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
1

School of Aquaculture, Tasmanian Aquaculture and Fisheries Institute, University of Tasmania, Launceston, Tasmania

7250, Australia
2
IASOS, University of Tasmania, Hobart, Tasmania 7001, Australia
3

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:

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 do 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:

1. To describe water quality and plankton densities
in shrimp ponds from mixed shrimp-mangrove
forestry farming systems in the Mekong Delta.
2. To establish important trends with season (wet,
dry), farm type (`mixed', `separate') and pond
water source (rivers, canals).
3. To identify situations where water quality may
be deleterious to shrimp production.
4. 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 6 ha 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

5

10

5

0

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

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am

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ay

184

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on

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im
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on
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South China
Sea

Figure 1 Map of Ca Mau province in southern Vietnam, the location of this study. The farms were located in State Fisheries
Forestry Enterprises Tam Giang 3 (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 3 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 5 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 6 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
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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 do 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 1  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

3
2
1
Wet
Mixed

0.15
Dry

Dry

Wet

Separate

Wet

2

Dry

Wet

Separate

× 104

1.5
1
0.5
0

Dry

Wet

Dry

Wet

Separate

Mixed

Macrobenthos (g m−2)

4

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

0

Wet

0.8

0

Wet

Mixed

NH4 (mgI−1)

DO (mg l−1)

3

5

Dry

0.4

4

0.1

27
26

Wet

5

0

28

Separate

Mixed

2

29

60

40
20
0

Dry

Wet
Mixed

Dry

Wet

Separate

Figure 2 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)

4
3
2
1
Wet

Canal

0.15
Dry

Dry

Wet
River

Wet

Dry

2

Wet
River

× 104

1.5
1
0.5
0

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

0

Wet

0.8

0

Wet

0.4

3

5

Dry

Canal

4

0.1

27
26

Wet

5

0

28

River

Canal

2

29

60

40
20
0

Dry

Wet
Canal

Dry

Wet
River

Figure 3 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

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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 2 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 0 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 + 7 gmÀ2) than `mixed' farms (10 +
2 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 1 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 5 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 2 (15 %)
3
SS

Zoop
Temp
Salinity
Do2
NH4 Phyto
Macroden
Macro biomass

Po4

Mixwet
Mixdry
−3

3

CD Axis 1 (72 %)

Sepwet

Sepdry

−3

Figure 4 Results of the canonical discriminant analysis
are displayed on the first two axes (CD axis 1 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

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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
4 mg lÀ1 at 2 m (Figs 5 and 6). At the deeper station
DO was constant from 2 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

0
2

Depth (m)

4
6
8
10
12
DO (mg L−1)
3

4

5

pH
3

6

4

5

6

7

0
2

Depth (m)

4
6
8
10

Pond 22
River

12
Figure 5 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 5 s as the datalogger was lowered through the water column.

792

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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

0

Depth (m)

4

8

12

DO (mg L−1)
2

3

4

pH
5

6

3

4

5

6

0

Depth (m)

1

2

Pond 23
River

3
Figure 6 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 5 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 7 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 6 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).
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Aquaculture Research, 2002, 33, 785±798

31
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Salinity

Water quality in extensive shrimp ponds D Johnston et al.

Time

Figure 7 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 min.

Readings were taken 20 cm above the pond bottom, approximately 6 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,
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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 (. 1 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 4 mg lÀ1 and saturation (Boyd & Fast
1992). Short-term exposure to DO concentrations
below 2 mg lÀ1 causes respiratory stress, with rapid
mortality at concentrations less than 1 mg lÀ1 (Fast
& Lannan 1992; Primavera 1993).
Pond pH varied considerably (around 1 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 5 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 Â 106 cells lÀ1, Fast & Lannan 1992) and fertilized
ponds in Auburn, Alabama (2 Â 107 cells lÀ1, Boyd
1990). Zooplankton densities were considerably
lower than semi-intensive shrimp ponds in tropical
Queensland, Australia (up to 1.4 Â 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 Â 104À5 Â 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; 6 Â 104À5 Â 105 no. mÀ3, McKinnon &
Klumpp 1998), but consistent with those reported
in the highly turbid Fly River in New Guinea
(1.5 Â 102À1.7 Â 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 (, 2 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

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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
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water quality and chlorophyll a of semi-intensive shrimp
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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 3 for their
guidance, support and cooperation. We thank Natalie Moltschaniwskyj for assistance with statistical
analyses.

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