Water quality, nutrient dynamics and sediment profile in shrimp farms of the Sundarbans mangrove forest, Bangladesh

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1 Indian Journal of Marine Sciences Vol. 33(2), June 2004, pp Water quality, nutrient dynamics and sediment profile in shrimp farms of the Sundarbans mangrove forest, Bangladesh M L Islam*, M J Alam, S Rheman, S U Ahmed & M A Mazid Bangladesh Fisheries Research Institute, Brackishwater Station, Paikgacha, Khulna 9280, Bangladesh [ latiful2000@yahoo.com] Received 6 March 2003, revised 30 December 2003 Investigation was carried out to observe the sediment-water characteristics and nutrient budget of traditional shrimp farms during January- December Most of the water quality parameters between inlet and outlet of shrimp farms were similar, but transparency, turbidity, total suspended solid (TSS), phytoplankton and zooplankton concentration and primary productivity were significantly (p < 0.05) different within inlet and outlet. Pollution indicator parameter, chemical oxygen demand (COD) was more or less similar between these two points indicating less or no chance of coastal eutrophication from shrimp farm effluent. Nutrient balance due to water exchange showed that about kg of nitrogen (N)/ha/culture cycle and kg of phosphorus (P)/ha/culture cycle were being deposited into the bottom sediment. Most of the sediment parameters between mangrove and shrimp farms were similar, except calcium (Ca), magnesium (Mg), potassium (K) and sulphur (S). Nitrogen (N), Phosphorus (P), Organic matter (OM) contents were low in the sediment sample indicating the sediment status poor and needs improvement through proper fertilization. [Key words: Water quality, nutrient dynamics, sediment profile, shrimp farm, mangrove] Introduction In Bangladesh, shrimp farming has taken place mainly on the reclaimed mangrove forest at the Sundarbans. The region has rich diversity of flora and fauna. The mangrove ecosystem in the Sundarbans is more dynamic, fragile, unique and complex where the plant and environmental factors interact in the process of energy fixation, accumulation of biomass, decomposition of dead organic matter and in nutrient recycling. The complex types of mangrove ecosystem provide an ideal resting and/or breeding ground to most of the marine and brackishwater fish and shellfish. In Bangladesh, shrimp culture was first introduced at the Sundarbans during The present culture area is around 1,41,353 ha 2. The rapid and horizontal expansion of shrimp farming, particularly in the greater Khulna region, caused a great threat to the natural mangrove ecosystem. Large volume of discharged effluents from shrimp farms can increase the salinity of receiving waters during period of low tide. In addition, shrimp farm effluents often consist of high organic matter, both suspended and dissolved, resulting in high biological oxygen demand (BOD) and chemical oxygen demand (COD), which can cause oxygen depletion in recipient water. Nutrient budget of intensive shrimp farms has been estimated by many authors 3-6, but nutrient budget of traditional shrimp farms not yet studied. Considering the above, the present study aimed to investigate the sediment-water characteristics and nutrient budget of shrimp farms under existing culture practices. Materials and Methods The study was carried out in the three shrimp farms and mangrove area (1 point) of Khulna region during January-December The study area is located near to the Sundarbans mangrove forest, a tropical ecosystem, covering an area of 5,772 km 2 of which 3,935 km 2 is land and 1,777 km 2 is water crisscrossed by rivers and canals. The area under the present study [between N and N and E] is located at three different Upazilas viz., Paikgacha, Dacop and Koira of Khulna district, the most south-western portion of Bangladesh. Three ghers were selected from three locations: Gher-1 in the Paikgacha upazila, Gher-2 in the Dacop upazila, and Gher-3 in the Koira upazila. In addition, one mangrove habitat was also selected at the Sundarbans, near the Hudda forest Station, about 0.5 km inside the forest to compare the sediment profile.

2 ISLAM et al.: WATER QUALITY IN SHRIMP FARMS 171 From the inlet and outlet point of each gher, water samples were collected at monthly intervals and analyzed. Water temperature ( mercury thermometer), transparency (standard secchi-disc 18 cm dia), turbidity (spectrophotometer DR 2000), water ph (hand water ph meter ph scan-2) and salinity (hand refractometer ATAGO S/Mill-E) were measured. The amount of water exchange (as level, cm) of the ghers was measured by the meter scale set up in each gher. Total dissolved solids (mg/l) was monitored directly by a combined conductivity meter (Jenway 4200). Dissolved oxygen (mg/l), total hardness (mg/l) and total alkalinity (mg/l) were monitored by the respective portable test kit (Hanna). While total suspended solids (g/l) was estimated using pre-weighed filter paper (GF/C) by weight reduction method, COD (mg/l), nitrate-nitrogen (mg/l), nitrite-nitrogen (mg/l), ammonia-nitrogen (mg/l) and phosphate-phosphorus (mg/l) were analyzed in laboratory 7. Plankton samples were collected and identified 8. Primary productivity was measured following the classical white and dark bottle method 9. Nutrient budget was estimated redacting the nutrient (N&P) present in the discharged water from nutrient in the intake water. Sediment samples from the three ghers and mangrove points were collected during April, July, September and December Sediment samples were dried in room temperature, powdered and stored in the desiccator. Sediment samples were analyzed (total analysis) following atomic absorption spectrophotometry method 10. ANOVA was done using standard statistical package to observe the difference between mangrove and shrimp farm habitat. Results and Discussion Management features of the ghers The management features of the selected farms are presented in Table 1. All the selected ghers were large in size ( ha) and very old in shrimp farming (8-16 years). Stocking and harvesting style was multiple type. Exchange of water was frequent (stocking to harvest) by tidal water directly from the adjacent rivers without any treatment. Most of the ghers produced shrimp during the high saline period (January-August) and rice during low saline period (August- December), depending upon the reclamation of soil and water salinity. Water quality Variations of inlet and outlet water quality of the ghers are given in Table 2. In the inlet point, water temperature ranged between C and C, but in the outlet it varied between C and C. The lowest and highest values were observed in February and May respectively. Outlet water temperature was always higher than that of the inlet water, which might be due to shallower depth into the ghers, as the lower water depth contributes to higher thermal conductivity of water 11. Intake of water due to water exchange ranged within 9.80 and cm, whereas discharge of water level ranged between 9.60 and cm. Initially the intake was higher, but the discharge was found higher than the intake at the end of the culture period. In inlet water, transparency ranged within cm, whereas in discharged point it ranged within cm. Transparency was found always relatively higher in outlet water than the inlet water. Turbidity of water was also found to vary within a wide range. In the inlet point, it varied between and FTU, but in outlet point it varied between and FTU. While, total suspended solid ranged between 0.33 and 1.22 g/l in the inlet water, it ranged between 0.24 and 0.42 g/l in outlet water. With a little exception, total suspended solid was always higher in inlet water than the outlet. Water ph was observed to be slightly alkaline. It ranged between 7.63 and 8.07 in the inlet water and 7.73 and 8.27 in the outlet water. Total dissolved solids were found to range widely. In the inlet water it ranged within g/l and in the outlet water it ranged within g/l. The highest salinity level was 15 ppt in both inlet and outlet in the month of May and June, but the lowest level was 0.00 ppt and 2.53 ppt in inlet and outlet respectively in September. Dissolved oxygen content was observed as moderate in both inlet and outlet water. It ranged mg/l in the inlet water and mg/l in the outlet water. The highest level in outlet water (7.07 mg/l) was observed in May when aquatic weeds (Najas spp) were found to grow influencing an increase in the dissolved oxygen content through photosynthesis. The lowest DO (4.2 mg/l) was found in the same gher in July, probably due to the decomposition of the aquatic weeds. COD of inlet water ranged within mg/l, whereas in outlet water it ranged between 2.57 and 3.54 mg/l, with an exception of 8.00 mg/l during the month of July, possibly at the time of decomposition of aquatic

3 172 INDIAN J. MAR. SCI., VOL. 33, No. 2, JUNE 2004 Table 1 Description, management features and land utilization of the selected ghers under study Items Gher-3, Hudda Gher-2, Dacop Gher-1, Paikgacha Farm description Farm area 12.7 ha 95 ha. 90 ha. Age of farm 16 years 8 years 11 years Water source Hudda river Sibsa river Kapataskha river Nos. of inlet and outlet 1 sluice serves as inlet and outlet purpose 1 inlet and 2 outlet, situated in the opposite side of the gher Water depth (cm) cm cm cm Distance from Sunderbans 0.2 km. 15 km 17 km 1 inlet and 2 outlet, situated in the same side of the gher. Farming and management Drying Yes Yes Yes Ploughing High portion (15%) High portion (25%) High portion (35%) Flushing Yes Yes Yes Liming and dose No Initially (35 kg/ha) Initially (15 kg/ha) Fertilization (Inorganic) No Initially (25 kg/ha, Urea:TSP=3:1 ) Initially (15 kg/ha, Urea:TSP=3:1 ) Fertilization (Organic) Cowdung Mustard Oil Cake Mustard Oil Cake 30 kg/ha 3.2 kg/ha 35 kg/ha Water exchange 20-30% in each new and full 20-30% in each new and full moon moon Stocking style Multiple Multiple Multiple Stocking density /m /m /m 2 Feeding No No No Culture duration (day) % in each new and full moon Land use (day/month) Shrimp farming (Jan.-Aug.) Agriculture (Aug.-Dec.) Unused (Dec.-Jan) (Jan.-Aug.) (Aug.-Dec.) (Dec.-Jan) (Jan.-Aug.) (Aug.-Dec.) (Dec.-Jan) Table 2 Mean values of monthly variation of inlet and outlet water quality of the studied ghers Parameters Highest Lowest Mean±SD inlet outlet inlet outlet inlet Outlet Temperature ( 0 C) ± ±3.454 Transparency (cm) ± ±5.173 Turbidity (FTU) ± ± Total suspended solids (g/l) ± ±0.058 Water ph ± ±0.199 Total dissolved solids (g/l) ± ± Salinity (ppt) ± ±4.438 Dissolved oxygen (mg/l) ± ±0.846 Chemical oxygen demand (mg/l) ± ±1.839 Alkalinity (mg/l) ± ± Total hardness (mg/l) ± ± Ammonia (mg/l) ± ± Nitrate (mg/l) ± ±0.375 Nitrite (mg/l) ± ± Phosphate (mg/l) ± ±0.119 Conductivity (ms/µs) ± ± Phytoplankton (cell/l) ± ± Zooplankton (unit/l) ± ± Primary productivity (gc/m 3 /hr) ± ±0.025

4 ISLAM et al.: WATER QUALITY IN SHRIMP FARMS 173 weeds (Najas sp). Total alkalinity in inlet water ranged within mg/l and in outlet water it ranged within mg/l. Total hardness showed a wide range all over the study period. In the inlet water, it ranged mg/l, but in the outlet water, it ranged between 341 and 1275 mg/l. The highest values in both inlet and outlet were found in May. Ammonia ( mg/l) and nitrite ( mg/l) were found in a limited concentration in both inlet and outlet water. Nitrate-nitrogen in the inlet water ranged between 0.86 and 1.73 mg/l, the highest was found in May. In the outlet water, nitratenitrogen ranged within mg/l with the highest values in July. Phosphate-phosphorus in the inlet water ranged within mg/l with the highest value in August, but in outlet water it ranged within mg/l with the highest value in May. Conductivity of water was found to vary widely. In inlet water, it ranged within ms/µs, whereas in outlet water, it ranged within ms/µs, with the highest levels in May and June. Water conductivity varied with the increase and decrease of the water salinity. Phytoplankton density (Table 2) in the inlet water ranged between 166 and 333 cells/l, but in the outlet water it ranged between 367 and 706 cells/l, with the peak in April. The concentration of zooplankton varied between 436 and 985 no/l in the inlet water, whereas it varied between 230 and 467 no/l in the outlet water. The concentration of phytoplankton was always higher in the outlet water, but zooplankton showed higher concentration in the inlet water. Primary productivity values in the inlet water ranged between and gc/m 3 /hr, but in outlet water, it ranged between and gc/m 3 /hr. Statistical analysis of water quality parameters between inlet and outlet are indicated in Table 3. Water transparency, phytoplankton concentration and primary productivity were significantly higher (P<0.05) in the outlet water, whereas, turbidity, TSS and zooplankton concentration were significantly higher (P<0.05) in the inlet water. The reasons behind that might be due to intake of silt and clay along with tidal water, which hindered the light penetration in the inlet water and subsequently resulted in low phytoplankton concentration and primary productivity. Moreover, entrance of ichthyoplankton and nauplii larvae along with tidal water intake caused higher zooplankton concentration in the inlet water. Nutrient budget The nutrient budget through water exchange are furnished in Table 4. The highest deposition of phosphorus (7405 kg) and nitrogen (17,623 kg) were found in April and May, respectively, during the initial phase of culture. The highest disposal of nitrogen (20,992 kg) and phosphorus (6255 kg) were observed during September at the time of end of the culture. On an average, kg/ha/culture cycle of nitrogen and kg/ha/culture cycle of phosphorus were entrapped into the gher which were being used during the agricultural cropping. Excessive use of feed and fertilizers in semiintensive 3 and intensive 4 culture systems causes considerable amount of nutrient input in the natural water through the discharge canal. Undoubtedly, the effluent loading is strongly affected by the water exchange rate throughout the growth cycle 5. The shrimp farming practice in Bangladesh is still traditional to improved traditional. Farmers use low amount of lime and fertilizers and the application of feed is rare or absent. The nutrient budget of the present study indicated that considerable amount of nitrogen and phosphorus entered through inlet water is entrapped into the gher and not discharged through outlet water. Traditional system for shrimp farming is not expected to provide any significant loading of organic matter and nutrients to the surrounding Table 3 ANOVA of water quality parameters within inlet and outlet in the ghers under study Parameters Source of variance Inlet versus Outlet MS F value Temperature ( 0 C) Water exchange (cm) Transparency (cm) ** Turbidity (FTU) ** Total suspended solids (g/l) ** Water ph Total dissolved solids (g/l) Salinity (ppt) Dissolved Oxygen (mg/l) Chemical Oxygen Demand (mg/l) Alkalinity (mg/l) Hardness (mg/l) Ammonia (mg/l) Nitrate (mg/l) Nitrite (mg/l) Phosphate (mg/l) Conductivity (ms/µs) Phytoplankton (cells/l) ** Zooplankton (unit/l) ** Primary productivity (gc/m 3 /hr) ** **Significant at 5% level, df= 1 & 4

5 174 INDIAN J. MAR. SCI., VOL. 33, No. 2, JUNE 2004 coastal environment 6. According to Edwards 12, in extensive and semi-intensive systems, typically a major part of nitrogen and phosphorus is accumulated in the sediments. These observations are strongly in the support of findings of the present study. Sediment profile Nutrient constituents and heavy metal content in sediment of mangrove and shrimp ghers are presented in Table 5. In the mangrove habitat, ph ranged between 8.1 and 8.5, but in shrimp farm it ranged between 7.8 and 8.0, with the exception of 6.0 in Gher-3. This exception might be due to the fact that, as the gher was very close to the mangrove (0.1km), semi-decomposed and decomposed litter influxed along with tidal water entering into the gher causing lower ph value. In both habitats ph was found to be alkaline in reaction. The ph levels of the present study differ from the findings of Murali & Smilde 13, but consistent with the observations of Matilal et al. 14. Organic matter (OM) content in mangrove sediment ranged within %, but in shrimp gher it ranged within %. Higher organic content was found in Gher-3, which might be due to the decomposition of aquatic weed (mainly Najas sp.) grown in the gher in considerable amounts. The amount of total nitrogen (N 2 ) in mangrove sediment ranged within %, but it ranged within % in shrimp farm. Total nitrogen content was slightly higher in shrimp ghers than in the mangrove habitat. Phosphorus (P) content ranged within µg/g sediment in mangrove, whereas it ranged between 5.12 and µg/g sediment in shrimp gher. Phosphorus content was found to be slightly higher in shrimp farms than in the mangrove habitat. Sulphur (S) was found to range between and µg/g in mangrove sediment, but it ranged within µg/g in shrimp farms sediment. Calcium (Ca) content was me/100g in mangrove sediment, whereas it ranged between 8.00 and me/100g in shrimp farm sediment. Magnesium (Mg) was found to range between 3.50 and 5.50 me/100g in mangrove sediment, but it ranged between 7.00 and 9.50 me/100g in the shrimp farms sediment. Potassium (K) in the mangrove habitat ranged between 0.53 and 0.93 me/100g, whereas it ranged between 0.66 and 1.39 me/100g in the shrimp farms sediment. Among the trace elements as well as heavy metal contents, Copper (Cu) ranged within µg/g

6 ISLAM et al.: WATER QUALITY IN SHRIMP FARMS 175 Collection period ph OM* (%) Table 5 Seasonal changes in sediment composition in Mangrove and shrimp farming ecosystem. Ca (me/100g) Mg (me/100g) K (me/100g) Characters Total N (%) P S Cu Fe Mn Mangrove April July September December Gher 1, Paikgacha region April July September December Gher 2, Dacop region April July September December Gher 3, Koira region April July September December OM*= Organic matter Zn in mangrove sediment, but it ranged within µg/g in the shrimp ghers sediment. Iron (Fe) content varied within µg/g in mangrove sediment and it ranged within µg/g in the shrimp ghers sediment. Manganese (Mn) ranged between and µg/g in mangrove, but it ranged within µg/g in shrimp farms sediment. Zinc (Zn) content varied between µg/g in mangrove sediment, whereas it ranged within µg/g in shrimp farms sediment. Nitrogen and phosphorus content in the gher were found to have an increasing trend with the increasing culture period. It might be due to the fact that nitrogen and phosphorus entered into the gher during water exchange and were entrapped within sediment-water interface. In extensive and semi-intensive systems within low water exchange rate, typically a major part of nitrogen and phosphorus are accumulated in the sediments 12. Organic matter, nitrogen and phosphorus contents in sediment sample in the present study were lower than the earlier observations 15,16, indicating poor sediment quality and productivity and a scope of its improvement through organic and inorganic fertilization 17. Statistical analysis of soil samples within mangrove and shrimp gher showed that Ca, Mg, K and S showed significant (p<0.05) difference within mangrove and shrimp farms. Other parameters were not statistically significant. Fig. 1 Salinity status of the mangrove river, mangrove sediment and gher sediment The salinity level of mangrove river, sediment salinity of mangrove and shrimp farm are depicted in Fig. 1. Water salinity was found to display an increasing trend from February to May, with a peak in April-May period and abruptly declined in June. The salinity level of sediment in the mangrove was found to gradually increase or decrease depending on the river water salinity. In the case of shrimp gher, sediment salinity, reduced nevertheless with the decrease of natural water salinity. During September- October, due to the heavy rainfall, river water salinity falls abruptly and the time is appropriate for seedling and cultivation of rice. The shrimp farmers tried their best to hold the saline water beyond the commencement of paddy cultivation season. As a result, a conflict between the shrimp farmers and

7 176 INDIAN J. MAR. SCI., VOL. 33, No. 2, JUNE 2004 landowners occurs very often. This salinity holding phenomenon as well as conflict between shrimp farmer and land owner is more prominent for the gher which takes water from the nearer ghers, due to the lack of sufficient freshwater for flushing and reclamation of soil salinity in time. Shrimp farming in rice fields leads to gradual build-up of soil salinity that may render a soil unfit for crop cultivation in the long run 18. The observation of the present study reveled that, the impact of shrimp farming could be minimized by flushing saline water to leach salinity of soil in time with available supply of freshwater. References 1 Kurien C V & Sebastain V D, Prawn and prawn fisheries in India. (Hindustan Publishing Corporation, Delhi, India) 1976, pp Ahmed M N U & Islam M R, Importance and development possibility of fisheries sector. Fish week conpendium (Directorate of Fisheries, Ministry of Fisheries and Livestock, Dhaka, Bangladesh) 1999, pp Paez-Osuna F, Guerreo-Galval S R, Ruiz-Fernandez A C & Espinoza-Angulo R. Fluxes and mass balances of nutrients in a semi-intensive shrimp farm in North-western Mexico, Mar Poll Bull No 34, (1997) Funge-Smith J S & Briggs M R P, Nutrient budget in intensive shrimp ponds: implications for sustainability. Aquaculture, 164 (1998) Hopkins J S, Hamilton II R D, Sandifer P A, Browdy, C L & Stokes, A D, Effects of water exchange on production, water quality, effluent characteristics and nitrogen budgets of intensive shrimp ponds, J World A Soc, 24 (1993) Phillips M J, Lin, C K & Beveridge M C M, Shrimp culture and the environment: lessons from the world's most rapidly expanding aquaculture sector, In: Environment and aquaculture in developing countries, edited by: R S V Pullin H Rosenthal and J L Maclean (ICLARM and GTZ, ICLARM Conf Proc) 31(1993) Strickland J D H & Parsons T R, A practical handbook of sea water analysis, Bull No 167, (Fish. Res. Bd. Canada, Ottawa, Canada) 1987, pp Prescott G W, Algae of the Western Great Lakes area, (Wn C Brawn Co, Inc Dubuque, Iowa,) 1987, pp APHA, Standard methods for the examination of water and waste water, (American Public Health Association, Washinton, DC) 1989, pp AOAC, Official methods of analysis, 13 th ed, (Association of Official Analytical Chemists, Washington, DC) 1980, pp Islam M S, Islam M A, Alam M S & Reshma B Z, Effects of shrimp farming on physico-chemical qualities of water in some medium saline areas in greater Khulna district, Bangladesh J. Fish., 21(1998) Edwards P, Environmental issues in integrated agricultureaquaculture and waste-water fed fish culture systems. In: Environment and aquaculture in developing countries, edited by: R S V Pullin, H Rosenthal and J L Maclean, (ICLARM and GTZ, ICLARM Conf Proc), 31(1993) Murali N S & Smilde K W, Agriculture and soil fertility: required imputs and their availability, Paper presented at AIT-LAWQQ Workshop on sustainable use of coastal land in southeast Asia. AIT, Bankok, Thailand, Matilal S, Mukherjee B B, Chaterjee N & Gupta M D, Studies on soil and vegetation of mangrove forests of Sundarbans, Indian J Mar Sci, 15 (1986) Smith P T, Physical and chemical characteristics of sediments from prawn farms and mangrove habitat on the Clarence river, Australia, Aquaculture, 164 (1996) Chien Y H & Ray W M, Study on the sediment chemistry dynamics of prawn ponds. Environmental survey and its improvement on shrimp grow out pond, COA Fish Ser, 28 (1991) Chattopadhyay G N & Mandal L N, Concept of fertilizing in brackishwater fish ponds, Fertilizer News, 27 (1982) Rahman A, Islam M A, Roy I, Azad L & Islam K S, Shrimp culture and environment in the coastal region. (Bangladesh Institute of Development Studies, E-17, Agargaon, Sher-e- Bangla Nagar) 1994, pp. 112.