Six shrimp culture ponds of 1 ha each, located between 101° 22′ E 2° 49′N −101° 22′ E 3° 17′N, were used to elucidate the effects of using eutrophic waters on phytoplankton communities. Three ponds were filled with unpolluted water, while the other three received eutrophic water. Water quality and phytoplankton populations were analyzed fortnightly over a period of 110 days to coincide with the shrimp culture cycle. In ponds with eutrophic water, the cyanobacteria (nine species) were the dominant phytoplankton group (>90% of the total phytoplankton density), followed by the green algae (seven species) and diatoms (six species). Ponds with originally unpolluted water were dominated by the diatoms with 18 species, followed by the cyanobacteria (six species) and one species of green algae. Shrimp production in ponds with unpolluted water was significantly higher (4,877.4 ± 438.5 kg ha-1 when compared to 1,385.0 ± 243.8 kg ha-1 in ponds using eutrophic water. This study illustrated that initial water quality supply influenced the phytoplankton dominance, which in turn determined the aquaculture production in shrimp culture ponds.
Shrimp culture production is frequently determined by the environmental conditions of the pond, especially water quality. Ultimately, it is the water quality that will influence optimal shrimp growth and yield (Casé et al., 2008). Water quality in the ponds is determined by the initial water quality used for the culture, as well as the organic loadings into the ponds in the form of feeds and fertilizers during the culture cycle. In shrimp culture ponds, usually nutrients such as phosphorus and nitrogen progressively increase with the culture period due to excess feed materials and excretory products (Matias et al., 2002). Increase of nutrients results in eutrophication characterized by low oxygen, high ammonia, high hydrogen sulphide, and high densities of cyanobacteria (Yusoff et al., 2002). In addition, large amount of organic matter in the water column finally settle on the pond bottom resulting in increased layer of anaerobic sludge which constantly release high concentrations of inorganic phosphorus and ammonia into the water column (Boyd, 1990; Yusoff et al., 2001). Studies of Hall et al. (1990), Holmer and Kristensen (1992) and Hansen et al. (1993) revealed that large amount of organic matter in the pond bottom can also lead to anaerobic processes like sulfate reduction and methanogenesis.
Phytoplankton communities undergo a continual succession of dominant species due to dynamic changes in growth factors such as light, temperature and nutrient concentrations (Harrison and Turpin, 1982; Casé et al., 2008). In areas where temperature is high and light is abundant, nutrient concentrations and ratios become important environmental factors influencing the dominance of various taxonomic groups (Harrison and Turpin, 1982; Hecky and Kilham, 1988; Yusoff and McNabb, 1997; Yusoff et al., 2002). Sanders et al. (1987) reported that nutrient–loading ratios can exert a strong selective effect on natural communities of phytoplankton. In natural sea-water in the tropics, diatoms are usually the dominant group contributing to more than 80% of the total phytoplankton populations (Zubaidah et al., 2003). With the onset of eutrophication, the diatom population decreases and other groups of algae, such as cyanobacteria and dinoflagellates persist. Officer and Ryther (1980) suggested that eutrophication of coastal waters by domestic wastes relatively poor in silica could lead to silica depletion and elimination of diatoms.
The availability and quality of water supply for shrimp farming is one of the most important factors in determining the location of the farm. Most shrimp farms in Malaysia obtain optimum yields between 4–6 tons ha−1 in the first 3–5 years of their operation. Unfortunately, most farms in the country do not treat their effluents, and often discharge both the water and sediment wastes into the same river from where they obtain their water supply. Over the years, self-eutrophication of the water supply causes a significant decline in the shrimp yields. This study was undertaken to demonstrate the consequence of using eutrophic waters on the phytoplankton communities in the culture ponds and its impacts on the shrimp production.
Six 1-ha marine shrimp culture ponds located on the west coast of Peninsular Malaysia, between 101° 22′ E 2° 49′N and 101° 22′ E 3° 17′N, were used for the experiment. Three ponds received relatively unpolluted seawater from the farm reservoir, whilst the other three received eutrophic water from the nearby estuary. Ponds were treated with agriculture lime at the rate of 2 t ha−1and hydrated lime at 1.5 t ha−1. Each pond was stocked with 28-day old Penaeus monodon post larvae at the rate of 37 individuals m−2. Water exchange in the ponds ranged from 2–25% every two days.
Shrimps were fed with commercial pellets twice a day for the first month, four times a day for the next two months and six times a day until harvest. The daily feeding rate was 6–10% of shrimp body weight during the first month and 4–5% during the rest of the culture period. A pair of paddlewheel aerators were used for the first two months (4–12 h day−1) and subsequently increased to four aerators (12–20 h day−1) to ensure adequate dissolved oxygen levels in the ponds.
Water quality parameters such as temperature, dissolved oxygen, pH and salinity were measured in situ fortnightly using the Hyrolab Surveyor 3 multimeter. Similarly, phytoplankton samples were collected fortnightly during the shrimp culture period of 110 days. Samples were collected using Van Dorn water sampler and transferred into labeled polyethylene bottles. Samples were immediately preserved by adding 10 ml Lugol's iodine solution to 1000 ml water samples. Phytoplankton samples were concentrated by sedimentation method (Clesceri et al., 1989), identified and counted using a counting chamber under Carl Zeiss inverted microscope. Phytoplankton identification followed Tomas (1997).
Water samples were filtered using 0.45 μm filters prior to analyses of dissolved reactive phosphorus (SRP), total ammonia, nitrate and nitrite. Total phosphorus and total nitrogen analyses were performed on unfiltered water samples. All analyses were done following Parsons et al. (1984).
Field data were analyzed using t-test to detect any significant difference in water quality parameters, phytoplankton abundance and chlorophyll a between treatments at p < 0.05.
In this study, the eutrophic water supply contained significantly higher (p < 0.05) total ammonia, nitrate, nitrite-N, SRP and total phosphorus than the unpolluted water (Table 1). The values of environmental parameters were relatively constant during the study period, with mean water temperature, day-time dissolved oxygen, pH and salinity ranging from 29.3°C – 31.0°C; 7.1 – 8.7 mg l−1; 7.4–8.4; 27.2–29.5 ppt, respectively.
As a result of higher nutrient levels, ponds with eutrophic water supply had significantly higher mean chlorophyll a concentrations (p < 0.05) than those in ponds with unpolluted water supply (Table 1). Phytoplankton communities in ponds with eutrophic water were dominated by the cyanobacteria, with nine species and a mean density of 122.37 × 103 cells ml−1 (Table 2). Cyanobacteria in these ponds formed more than 90% of the total phytoplankton populations from the initial stage of the culture until the end (Figure 1a). In ponds with eutrophic water, there were six species of diatoms with a mean density of 3.07 × 103cells ml−1(Table 2). The chlorophytes were the second most abundant group in eutrophic ponds with seven species and a mean density of 17.19 × 103cells ml−1(Table 2). Other groups of algae such as euglenoids and dinoflagellates had fewer species with mean densities of less than 1.0 × 103cells ml−1 (Table 2).
In ponds with water from an unpolluted source, diatoms were more abundant with 18 species and a mean density of 9.19 × 103 cells ml−1, followed by cyanobacteria with six species and a mean density of 7.20 × 103 cells ml−1 (Table 2). The chlorophytes were the third most dominant group whilst the other algal groups, euglenoids and dinoflagellates were found with less than 1.0 × 103cells ml−1 (Table 2). In these ponds, diatoms formed 86.93% of the total phytoplankton in the initial phase of the culture cycle (first 45 days) (Figure1b). Due to increasing levels of nutrients as the culture progresses, the percentage of cyanobacteria increased to 48.31%. However, this value was significantly lower than that observed in the ponds using eutrophic waters.
The nutrient concentrations in the water supply depend on the degree of contamination of the water source by anthropogenic discharges from domestic, industrial and agricultural activities in the upstream. In fact, aquaculture farms themselves often pollute by releasing effluent into the environment from where they receive the water supply. Costanzo et al. (2004) reported that elevated nutrient and phytoplankton concentrations in a tidal mangrove creek in north-east Australia were caused by the discharge of shrimp pond effluents. Trott and Alongi (2000) also noted that chlorophyll a and biological oxygen demand at the discharge site of shrimp farm effluent were significantly higher than in the control estuaries. However, study of Tahir et al. (1997) showed that the level of pollution due to hydrocarbons in the estuarine and near-coastal waters in the Peninsular Malaysia was insignificant and the waters were considered unpolluted.
In the aquaculture ponds, nutrient concentrations progressively increased with the culture period due to excess feeds and accumulated metabolites from the cultured shrimp (Matias et al., 2002). In this present study, mean nutrient values such as ammonia, nitrite, nitrate and total phosphorus were significantly higher (p < 0.05) in ponds receiving eutrophic water supply. Due to organic loading from aquaculture practice, even ponds with water from unpolluted source had relatively high nutrient concentrations by the second month of the culture period.
In the ponds receiving polluted water source, cyanobacteria (specifically from the genus Oscillatoria) dominated the phytoplankton community comprising >90% of the total population even in the beginning of the culture period. Alonso-Rodriguez and Paez-Osuna (2003) reported that cyanobacteria was also the dominant group (>88%) in NW Mexican shrimp ponds. Similarly, Casé et al. (2008) reported cyanobacteria as mostly responsible for phytoplankton blooms in tropical marine shrimp ponds in Brazil.
In shrimp ponds which received unpolluted water, diatoms (mostly Chaetoceros and Rhizosolenia) dominated (87%) in the beginning of the culture period (1–45 days) (Figure 1b). Yusoff et al. (2002) also reported that shrimp ponds which received unpolluted water were dominated by diatoms at the beginning of the culture period and cyanobacteria only appeared after the first month of the culture. According to Casé et al. (2008), in marine shrimp culture ponds diatom dominance was replaced by cyanobacteria as nutrient concentrations increased and silica was depleted with the culture period. Similarly in this study, the percentages of diatoms decreased as a response to increasing nutrients. Despite of the decrease, diatoms remained >40% of the total phytoplankton throughout the culture period in the ponds receiving unpolluted water supply (Figure 1).
The presence of phytoplankton such as diatoms at the beginning of the culture period was crucial as they formed the beneficial natural feed for the small shrimp post-larvae either as direct food source or indirectly through the food chain. In addition, good water quality in the initial phase of culture was also important to allow the growth and proliferation of beneficial macrobenthos which would become major natural food items for the shrimp in their juvenile and adult stages. According to Shishehchian et al. (2001), highest density of edible and beneficial benthos such as polychaetes and insect larvae were available in ponds with good environmental condition. These factors were of primary importance to allow the maintenance of good environment for the growth and survival of the shrimp post larvae. In the present study, the ponds with unpolluted water produced 3.5 times more shrimp (4,877.4 ± 438.5 kg shrimp ha−1) compared to ponds using eutrophic water (1,385 ± 243.8 kg ha−1). This is probably due to the fact that shrimp in the ponds receiving unpolluted water had been provided with good environment and quality natural food during the early stage of the culture cycle. Studies of Shishehchian et al. (2001) showed higher shrimp production in ponds with higher densities of natural food. In addition, they found that shrimp provided with good quality natural food during the early stage of culture cycle had significantly higher feed conversion ratio.
The present study illustrated that using eutrophic water supply caused rapid water quality deterioration in aquaculture ponds accelerated by the additional organic loadings from feed, feces and decomposed microalgal mats. Alonso-Rodriguez and Paez-Osuna (2003) reported that phytoplankton was most abundant in the advanced stage of the culture cycle. Eutrophic conditions of the ponds in the present study, characterized by high nutrient concentrations and dominance of cyanobacteria, resulted in low shrimp production. Although using unpolluted water eventually culminated in eutrophic conditions, the good water quality and beneficial phytoplankton species in the initial culture phase has probably contributed to better growth rate and high survival of the cultured shrimp.
This research was supported by the Government of Malaysia under IRPA (Intensification of Research in Priority Areas) Grant, Project No. 01-02-04-165 and the International Foundation for Science Grant No. A/2496-IFS.