Only fish production and the water depth factors had significant effects on DO saturation (P b 0.001). Furthermore, only a
Fig. 2. Diel stratification of DO saturation (%) (mean ± S.D) during the dry season at depths of 20, 50 and 80 cm (a) in the pond with fish and (b) in the pond without fish.
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Fig. 3. Diel stratification of DO saturation (%) (mean ± S.D) during the rainy season at depths of 20, 50 and 80 cm (a) in the pond with fish and (b) in the pond without fish.
highly significant interaction existed between water depth and fish production factors. At water depths of 50 and 80 cm, DO saturation percentages of the treatment were significantly different from those of the control (DMRT, P b 0.05) in both seasons. 4. Discussion 4.1. Variations of water temperature and effects of fish production The treatment efficiencies of the waste stabilization ponds could be strongly affected by environmental parameters, especially water temperature and DO due to their essential role in the bacteria-algae symbiotic treatment processes of ponds (Mara, 1997; Hosetti and Frost, 1998). However, knowledge about the effects of fish production on these abiotic conditions in stabilization ponds is still limited. In the present study, a range of water temperatures from 28.7 to 30.8 °C in all experimental ponds during both rainy and dry seasons was recorded, favorable for the growth of tilapia (Chervinski, 1982) and for the development of algae and aquatic invertebrates as observed in tropical sewage-fed ponds (Liang et al., 1998). When comparing the effect of the three factors (season, water depth and fish production) on water temperature, it was shown that all had significant effects on the variation of water temperature. The effects of season and water depth factors were highly significant (P b 0.001) in comparison to that of fish production (P b 0.01). As a result, the former two would be predominant over the fish production factor on the variation of water temperature.
In the rainy season there was no significant difference in water temperature between water depths and between the treatment (with fish) and control (without fish) (Table 1). This indicates that the whole system, a shallow aquatic ecosystem, was partially mixed by the impacts of wind and rain. Therefore, there was no thermal stratification in the rainy season in all experimental ponds. In the dry season, contrarily, the system exhibited a clear thermal stratification as seen in the ponds without fish. The temperature of the upper layer was significantly higher than the one near the bottom. However, in the pond with fish no thermal stratification was recorded (Table 1). The activities of tilapia (such as feed scavenging, mating and building nests on the pond bottom) appeared to agitate the whole water column and therefore disperse the high temperature of the upper layer down to the lower ones. Recently, Rasmussen et al. (2005), using Rhodamine tracer, demonstrated that irrespective of flow rate, the presence of fish enhanced the mixing process, with the mixing time in tanks with fish being one-third that for tanks without fish. The in-tank dispersion coefficients and dispersion numbers differed significantly in the presence of fish. 4.2. Variations of DO saturation and effects of fish production Low DO concentrations are normally the main cause of fish die-off in sewage-fed fishponds (Bartone and Khouri, 1990; Ghosh et al., 1999). Therefore, investigation of the role the DO regime in ponds plays is important in integrating polluted water reclamation with fish production (Ghosh et al., 1999).
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In the present study, 1 week after the retention of the polluted water in ponds, a broad range of DO saturation values was recorded in the diel DO regime, depending on water depth. Generally, DO oversaturation (N8 mg O2 l− 1) occurred in the early afternoon in the surface water layer whereas DO depletion (less than 12% or b1 mg O2 l− 1) occurred at dawn in the whole water column. In a temperate climate, Ghosh et al. (1999) observed that a DO concentration as low as 0.2 to 2 mg O2 l− 1 occurred at the bottom zone (70 cm) in the daytime (8:00 am–6:00 pm), causing the death of silver carp (Hypopthalmichthys molitrix) and common carp (Cyprinus carpio) during rainy days. In the present study such daytime DO concentration levels (below 20% DO saturation) were also recorded at about the same water depth in the rainy season but only in the pond without fish. Dawn DO (2:00 to 6:00 am) less than 20% saturation (b2 mg O2 l− 1) was common in the diel DO regime of the experimental ponds. However, no fish mortality was observed. Tilapia can survive at DO concentrations as low as 1 mg O2 l− 1 by utilizing atmospheric oxygen, and its mortality only occurred when the DO saturation was below 20% for more than 2 to 3 days (Chervinski, 1982). Statistical analyses showed that the effect of fish production factor on DO was highly significant (P b 0.001) in the single-pond system. When fish were present, the DO saturation of the pond water increased significantly in the two lower water depths (50 and 80 cm). The bioturbatory activity of fish supported the diffusion of DO throughout the water column of the ponds. The quiescence of DO variations (invariable linear patterns) in the pond without fish at depths of 50 and 80 cm in the rainy season demonstrated this effect (Fig. 3b). At the water surface layer, however, intense photosynthesis likely led to the establishment of a regime of high DO, over riding the significant effect of fish on the difference in DO between the ponds with and without fish (as shown by a highly significant interaction, P b 0.001, between the water depth and the fish production factor). Also, similar DO saturation levels of the water surface layer in both seasons showed the non-significant effect of season on DO. It appeared that solar radiation in the study area was in a range favourable for algae to develop equally in both dry and rainy seasons. This resulted in active photosynthesis in the upper layers of the ponds, thereby producing daily over-saturated levels of DO in the early afternoon in both seasons (Fig. 2a,b) and generating a sufficient source of DO for diffusion downward to the lower layers of pond water through fish bioturbation. It is clear that the presence of tilapia improved the aerobic conditions in the water column where intense photosynthesis co-existed without significantly adverse seasonal impacts (data about phytoplankton are available in Phan-Van, 2006). The results of the present study supported the work by Rasmussen et al. (2005) that fish presence and any active moving objects in the water, can generate mixing of the water column, thereby enhancing the dispersion/diffusion of dissolved substances throughout its environment. There is evidence that fish bioturbation can impact water layers as deep as the water–soil interface at the superficial layer of pond bottom. Fish bioturbation enables oxidation of sedimenting organic matter by resuspending it into aerobic upper water layers (Scheffer, 1997), enhances the release of phosphorus (Jana and Das, 1992; Jana and Sahu, 1993; Saha and Jana, 2003), changes the structure of soil particles and decreases the concentrations of total dissolved sulphide, soluble manganese, easily oxidized matter, and exchangeable NH4-N at the superficial layer (Ritvo et al., 2004). 5. Conclusion Based on this study and previous research discussed above, it can be concluded that fish bioturbation not only enhances the aerobic
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conditions inside the whole water column but also in the pond bottom soils. Furthermore, fish bioturbation probably also affects other ecological aspects of the fishculture-integrated waste stabilization system such as the water quality reclamation process, nutrient recycling and variations of food web structure but these need to be further studied. The bioturbutary activities of tilapia can over ride the vertical distribution of water temperature in stabilization ponds to a depth of 1 m during the dry season. At the same time it can bring some DO downwards to the lower layers of the water column, improving aerobic conditions on the pond bottom. Acknowledgements The authors would like to express sincere thanks to the Ho Chi Minh City Department of Science and Technology and the Belgium Technical Cooperation for granting this research. References Avnimelech, Y., Kochva, M., Hargreaves, J.A., 1999. Sedimentation and resuspension in earthern fish ponds. J.World Aquacult. Soc. 30, 401–409. Bartone, C.R., Khouri, N., 1990. Reuse of stabilization pond effluents for fish culture in Lima, Peru. Preliminary experiment. In: Edwards, P., Pullin, R.S.V. (Eds.), Wastewater-fed Aquaculture, Proceedings of the International Seminar on Wastewater Reclamation and Reuse for Aquaculture, Calcutta, India. Environmental Sanitation Information Center, Asian Institute of Technology, Bangkok, Thailand, pp. 135–146. 6–9 December 1988, xxix+296 pp. Brix, H., 1999. How ‘green’ are aquaculture, constructed wetlands and conventional wastewater treatment systems? Water Sci. Technol. 40, 45–50. Chervinski, J., 1982. Environmental physiology of tilapias. In: Pullin, R.S.V., LoweMcConnell, R.H. (Eds.), ICLARM Conference Proceedings 7. The Biology and Culture of Tilapias. International Center for Living Aquatic Resources Management, Manila, Philippines, pp. 119–128. 432pp. De Pauw, N., Salomoni, C., 1991. The use of microalgae in wastewater treatment: achievements and constraints. In: Madoni, P. (Ed.), Biological Approach to Sewage Treatment Process: Current Status and Perspectives, Perugia, pp. 329–352. Edwards, P., 1992. Reuse of human wastes in aquaculture: A technical review. UNDP-World Bank Water and Sanitation Program. The World Bank, Washington, D.C., U.S.A. 350 pp. Edwards, P., 2005. Development status of, and prospects for, wastewater-fed aquaculture in urban environment. In: Costa-Pierce, B., Desbonet, A., Edwards, P., Baker, D. (Eds.), Urban Aquaculture. CABI Publishing, Wallingford, Oxford, U.K., pp. 45–59. Ghosh, C., Frijns, J., Lettinga, G., 1999. Performance of silver carp (Hypopthalmichthys molitrix) dominated integrated post treatment system for purification of municipal waste water in a temperatate climate. Bioresource Technol., 69, 255–262. Hosetti, B., Frost, S., 1998. A review of the control of biological waste treatment in stabilization ponds. Crit. Rev. Environ. Sci. Technol. 28, 193–218. Jana, B.B., Das, S.K., 1992. Bioturbation induced changes of fertilizer value of phosphate rock in relation to alkaline phosphatase activity. Aquaculture 103, 321–330. Jana, B.B., Sahu, S.N., 1993. Relative performance of three bottom grazing fishes (Cyprinus carpio, Cirrhinus mrigala, Heteropneustes fossilis) in increasing the fertilizer value of phosphate rock. Aquaculture 115, 19–29. Liang, Y., Cheung, R.Y.H., Everitt, S., Wong, M.H., 1998. Reclamation of wastewater for polyculture of freshwater fish: wastewater treatment in ponds. Water Res. 32, 1864–1880. Mara, D.D., 1997. Design manual for waste stabilization ponds in India, Lagoon Technology International Ltd. University of Leeds, Leeds, U.K. 150 pp. Oswald, W.J., 1995. Ponds in the twenty-first century. Water Sci. Technol. 31, 1–8. Phan-Van, M. (2006). Single pond system for integration of polluted water reclamation with tilapia fingerling production. PhD thesis, Ghent University, 304pp. Phan-Van, M., De Pauw, N., 2005. Wastewater-based urban aquaculture systems in Ho Chi Minh City, South Vietnam. In: Costa-Pierce, B., Desbonet, A., Edwards, P., Baker, D. (Eds.), Urban Aquaculture. CABI Publishing, Wallingford, Oxford, U.K, pp. 77–102. Rasmussen, M.R., Laursen, J., Craig, S.R., McLean, E., 2005. Do fish enhance tank mixing? Aquaculture 250, 162–174. Riise, J.C., Roos, N., 1997. Benthic metabolism and the effects of bioturbation in a fertilized polyculture fish pond in northeast Thailand. Aquaculture 150, 45–62. Ritvo, G., Kochba, M., Avnimelec, Y., 2004. The effects of common carp bioturbation on fishpond bottom soil. Aquaculture 242, 345–356. Saha, S., Jana, B.B., 2003. Fish macrophyte association as a low-cost strategy for wastewater reclamation. Ecol. Eng. 21, 21–41. Scheffer, M., 1997. Ecology of Shallow Lakes. Kluwer Academic Publishing, Dordrecht, The Netherlands. 384pp. Steel, R.G.D., Torrie, J.H., 1960. Principles and Procedures of Statistics. McGraw-Hill Book Company, Inc, New York, U.S.A. 481 pp.