Water quality and plankton densities in

Aquaculture Research, 2002, 33, 785±798 Water quality and plankton densities in mixed shrimp-mangrove forestry farming systems in Vietnam D Johnston1, M Lourey2, D Van Tien3, T T Luu3 & T T Xuan3 School of Aquaculture, Tasmanian Aquaculture and Fisheries Institute, University of Tasmania, Launceston, Tasmania 7250, Australia IASOS, University of Tasmania, Hobart, Tasmania 7001, Australia Research Institute for Aquaculture no 2, Ho Chi Minh City, Vietnam Correspondence: Danielle Johnston, School of Aquaculture, Tasmanian Aquaculture and Fisheries Institute, University of Tasmania, Locked Bag 1±370, Launceston, Tasmania 7250, Australia E-mail: danielle.johnston@utas.edu.au Abstract Water quality and plankton densities were monitored in shrimp ponds at 12 mixed shrimpmangrove forestry farms in Ca Mau province, southern Vietnam, to detail basic water chemistry and assess whether conditions are suitable for shrimp culture In general, water quality was not optimal for shrimp culture In particular, ponds were shallow (mean + 1SE, 50.5 + 2.8 cm), acidic (pH , 6.5), had high suspended solids (0.3 + 0.03 g lÀ1), low chlorophyll a/phytoplankton concentrations (0.2 + 0.05 mg lÀ1 and 8600 + 800 cells lÀ1 respectively) and low dissolved oxygen (DO) levels (3.7 + 0.15 mg lÀ1) Eight out of the 12 farms sampled had potentially acid sulphate soils (pH , 4.2) Salinity, DO and pH were highly variable over short time-periods (hours); DO in particular was reduced to potentially lethal levels (1±2 mg lÀ1) Seasonal variations in water chemistry and plankton communities (i.e salinity, DO, phosphate, temperature, phytoplankton and zooplankton densities) appear to be driven by differences in rainfall patterns The presence or absence of mangroves on internal pond levees (`mixed' versus `separate' farms) and the source of pond water (rivers versus canals) were of lesser importance in determining water quality patterns and plankton biomass Zooplankton and macrobenthos densities were sufficient to support the current (low) stocking densities of shrimp However, natural food sources are not adequate to support increases in production by stocking hatchery reared post larvae Increasing productivity by fertilization and/ ß 2002 Blackwell Science Ltd or supplemental feeding has the potential for adverse water quality and would require improvements to water management practices Some practical strategies for improving water quality and plankton densities are outlined Keywords: shrimp, water quality, Vietnam, integrated farming, mangroves, extensive shrimp culture, shrimp aquaculture Introduction Shrimp aquaculture in Vietnam has undergone rapid expansion over the past two decades, particularly in the Mekong Delta (Lovatelli 1997; Phuong & Hai 1998) Despite this expansion, shrimp yields per unit area are in decline (de Graaf & Xuan 1998; Johnston, Trong, Tuan & Xuan 2000a, b) Poor shrimp yields in the Mekong Delta and other countries with similar farming systems have been attributed to several factors, including low quality and quantity of shrimp seed, poor pond management and infrastructure, overexploitation of wildstock and whitespot disease outbreaks (Sinh 1994; Binh, Phillips & Demaine 1997; Primavera 1998; de Graaf & Xuan 1998; Johnston, Clough, Xuan & Phillips 1999; Johnston et al 2000a, b) However, the extent to which poor water quality has contributed remains largely unstudied, particularly in remote regions such as the Mekong Delta in Vietnam Good water quality in shrimp ponds is essential for survival and adequate growth (Boyd 1990; 785 Water quality in extensive shrimp ponds D Johnston et al Aquaculture Research, 2002, 33, 785±798 Burford 1997) In the Mekong Delta, low primary production, rapid rates of water column respiration, and low rates of benthic decomposition have already been suggested as possible factors limiting shrimp production (Alongi, Dixon Johnston, Tien & Xuan 1999a; Alongi, Tirendi, Trott & Xuan 1999b) On other South-east Asian shrimp farms, disease problems have been attributed to poor water quality (Phillips 1998) This study aims to address the lack of basic information on water quality in extensive shrimp ponds in Vietnam and comment on the potential for deleterious effects on shrimp aquaculture Shrimp ponds in Vietnam are primarily extensive shrimp±rice and shrimp-mangrove integrated systems, although there has been an increase in the number of improved extensive and semi-intensive ponds (Binh & Lin 1995; Binh et al 1997) The extensive ponds in the Mekong Delta rely on tidal flushing for water exchange and post-larval recruitment, so farmers have little control over the water quality in their ponds Fortunately, extensive shrimp farms such as those in southern Vietnam have low stocking densities, little or no fertilization and no supplementary feeding, so not generate significant amounts of organic effluent However, mangrove deforestation, due to the uncontrolled increase in the number of aquaculture ponds and increasing population pressure, has emerged as a threat to water quality in the region (de Graaf & Xuan 1998; Johnston et al 1999; 2000b) The effects of deforestation include acidic run-off and discharges from ponds constructed on acid sulphate soils (Phillips 1998), increased coastal erosion, salinity intrusion and loss of shrimp nursery grounds (Hong 1993; Macintosh 1996) Population pressure and reliance by local communities on the waterways for transport and market locations may have important impacts on water quality on a regional basis Data on shrimp pond water quality in the Mekong Delta is limited to investigations of water column (Alongi et al 1999a) and benthic (Alongi et al 1999b) processes in just two shrimpmangrove ponds We introduce data from 12 ponds on 12 shrimp-mangrove forestry farms and cover a range of environments, farm types and both wet and dry seasons We present the first information on phytoplankton, zooplankton and macrobenthos densities, which are particularly important as they form the basis of the natural food webs in extensive ponds and, in some cases, may limit shrimp productivity The specific aims of this study were: To describe water quality and plankton densities in shrimp ponds from mixed shrimp-mangrove forestry farming systems in the Mekong Delta To establish important trends with season (wet, dry), farm type (`mixed', `separate') and pond water source (rivers, canals) To identify situations where water quality may be deleterious to shrimp production Make recommendations to improve pond water quality and plankton densities 786 Materials and methods Sample collection The study was conducted in 12 shrimp ponds ranging in size from 0.5 to at 12 (there is traditionally one pond per farm) mixed shrimpmangrove forestry farms in the Ca Mau province of the Mekong Delta of Vietnam (Fig 1) (see Johnston et al 1999) These are integrated extensive farming systems where ponds are effectively ditches dug either separate to or through mangroves Each pond consists of a series of long (250±800 m), narrow (3±4 m) interconnected channels separated by internal levees and surrounded by a dyke Ponds are connected to external waterways via a single sluice gate through which water is exchanged Exchange during grow out is generally minimal although water levels can be maintained during tides of sufficient height and losses due to leakage are common Recruitment and harvesting of wild shrimp (primarily Metapenaeus spp.) occur on consecutive flood and ebb tides of the spring tide period Recruitment is followed by 10±12 days of grow out (Johnston et al 1999) There is little or no supplementary feeding, aeration, liming or fertilizer treatment Water samples or in situ measurements were collected from 20 cm below the surface at two stations within each pond, one 5±6 m from the sluice gate and one in the middle to back of the pond Two farm types were sampled: `separate' farms have separate shrimp pond and mangrove areas so the internal levees within each pond are devoid of mangroves; on `mixed' farms the internal levees have mangroves planted at high densities Farms of each type were further categorized based on their location and source of pond water, i.e from a large river (rivers) or a small canal (canals) Sampling ß 2002 Blackwell Science Ltd, Aquaculture Research, 33, 785±798 Aquaculture Research, 2002, 33, 785±798 Water quality in extensive shrimp ponds D Johnston et al China 10 Hanoi 10 N Kilometres W Laos E Ca Mau S Thailand Vietrlam Song Granh Hao Cambodia District Tran Van Thoi Kinh Sau Dong Kinh Doi Cuo ng District Cai Nuoc gD Son Ca Mau District Dam Doi ng Cu am So ng D i m Da Do Ch am nh Ki Kinh Ong Don S Ca ay 184 Son gC ua District Ngoc Hien Kinh 17 g on g iN L hap N hC Kin ap ay H gB Son Bien Tay im Ho Chi Minh on So TG3 ng m Da ng So Bo D im Ch e Bien Bong Song Cua Lon Hoc Nang Ra ch Du on g Ke o Kien Vang South China Sea Figure Map of Ca Mau province in southern Vietnam, the location of this study The farms were located in State Fisheries Forestry Enterprises Tam Giang (TG3) and 184 was carried out in the morning between 08.00 and 10.00 hours, twice during the dry season (April and June 1996) and twice during the wet season (August and October 1996) Vertical and diurnal profiles for pH, salinity, temperature and DO were recorded from ponds and their adjacent river/canals (source waters) Both profiles were measured in situ using Hydrolabß Datasonde dataloggers calibrated to factory specifications Vertical profiles were recorded at approximately 11.00 hours in the morning during October 1996 (wet season) Data was recorded every s as the datalogger was lowered through the water column Diurnal profiles in the ponds were recorded for 7-day periods using dataloggers deployed 20 cm from the bottom and approximately m from the sluice gate Profiles representative of the general patterns were selected and are presented here ß 2002 Blackwell Science Ltd, Aquaculture Research, 33, 785±798 Water quality analyses Temperature and DO were measured using an Orion oxygen meter; redox potential with Orion electrodes and salinity with a refractometer Replicate water samples from each station were filtered through a 0.45-mm filter and analysed for dissolved ammonium (NH4‡), nitrite (NO2-N), phosphate (PO4-P) and iron (Fe) using Pharmacia Biochrom (Palintest) test kits designed for a Novaspec II spectrophotometer The ammonia and nitrite tests were rated for saline water with incorporation of a conditioning agent to prevent the precipitation of salts Rudimentary facilities precluded the use of standard methods for total ammonia, NO2-N, PO4-P and total Fe analyses (Grasshoff, Ehrhadt & Kremling 1983) Replicate 100-mL samples were filtered onto preweighed GFC filter papers, dried at 60  C for 6±8 h and 787 Water quality in extensive shrimp ponds D Johnston et al Aquaculture Research, 2002, 33, 785±798 reweighed for determination of total suspended solids Particulate matter from 100 mL of each sample was filtered onto a GFC filter, the chlorophyll was extracted in 90% acetone and quantified by spectrophotometry (Parsons, Maita & Lalli 1984; Stirling 1985) Pond sediment from each sampling station was collected and dried Dry soil pH and redox potential were determined on slurries of the dried sediment mixed with deionized water included in the analysis, resulting in a two-way orthogonal design For each parameter measured, the assumptions of anova were checked using residual plots; where the assumptions had been violated, a square-root transformation was used In those analyses that had significant factors, Tukeys HSD post hoc test was used to determine the nature of the differences Additionally a manova (multivariate analysis of variance) was used to explore these structured data because more than one parameter was measured at each farm (pond) In this analysis, differences among levels in a factor (season, farm type, location) could be explored in multivariate space allowing differences to be found that would not be seen in univariate space Following the manova, significant differences were explored using a Canonical Discriminant Analysis (CDA) Each group was plotted in the reduced multivariate space, in which the new axes (CD1 and CD2) explained a proportion of the total variability in the data Superimposed on this plot was the association between the new axes [which display the differences among the groups (farms)] and the parameters that were measured This is displayed as a vector diagram in which the direction and length of the vector is a measure of the association between the parameter and the axes This allowed differences among the groups (farms) to be interpreted with respect to the water quality parameters measured Plankton and macrobenthos density Water samples (1000 mL) were collected at each station and fixed in 4% seawater±formalin for phytoplankton density determination The samples were allowed to settle for 24±48 h in the laboratory and excess water was removed to a final volume of 100 mL Phytoplankton density (cells lÀ1) was determined on replicate 0.1-mL subsamples using a Palmer±Maloney plankton counter Water (60 L) was collected from 20 cm depth using a 15-L bucket and filtered through a 30-mm plankton net The zooplankton collected were fixed in 4% formalin in seawater and the solution made up to 30 mL The number of zooplankton in two 1-mL replicate subsamples were counted and the density of zooplankton (no mÀ3) was extrapolated The contents of two benthic grab samples (area 0.025 m2) were pooled and fixed in 4% seawater±formalin for macrobenthos density determination The organisms were removed from the sample and the total wet weight biomass (g mÀ2) and density (no mÀ2) were determined Statistics Univariate anovas were used to explore seasonal differences in the parameters measured between the farm locations (river versus canal) and farming type (`mixed' versus `separate') Given that the design was not fully balanced, it was not possible to a single analysis involving all factors of interest simultaneously, i.e season, farm type and location Therefore, separate analyses were conducted to explore the interaction between season and farm type effects and the interaction between season and location In the season, location analysis, farms were nested within location; therefore, the final anova design was a three-factor mixed model However, in the season, farm type analysis there was an unbalanced number of farms, therefore, farm was not 788 Results The shrimp ponds studied here were typically shallow, averaging just 50.5 + 2.8 cm (range 10±140 cm) On average, salinity was higher in the dry season (mean + 1SE 27.4 + 0.7) than the wet season (16.7 + 0.7) (F1, 89 ˆ 121; P , 0.001) and higher at `mixed' farms (22.9 + 0.8) than `separate' farms (20.3 + 1.5) (F1, 89 ˆ 6.3; P ˆ 0.014) (Fig 2) The reduction in salinity during the wet season was more pronounced in ponds that source their water from rivers than in those that source water from canals (F1, 69 ˆ 5.6; P ˆ 0.039) (Fig 3) Temperature was approximately  C higher in ponds that source water from canals (28.5 + 0.3  C) than from rivers (27.4 + 0.3  C) (F1, 69 ˆ 7.2; P ˆ 0.022) (Fig 3) There were no significant seasonal or farm type trends in temperature Although variable (0.5±9.6 mg lÀ1), DO concentrations were generally low (3.7 + 0.15 mg lÀ1) ß 2002 Blackwell Science Ltd, Aquaculture Research, 33, 785±798 Aquaculture Research, 2002, 33, 785±798 Water quality in extensive shrimp ponds D Johnston et al 30 Temp (ЊC) 30 Salinity 25 20 15 10 Dry Wet Dry Dry Wet Dry 0.4 0.2 Dry Wet Dry 0.2 Wet Dry Wet Wet Mixed 0.15 Dry Dry Wet Separate Wet Dry Wet Separate × 104 1.5 0.5 Dry Wet Dry Wet Separate Mixed Macrobenthos (g m−2) Wet 0.2 Separate Mixed × 104 Dry Separate Mixed Phytoplankton (cells I−1) 0.3 Wet 0.25 Separate 0.4 Dry Dry 0.3 0.1 Wet 0.5 Dry 0.1 Mixed Chlorophyll a (µgI−1) PO4 (mg l−1) 0.6 Mixed Susp Solids (g l−1) Wet Separate 0.2 Separate Mixed Zooplankton (no m−3) Dry 0.3 Wet 0.8 Wet Mixed NH4 (mgI−1) DO (mg l−1) Dry 0.4 0.1 27 26 Wet 28 Separate Mixed 29 60 40 20 Dry Wet Mixed Dry Wet Separate Figure Mean + SE of a range of water quality parameters in ponds from two types of shrimp-mangrove farm (`mixed' and `separate') in the wet and dry seasons ß 2002 Blackwell Science Ltd, Aquaculture Research, 33, 785±798 789 Water quality in extensive shrimp ponds D Johnston et al Aquaculture Research, 2002, 33, 785±798 30 Temp (ЊC) 30 Salinity 25 20 15 10 Dry Wet Dry DO (mg l−1) NH4 (mgI−1) Dry Wet Dry Chlorophyll a (µgI−1) PO4 (mg l−1) 0.4 0.2 Dry Wet Dry Phytoplankton (cells I−1) 0.2 Dry Wet Macrobenthos (g m−2) Wet Canal 0.15 Dry Dry Wet River Wet Dry Wet River × 104 1.5 0.5 Dry Wet Dry Wet River Canal × 104 Wet River 0.2 River Canal Dry Canal 0.3 Wet Wet 0.25 River 0.4 Dry Dry 0.3 0.1 Wet 0.5 Dry 0.1 Canal 0.6 Canal Susp Solids (g l−1) Wet River 0.2 River Canal Zooplankton (no m−3) Dry 0.3 Wet 0.8 Wet 0.4 Dry Canal 0.1 27 26 Wet 28 River Canal 29 60 40 20 Dry Wet Canal Dry Wet River Figure Mean + SE of a range of water quality parameters in ponds of shrimp-mangrove farms that obtain their water from rivers and canals in the wet and dry season 790 ß 2002 Blackwell Science Ltd, Aquaculture Research, 33, 785±798 Aquaculture Research, 2002, 33, 785±798 Water quality in extensive shrimp ponds D Johnston et al There were no significant temporal or seasonal trends in either ammonia and nitrite concentrations (Figs and 3), with mean concentrations of 0.13 + 0.02 mg lÀ1 and 0.01 + 0.002 mg lÀ1 respectively Phosphate concentration in the wet season (0.41 + 0.03 mg lÀ1) was double that of the dry season (0.21 + 0.02 mg lÀ1) (F1, 45 ˆ 21.87; P ˆ 0.001), but the wet season increase was larger at `separate' (levees bare of mangroves) farms than `mixed' (levees with mangroves) farms (F1, 65 ˆ 4.5, P ˆ 0.038) (Fig 2) Ponds were turbid (suspended solid loads of 0.3 + 0.03 g lÀ1; range 0.03±1.54 g lÀ1) in both seasons and regardless of farm type and water source Chlorophyll a concentrations were generally low averaging 0.2 + 0.05 mg l±l and ranged from to 0.5 mg l±l Phytoplankton densities encountered during this study were highly variable ranging from 1000 to 36 500 cells lÀ1 Similarly, zooplankton densities ranged from 1900 to 119 000 no mÀ3 Phytoplankton and zooplankton densities were around twofold higher in the dry season (11000 + 1300 cells lÀ1 and 33 600 + 4000 no mÀ3 respectively) than the wet season (7000 + 1000 cells lÀ1 and 16 400 + 1700 no mÀ3 respectively) (F1, 89 ˆ 12.1; P ˆ 0.001 for phytoplankton and F1, 89 ˆ 15.3; P , 0.001 for zooplankton) (Fig 3) Zooplankton densities were also 1.6 times higher in ponds at `mixed' farms (28 300 + 1300 no mÀ3) than `separate' farms (18 200 + 3000 no mÀ3) (F1, 89 ˆ 6.14; P ˆ 0.015) (Fig 2) In contrast, macrobenthos biomass was threefold higher in ponds at `separate' farms (26 + gmÀ2) than `mixed' farms (10 + gmÀ2) (F1, 89 ˆ 7.13; P ˆ 0.009) (Fig 2) Pond sediments were not highly reducing with a mean eH of À7 mV Soil surrounding the ponds was acidic at eight out of the 12 farms sampled (pH , 4.2), indicating that the majority of farms had acid sulphate soils The CDA explained 87% of the variation among the groups (farms) on the first two axes (Fig 4) The greatest difference among the groups was along the first axis (CD axis 1), which explained 72% of the variation This difference was largely due to the difference between the wet versus the dry season groups, with the two wet season groups situated at one end of CD axis and the two dry season groups at the other end The vector diagram of parameters (Fig 4) suggests that high salinity and zooplankton density and to a lesser extent high temperature and high DO occurred during the dry season (these four parameters have vectors that are pointing positively along CD axis 1) The second axis (CD axis 2) explained 15% of the variation among groups and separated the `separate' from the `mixed' farms Macrobenthos biomass is higher in the `separate' farms than the `mixed' farms and to a lesser extent NH4, zooplankton density and phytoplankton density were also higher in `separate' farms The `mixed' farms during the wet season also seemed to be characterized by higher PO4 and suspended solids concentrations Vertical profiles of DO, pH, salinity and temperature within two ponds and their adjacent river/ canals are presented (Figs and 6) These stations represent a `separate' pond (pond 22) and a `mixed' pond (pond 23) Both profiles were measured in the same season to allow for comparisons The general patterns were typical of others measured in the area and in other seasons The salinity in pond 22 was CD Axis (15 %) SS Zoop Temp Salinity Do2 NH4 Phyto Macroden Macro biomass Po4 Mixwet Mixdry −3 CD Axis (72 %) Sepwet Sepdry −3 Figure Results of the canonical discriminant analysis are displayed on the first two axes (CD axis and 2) The mean and 95% confidence limits for each group is displayed in the reduced multivariate space In the top righthand corner of the graph is a vector diagram for the parameters measured The direction and length of the vector for each parameter is an indication of the association between the parameter and the CD axes and can be used to interpret the differences among the groups (farms) The vector for ammonia lies along the macrobenthos biomass vector and is not seen in this figure DO2, dissolved oxygen concentration; Macroden, macrobenthos density; Macro Biomass, macrobenthos biomass; MixDry, `mixed' farms dry season; MixWet, `mixed' farms wet season NH4, ammonia concentration; Phyto, phytoplankton; PO4 phosphate concentration; SepDry, `separate' farms dry season; SepWet, `separate' farms wet season; SS, suspended solids; Temp, temperature; Zoop, zooplankton density ß 2002 Blackwell Science Ltd, Aquaculture Research, 33, 785±798 791 Water quality in extensive shrimp ponds D Johnston et al Aquaculture Research, 2002, 33, 785±798 similar to that below '3 m in the adjacent river and increased with depth in both the river and the pond (Fig 5) The salinity in pond 23 was higher than the adjacent river (Fig 6) but in this case the water column in both pond and canal appeared to be well mixed In the river adjacent pond 22, temperature increased with depth in a similar pattern to salinity In the pond however, thermal stratification was evident with a sharp decrease in temperature with depth In pond 23 a thermal gradient was absent There were vertical gradients of DO in all source waters (canals/rivers) and ponds DO levels in the canals/rivers decreased with depth from around 5±5.5 mg lÀ1 at the surface to around mg lÀ1 at m (Figs and 6) At the deeper station DO was constant from to 10 m Dissolved oxygen levels in deeper waters in pond 23 were depleted to a greater extent than pond 22 Both ponds and the canals/rivers were acidic with pH of around 4.5±6 OpH in pond 22 was similar to its adjacent river waters, whereas pH in pond 23 was higher than its adjacent canal waters 13 Salinity 15 16 14 17 18 28.0 Temperature (ЊC) 28.5 29.0 29.5 30.0 Depth (m) 10 12 DO (mg L−1) pH 6 Depth (m) 10 Pond 22 River 12 Figure Vertical profiles of DO, pH, salinity and temperature in the pond at farm 22 (a `separate' farm) and in the adjacent river from which water is obtained Profiles were recorded at approximately 11.00 hours during October 1996 (wet season) Data are individual recordings taken every s as the datalogger was lowered through the water column 792 ß 2002 Blackwell Science Ltd, Aquaculture Research, 33, 785±798 Aquaculture Research, 2002, 33, 785±798 11 12 13 Water quality in extensive shrimp ponds D Johnston et al Salinity 14 15 16 17 28.0 Temperature (ЊC) 28.5 29.0 29.5 30.0 Depth (m) 12 DO (mg L−1) pH 6 Depth (m) Pond 23 River Figure Vertical profiles of DO, pH, salinity and temperature in the pond at farm 23 (a `mixed' farm) and in the adjacent river from which water is obtained Profiles were recorded at approximately 11.00 hours during October 1996 (wet season) and in close proximity to the sluice gate in the pond Data are individual recordings taken every s as the datalogger was lowered through the water column Salinity, pH, temperature and DO in ponds varied considerably over a time-frame of hours The diurnal profile in Figure is typical of patterns displayed in both seasons, in both `mixed' and `separate' ponds and regardless of water source Water depth in pond 22 was maintained at a reasonably constant level throughout the 7-day period when measurements were taken However, dramatic, short-term reductions and increases in pond depth occurred with the tide (about every h; see diagonal arrows in Fig 7), ß 2002 Blackwell Science Ltd, Aquaculture Research, 33, 785±798 followed by a slow decrease in depth in the period between these larger events The rapid reductions in the depth coincided with increases in DO (see diagonal arrows in Fig 7), whereas rapid increases in pond depth led to smaller increases in DO levels Between these rapid events there were two distinct patterns; DO and pH were maintained or increased during the day or were drawn down during dark periods DO concentrations were lowest (1±2 mg lÀ1) shortly before sunrise (vertical arrows in Fig 7) 793 Depth (m) Depth DO 0.8 0.6 0.4 1800 2100 300 600 900 1200 1500 1800 2100 300 600 900 1200 1500 1800 2100 300 600 900 1200 1500 1800 2100 300 600 900 1200 1500 1800 2100 300 600 900 1200 1500 1800 2100 300 600 900 0.2 8.5 pH 7.5 pH Salinity 6.5 1800 2100 300 600 900 1200 1500 1800 2100 300 600 900 1200 1500 1800 2100 300 600 900 1200 1500 1800 2100 300 600 900 1200 1500 1800 2100 300 600 900 1200 1500 1800 2100 300 600 900 14 12 10 DO (mg L−1) Aquaculture Research, 2002, 33, 785±798 31 30 29 28 27 26 25 Salinity Water quality in extensive shrimp ponds D Johnston et al Time Figure Diurnal cycles of depth, DO, salinity and pH over seven days within a grow-out cycle in the pond at farm 22 in March 1997 (dry season) Curves are means integrated by 3-h intervals of datalogger readings taken every 20 Readings were taken 20 cm above the pond bottom, approximately m from the sluice gate Vertical arrows indicate lowest levels of DO during the 7-day grow-out period and diagonal arrows indicate rapid fluctuations in pond depth and DO Discussion Water quality Most of the seasonal patterns of water quality and food chain dynamics observed here were driven by differences in rainfall patterns between the wet and dry season Low pond salinity and vertical stratification (Fig 5) during the wet season were attributed to run-off from monsoon rainfall events Lower salinity at `separate' than `mixed' farms was most likely due to higher run-off from bare internal levees on the `separate' farms During the wet season, salinity in ponds that obtain water from rivers was lower than in ponds that obtain water from canals Water exchange is achieved on the rising tide, so pond waters fill with the surface water of the adjacent river or canal If water exchanges are made while the layer of freshwater dominates the surface (Fig 5), then a considerable reduction in pond salinity may occur This suggests that the dilution of river waters by run-off is greater than in canals and that these differences are passed on to pond waters This is consistent with the idea that freshwater fluxes would be greater in rivers than canals because rivers have larger catchments Pond temperature was higher at farms that obtain their water from canals rather than rivers, due to greater heating of the smaller water mass in the shallow canals compared with the larger rivers 794 (combined with reduced tidal flushing) Shading may moderate temperature changes, as thermal stratification in ponds with no mangroves was greater than in ponds where mangroves lined the levee banks A thermally stratified water column results in poor circulation and possibly stagnation from benthic heterotrophic processes (Alongi et al 1999a) Poor pond design (one sluice gate, long narrow shallow channels) and lack of mechanical aeration contribute to stratification Thermal stratification was evident in pond 22 and may maintain the vertical dissolved oxygen gradient (Fig 5) However, the presence of a similar DO gradient in pond 23 in the absence of a vertical temperature (or salinity) gradient (Fig 6) suggests that stratification is not necessary for high surface-water DO concentrations to develop Ammonia and nitrite concentrations in ponds were well below toxic levels recorded for shrimp (, 1.3 mg lÀ1 NO2-N and , 7.7 mg lÀ1 NH4-N for Metapenaeus macleayi Haswell and , 4.1 mglÀ1 NH4-N for Peneaus monodon Fabricius (Allan Maguire & Hopkins 1990) and are consistent with levels recorded previously in integrated shrimpmangrove ponds and canals in Ca Mau province (Hong 1996; Binh et al 1997) The highest levels of nitrogenous nutrients encountered in this study were most likely due to point sources of pollution in the source waterways rather than shrimp excretion, ß 2002 Blackwell Science Ltd, Aquaculture Research, 33, 785±798 Aquaculture Research, 2002, 33, 785±798 Water quality in extensive shrimp ponds D Johnston et al as stocking densities in ponds were low (Johnston et al 2000a) Rivers are under particular pressure from point sources of pollution due to boat traffic and effluent from community markets; indeed, the highest ammonia concentrations were detected at farms in close proximity to markets Phosphate concentrations were considerably elevated in the wet season, presumably due to run-off from agricultural land close to shrimp- mangrove farms During the wet season, phosphate concentrations were also higher at `separate' than `mixed' farms `Separate' farms had no mangroves on the levees so there was presumably more erosion of phosphate-rich soils during periods of rainfall Of considerable concern to farmers were the rapid fluctuations in pond depth, DO, salinity and pH Rapid variations in water quality are universally accepted as having severe impacts on shrimp health and survival (Boyd 1990) and the extreme nature of the changes, and the levels to which DO, in particular, was reduced suggest that some reduction in yields should be expected Many of the marked fluctuations over short periods can be attributed to shallow pond depth (mean 50.5 + 2.8 cm), exacerbated by water losses from pond leakage problems (Johnston et al 1999, 2000b) Although the pond depths encountered here are typical of extensive brackish water aquaculture ponds in other parts of South-east Asia (Kungvankij 1984), the fluctuations in pond depth over periods as short as a few hours are unique (Fig 7) Two distinct patterns were observed: the gradual reduction in pond depth during an intertidal period attributed to leakage, and dramatic increases and reductions in depth occurring with the tide (about every 6±8 h) as the farmer replenished and exchanged water Deeper ponds ( m) and reducing leakage by increasing structural support to the sluice gates, positioning them in the most stable regions of the pond and preventing erosion (using mesh, nipa palm leaves or mangrove poles) would stabilize depth and reduce the need to exchange water during grow out Exchanging water during grow out has the potential to import water of poor quality or rapidly change water chemistry within a pond, i.e by introducing low salinity water after rain Fortunately, there were no significant seasonal or spatial differences in DO suggesting that, from this perspective, both water sources (canals and rivers), farm types (`mixed' and `separate') and seasons sampled are similar for this type of shrimp culture However, DO levels were low (mean 3.7 mg lÀ1) and fell to potentially lethal concentrations of 1±2 mg lÀ1 at dawn as a result of respiration and microbial oxidation of organic matter Low DO is one of the most common causes of mortality and reduced growth in shrimp ponds (Primavera 1993) Best survival and growth is obtained at DO concentrations between mg lÀ1 and saturation (Boyd & Fast 1992) Short-term exposure to DO concentrations below mg lÀ1 causes respiratory stress, with rapid mortality at concentrations less than mg lÀ1 (Fast & Lannan 1992; Primavera 1993) Pond pH varied considerably (around pH unit) over a period of just a few hours These short-term changes in pH were in concert with variations in DO and are due to consumption and release of carbon dioxide (CO2) during photosynthesis and respiration Photosynthesis raises pH by two mechanisms: first, protons are removed directly as part of the photosynthesis reaction and second, the CO2 removed is a weak acid The reverse is the case during respiration Large fluctuations in pH are common in poorly buffered, low alkalinity ponds (Fast & Lannan 1992) Shrimp become stressed outside their optimal pH range of 7±9 (Boyd & Fast 1992) Although low pond water pH (during the wet season, see Figs and 6) possibly arises from several factors including lateral inputs from interstitial mangrove water, acid rain and excavation and subsequent oxidation of pond sediments (Alongi et al 1999a; Johnston et al 1999), low pH in these ponds is probably due to leaching of acid sulphate soils that are endemic to mangrove regions (Binh et al 1997; Phillips 1998) Clearing mangroves and exposing acid sulphate soils has previously been associated with low yields and acidic conditions in shrimp ponds in Ca Mau province (Binh et al 1997) In this study, eight out of the 12 farms had potentially acid sulphate soils, making the situation difficult to remedy Lime could be applied to ponds and adjacent soils to neutralize pH; however, this option would be limited by financial constraints (Johnston et al 1999; 2000b) Removing excavated bottom sediments from the area, as opposed to the current practice of placing sediments onto levees where it can be washed back into the pond, may be a viable solution ß 2002 Blackwell Science Ltd, Aquaculture Research, 33, 785±798 Plankton densities Few studies have focused on primary and secondary productivity in extensive aquaculture ponds, 795 Water quality in extensive shrimp ponds D Johnston et al Aquaculture Research, 2002, 33, 785±798 particularly in tropical areas Those that have suggest that low levels of primary and secondary production are typical (Boyd 1990; Fast & Lannan 1992) Chlorophyll a concentrations in shrimp ponds in Ca Mau province were low compared with semi-intensive tropical (4±114 mg lÀ1, Burford 1997; 36 mg lÀ1, Ziemann, Walsh, Saphore & Fulton-Bennett 1992) and subtropical shrimp ponds studied elsewhere (6.3±31.3 mg lÀ1, Guerrero-GalvaÂn, PaÂez-Osuna, Ruiz-FernaÂndez & Espinoza-Angulo 1999) Phytoplankton densities in ponds (mean of 8600 cells lÀ1) were also low compared with semi-intensive shrimp ponds (1  106 cells lÀ1, Fast & Lannan 1992) and fertilized ponds in Auburn, Alabama (2  107 cells lÀ1, Boyd 1990) Zooplankton densities were considerably lower than semi-intensive shrimp ponds in tropical Queensland, Australia (up to 1.4  105 no mÀ3, Burford 1997) In contrast with these above studies, the ponds in Ca Mau province are not fertilized and are likely to display plankton population densities that are closer to natural abundance Chlorophyll a was at the lower end of the range recorded in the Fly River in Papua New Guinea (0.15±5.07 mg lÀ1, Robertson & Blaber 1992) and phytoplankton densities were also lower than mangrove creeks in the Americas (2  104À5  108 cells lÀ1, Ricard 1984) Zooplankton densities in our ponds were considerably lower than mangrove systems in Australia (6.1  104À2  106 no mÀ3; Robertson, Dixon & Daniel 1988;  104À5  105 no mÀ3, McKinnon & Klumpp 1998), but consistent with those reported in the highly turbid Fly River in New Guinea (1.5  102À1.7  104 no mÀ3, Robertson, Alongi, Christoffersen, Daniel, Dixon & Tirendi 1990) Plankton studies in tropical estuarine waters, including aquaculture ponds, have shown that highinorganic, suspended solid loads diminish phytoplankton production (Teichert-Coddington, Green & Phelps 1992; Alongi et al 1999a) Suspended solids block the transmission of light and effectively minimize photosynthetic efficiency, particularly at depth In this study, high suspended solid levels during the wet season, coincided with lower phytoplankton densities that were highest during the dry season Although suspended solid loads in ponds in Ca Mau province are similar to those recorded for the Fly River, a mangrove-lined estuarine river in Papua New Guinea (Robertson & Blaber 1992), and are characteristic of shrimp ponds and water ways in the Mekong Delta (Binh et al 1997), levels are generally high compared with semi-intensive shrimp ponds (Burford 1997) Limiting water exchange during grow out or adding settlement ponds could possibly reduce suspended solid levels and lead to higher levels of primary production (also improving DO levels and providing a food source for secondary production) However, space and financial resources for this type of improvement to infrastructure were generally limited in these farms Reducing colloidal material entering the ponds would be more difficult, probably requiring chemical treatments beyond the scope of this type of aquaculture Low densities of phytoplankton may be the factor that limits grazers that rely on them as a food source However, zooplankton and macrobenthos may be further limited by low pH in pond waters, oxygen stress caused by decomposing organic matter from the mangrove canopy, periodic changes in salinity in ponds and flushing during rainfall events Indeed, zooplankton densities were higher in the dry than the wet season and higher in ponds at `mixed' than `separate' farms, consistent with a constant salinity and reduced run-off influence In contrast, macrobenthos biomass was considerably lower in `mixed' than `separate' ponds, possibly due to increased microbial draw down of oxygen from a greater influx of mangrove organic litter Although zooplankton and zoobenthos densities (by biomass) were low, they appeared to be sufficient to support the current stocking densities in these ponds (, post larvae mÀ2, Johnston et al 2000a) However, the natural food chain was not sufficient to support higher stocking densities as a means of increasing productivity (i.e by stocking hatchery reared post larvae) To support higher stocking densities in ponds, farmers would need to increase plankton levels naturally, fertilize their ponds or use supplemental feed Stimulating productivity with fertilizers, supplemental feeding and high shrimp biomass often comes at a cost to water quality; deoxygenation can result from increased amounts of organic matter and ammonia excreted by the animals can accumulate We suggest that fertilization and reliance on supplemental feeding as a means of increasing pond production should be discouraged under the current circumstances because of the high potential for adverse water quality in ponds and subsequently the environment at large If fertilization and supplemental feeding were adopted as a means of increasing pond production, improved water quality management practices would be required to ensure pond 796 ß 2002 Blackwell Science Ltd, Aquaculture Research, 33, 785±798 Aquaculture Research, 2002, 33, 785±798 Water quality in extensive shrimp ponds D Johnston et al conditions did not deteriorate and environmental impacts were minimized the southern provinces of Vietnam Mangroves and Salt Marshes 2, 159±166 Grasshoff K., Ehrhardt M & Kremling K (1983) Methods of Seawater Analysis, 2nd edn Verlag Chemie, Weinham Guerrero-GalvaÂn S.R., PaÂez-Osun F., Ruiz-FernaÂndez A.C & Espinoza-Angul R (1999) Seasonal variation in the water quality and chlorophyll a of semi-intensive shrimp ponds in a sub-tropical environment Hydrobiologia 391, 33±45 Hong P.N (1993) Mangroves in Vietnam International Union for the Conservation of Nature, Regional Wetlands Office, Bangkok, Thailand Hong P.N (1996) The impact of pond construction along the mangrove coastal accretion at southwest Ca Mau Cape, Viet Nam SEAFDEC Asian Aquaculture 28 (4), 3±8 Johnston D.J., Clough B., Xuan T.T & Phillips M.J (1999) Mixed shrimp-mangrove forestry farming systems in Ca Mau province, Vietnam Aquaculture Asia (2), 6±12 Johnston D.J., Trong N.V., Tuan T.T & Xuan T.T (2000a) Shrimp seed recruitment in mixed shrimp and mangrove forestry farms in Ca Mau province, Southern Vietnam Aquaculture 184, 89±104 Johnston D.J., Trong N.V., Tien D.V & Xuan T.T (2000b) Shrimp yields and harvest characteristics of mixed shrimp-mangrove forestry farms in southern Vietnam: factors affecting production Aquaculture 188, 263±284 Kungvankij P (1984) Overview of penaeid shrimp culture in Asia Proceedings of the First International Conference on the Culture of Penaeid Prawns/Shrimps, Iloilo City, Philippines SEAFDEC Aquaculture Department Philippines, pp 11±21 Lovatelli A (1997) Status of aquaculture in Vietnam Aquaculture Asia (3), 18±24 Macintosh D.J (1996) Mangroves and coastal aquaculture: Doing something positive for the environment Aquaculture Asia (2), 3±8 McKinnon A.D & Klumpp D.W (1998) Mangrove zooplankton of North Queensland, Australia I Plankton community structure and environment Hydrobiologia 362, 127±143 Parsons T.R., Maita Y & Lalli C.M (1984) A Manual of Chemical and Biological Methods for Seawater Analysis Pergamon Press, Oxford Phillips M.J (1998) Tropical mariculture and coastal environmental integrity In: Tropical Mariculture (ed by S.S De Silva), pp 17±69 Academic Press, London Phuong N.T & Hai T.N (1998) Coastal aquaculture and environmental issues in the Mekong Delta, Vietnam In: TCE-P Workshop No II `Coastal Environmental Improvement in Mangrove/Wetland Ecosystems' (ed by D.J Macintosh), pp 120±127 Ranong, Thailand Primavera J.H (1993) A critical review of shrimp pond culture in the Philippines Reviews in Fisheries Science (2), 151±201 Acknowledgments This study was conducted as part of the project `Mixed shrimp-mangrove forestry models in the Mekong Delta' (FIS/94/12), supported by The Australian Centre for International Agricultural Research (ACIAR) Field work was completed while the senior author (D.J.) was employed at the Australian Institute of Marine Science (AIMS) We thank Barney Smith (ACIAR), Barry Clough (AIMS), Michael Phillips (NACA), the Ministry of Fisheries (Vietnam), and farmers and staff at State Fisheries Forestry Enterprises 184 and Tam Giang for their guidance, support and cooperation We thank Natalie Moltschaniwskyj for assistance with statistical analyses References Allan G.L., Maguire G.B & Hopkins S.J (1990) Acute and chronic toxicity of ammonia to juvenile Metapenaeus macleayi and Penaeus monodon and the influence of low dissolved-oxygen levels Aquaculture 91, 265±280 Alongi D.M., Dixon P., Johnston D.J., Tien D.V & Xuan T.T (1999a) Pelagic processes in extensive shrimp ponds of the Mekong 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Alongi D.M., Christoffersen P., Daniel P.A., Dixon P & Tirendi F (1990) The Influence of Freshwater and Detrital Export from the Fly River System on Adjacent Pelagic and Benthic Systems Australian Institute of Marine Science Report no 4, Townsville, Australia Sinh L.X (1994) Mangrove forests and shrimp culture in Ngoc Hien District, Minh Hai Province, Vietnam NAGA 17 (4), 15±16 Stirling H.P (1985) Chemical and Biological Methods of Water Analysis for Aquaculturalists Institute of Aquaculture, University of Stirling, Scotland Teichert-Coddington D.R., Green B.W & Phelps R.P (1992) Influence of site and season on water quality and tilapia production in Panama and Honduras Aquaculture 105, 297±314 Ziemann D.A., Walsh W.A., Saphore E.G & Fulton-Bennett K (1992) A survey of water quality characteristics of effluent from Hawaiian aquaculture facilities Journal of the World Aquaculture Society 23, 180±191 798 ß 2002 Blackwell Science Ltd, Aquaculture Research, 33, 785±798 ... out Exchanging water during grow out has the potential to import water of poor quality or rapidly change water chemistry within a pond, i.e by introducing low salinity water after rain Fortunately,... caused by decomposing organic matter from the mangrove canopy, periodic changes in salinity in ponds and flushing during rainfall events Indeed, zooplankton densities were higher in the dry than... leakage, and dramatic increases and reductions in depth occurring with the tide (about every 6±8 h) as the farmer replenished and exchanged water Deeper ponds ( m) and reducing leakage by increasing