Mesozooplankton Composition and Abundance in a Tropical Estuary During Monsoon Season
The study was undertaken to determine zooplankton composition and abundance
along Perak River estuarine system during the Northeast (NE) monsoon. Monthly
samplings were conducted from November 2009 until February 2010. Four sampling
stations were selected along the salinity gradient covering stations from the
upstream down to 1 km off the coastal sea area. The zooplankton samples were
collected by vertical tows using conical plankton net (100 μm mesh size).
Throughout the monsoon season, a total of 11 higher taxa of zooplankton groups
were recognized, of which copepods were the dominant group comprising 96% (including
nauplii and copepodites) 95 and 99% of the total zooplankton abundance in the
marine, estuarine and upstream station, respectively. Seventeen species of copepods
from 10 genera were identified. Total zooplankton abundances recorded during
the sampling period were 45.5±18.9x103, 34.9±25.9x103
and 45.9±30.5x103 individual m-3 in the upstream,
estuarine and marine station, respectively. Multidimensional Scaling (MDS) analysis
based on zooplankton abundance revealed three distinct groups separating the
upstream from the estuarine and marine stations. Similarly, lower diversity
index was recorded in the estuarine station (H
= 2.04±0.28) compared to marine area (H
= 2.20±0.26), suggesting that less copepod species were tolerant to the
drastic changes of the estuarine environment. Low copepod densities were recorded
in the beginning of the monsoon season, but progressively increased with a peak
in the middle of the monsoon season. The results demonstrated that the Perak
estuary was a turbid environment with drastic changes in salinity levels due
to high inflow of freshwater during the monsoon season which affect the distribution
of the zooplankton populations.
Received: September 19, 2012;
Accepted: September 28, 2012;
Published: February 09, 2013
Perak river is the second longest river in Peninsular Malaysia after Pahang
river with an approximate length of 400 km (Yusoff
et al., 2006). The river system originates from Titiwangsa range from
the North which flows and drains into the Straits of Malacca at Bagan Datoh
village (3°59N-100°46E). Due to its large watershed (14,700
km2), the upstream of the Perak river provides a large volume of
freshwater input into the riverine system which results in the wide range of
salinity variations in its estuary and the associated coastal sea. Perak estuary
has been known as an important fishing site for the jellyfish, Acromitus
hardenbergi which is one of the most important fishery products in the area
(Nishikawa et al., 2008; Yusoff
et al., 2010).
In an ecotone, such as an estuary ecosystem, zooplankton composition is strongly
determined by the environmental factors. According to Rezai
et al. (2005), salinity was one of the main factors that determine
the composition of zooplankton in that particular area. Zooplankton community
structure is influenced by salinities and water circulation and there are many
species which are tolerant to drastic salinity fluctuations in the estuaries.
Northeast (NE) monsoon is one of two main monsoon seasons that characterised
the weather in Malaysia. Northeast monsoon usually brings heavy rainfall from
November until early March (Yoshida et al., 2006).
This wet season becomes important as there will be more water input into the
estuarine systems and will be a main dilution factor to this transition zone
between the river and the sea. This study therefore was conducted to evaluate
the influence of salinity fluctuations during the NE monsoon on zooplankton
abundance and community structure in Perak river.
MATERIALS AND METHODS
Four sampling stations were allocated in Perak estuary based on salinity distribution
during high tide (Fig. 1). Station 1 was located 1 km away
from the river mouth, two stations (Stations 2 and 3) in the estuary and Station
4 was in the upstream. Since there were no significant differences between Stations
2 and 3, they were pooled together to represent the estuarine area.
||Map of the sampling location in Perak river showing the sampling
The monthly samplings were carried out during the NE Monsoon starting from
November 2009 until February 2010. Environmental parameters such as salinity,
temperature, dissolved oxygen, pH and turbidity at each station were investigated
using the Hydrolab multiparameter probe (Surveyor DS4). Water samples were taken
using Niskin bottle and the concentrations of total ammonium nitrogen (TAN)
and nitrite and nitrate (NO2+NO3) were measured according
to the methods described by Kitamura et al. (1982).
Soluble reactive phosphorus (SRP) concentrations were also measured using the
spectrophotometric methods described by Parsons et al.
(1984). Zooplankton samples were obtained by vertical hauling of conical
plankton net (0.3 m mouth diameter, 100 μm mesh size) from the deepest
depth of each station (1 m above the bottom) to the surface. Three replicates
were taken for each station. The samples were immediately fixed with 4-5% buffered-formalin/seawater
solution just after collection.
The zooplankton samples were then transferred to the laboratory for identification
and enumeration. A 10 mL subsample was splitted from an original sample and
examined using the Bogorov counting chamber under dissecting microscope. Zooplankton
were identified to the lowest possible taxa and further identification to species
were made to copepods, the most dominant taxa. Number of each taxon of zooplankton
was counted and numerical density was expressed as individuals m-3.
Non-metric Multidimensional Scaling (MDS) was applied to examine the zooplankton
community structure using the software package, PRIMER (ver. 6.1.9, PRIMER-E
Ltd). Statistical test between parameter factors and zooplankton assemblage
were analyzed using Analysis of Variance (ANOVA). Shannon-Wiener diversity index
(H) was used to evaluate the
species diversity of dominant zooplankton using the software package, SPSS Statistics
(version 17.0, IBM).
Water temperature in the study area varied from 27.7 to 31.7°C throughout
the sampling season in all the sampling stations (Table 1).
However, mean temperature values were similar at all stations, 29.72±1.01,
29.58±0.8 and 29.66±1.02°C for the upstream, estuarine and
marine stations, respectively.
||Physicochemical parameters in the different sampling stations
in Perak river during northeast monsoon 2009-2010
|DO: Dissolved oxygen, TDS: Total dissolved solid, TAN: Total
ammonia nitrogen, SRP: Soluble reactive phosphorus, NTU: Nephelometric turbidity
unit, Ranges are given in the parenthesis
|| Rainfall data for Perak river during February 2009 until
Likewise, dissolved oxygen showed only a little variation throughout the sampling
months at all stations (Table 1). However, there were significant
differences (ANOVA: p<0.05) in dissolved oxygen between stations. The highest
dissolved oxygen was recorded in the upstream station (5.45±0.03 mg L-1)
followed by marine station (4.8±0.3 mg L-1) and the lowest
was recorded in estuarine station (4.25±0.38 mg L-1). In addition,
a wide range of salinity variation was observed in both the estuarine and marine
stations. In the estuarine stations, the salinity varied from 1.2 to 20.6 ppt,
whereas, in the marine station, salinity ranged from 14.80 to 25.39 ppt (Table
1). On the other hand, relatively constant salinities were observed in the
upstream station ranging from 0.00 to 0.02 ppt. Highest turbidity levels were
recorded in estuarine station compared to other stations with the turbidity
readings ranging from 10.3 to >1000 NTU (mean turbidity = 505.2±247.5
NTU). This was followed by the upstream station (50.75±20.4 NTU), while
the lowest turbidity reading was recorded in the marine station (0.3±0.05
NTU). Monthly total rainfall recorded for Perak river varied from 23.4 mm to
311.9 mm (Fig. 2) with the highest rainfall recorded in November
2009 (311.9 mm) and decreased towards the inter-monsoon season.
Nutrient concentrations during the study period did not show significant variations
(ANOVA: p>0.1) amongst all the sampling stations (Table 1).
Overall, marine station showed the lowest concentration of total ammonia nitrogen,
TAN (0.02±0.01 mg L-1) and nitrite and nitrate, NO2+NO3
(0.03±0.01 mg L-1) followed by the estuarine stations
(0.03±0.01 and 0.11±0.05 mg L-1 for TAN and NO2+NO3,
respectively) and the highest in the upstream stations (0.04±0.01 and
0.23±0.04 mg L-1 for TAN and NO2+NO3,
respectively). On the other hand, the estuarine station showed the lowest concentration
in soluble reactive phosphorus, SRP (0.02±0.00 mg L-1) followed
by the estuarine stations and the highest in the marine station (0.03±0.01
Total zooplankton abundance fluctuated throughout the monsoon season with the
peak appearing in the middle of the monsoon. Eleven groups of zooplankton were
found from the Perak river estuary. Among them, copepods (including nauplii
and copepodites) were the most numerically dominant group throughout the sampling
period, contributing 96% of the total zooplankton abundance on average. Copepods
also dominated zooplankton count in all sampling stations with the percentage
of 98.5, 94.7 and 96.0% in the upstream, estuarine and marine stations, respectively
(Fig. 3). Cirripedia was the second most abundant group, contributing
only 1.5% of the total zooplankton abundances and the remaining percentage of
less than 1% were contributed by other zooplankton groups (Fig.
3). Among the copepods (Fig. 4), calanoids were the most
abundant (36.2% of total), followed by cyclopoids (33.3%), harpacticoids (28.6%)
and poecilostomatoids (2.0%).
||Percentages of zooplankton groups in (a) Upstream, (b) Estuarine
and (c) Marine stations of Perak river, during northeast monsoon season
|| Density (individuals m-3) of copepods at different
sampling stations in Perak river during the monsoon season 2009-2010
|| Percentage of copepods during northeast monsoon season in
Perak river, Malaysia
In upstream station, copepod community was dominated by Oithona aruensis
(77.5%) and Hemicyclops sp. (22.1%). On the other hand, estuarine station
was dominated by Microsetella norvegica which contributed 19.6% of total
copepods count (Table 2), followed by Parvocalanus crassirostris
(18.7%) and Oithona aruensis (15.8%). Similarly, marine station was also
dominated by Microsetella norvegica (33.1%), followed by Oithona simplex
(30.6%) and Euterpina acutifrons (14.7%).
The highest mean density of 45.94x103±30.52x103
individuals m-3 was recorded in the marine station (Fig.
5), while the lowest was recorded in the estuarine station at 34.88x103±25.85x103
individuals/m3. However, there was no significant difference (p>0.05)
in terms of zooplankton density in all stations.
||Zooplankton density and salinity in different sampling stations
during northeast monsoon season in Perak river, Malaysia, Values are Mean±SE
||Density of zooplankton in different months and stations during
northeast monsoon season in Perak river, Malaysia, Values are Mean±SE
The highest zooplankton density was recorded in December 2009 for upstream
station (86.35x103±28.63x103 individuals m-3)
while highest zooplankton density in estuarine and marine stations were recorded
in January 2010 (112.01x103±18.57x103 individuals/m3
and 136.37x103±39.81x103 individuals m-3
for estuarine and marine stations, respectively) (Fig. 6).
On the other hand, the lowest zooplankton density for upstream and estuarine
station were recorded in November (3.48x103±1.12x103
individuals m-3 and 1.77x103±0.33x103 individuals
m-3 for upstream and estuarine station, respectively) while lowest
zooplankton density for marine station was recorded in February 2010 (5.00x103±1.07x103
Highest copepod diversity index, H
(2.20±0.23) was recorded at the marine station while the lowest was recorded
at the upstream station (H =
1.59±0.01) (Fig. 7). Multi dimensional scaling based
on zooplankton density showed three major groups at 43% similarities, stress
value: 0.17 (Fig. 8). The first group consisted mainly of
the zooplankton from the upstream station and a few from the estuarine area,
whereas the marine group contained mainly of marine species and a few from the
estuarine area. On the other hand, the estuarine group consisted of the combination
of the zooplankton found in both estuary and marine stations. In the upstream
station, the zooplankton community mainly consisted of zooplankton that adapted
to freshwater including some copepods, polychaetes and cladocerans.
|| Copepod diversity index (Mean±SE) at Perak river,
||Multi dimensional scaling analysis based on zooplankton abundance
at Perak River sampling stations, Malaysia
A dynamic ecosystem such as an estuary is subjected to the influence from the
marine and freshwater influxes, preserving nutrient and other substances that
stimulate its productivity (Elliott and McLusky, 2002).
In Perak river mouth, the salinity was lower than other estuaries bordering
the Straits of Malacca probably due to the large size of this river and its
catchment area. Huge flush of freshwater from the upstream, surface runoff and
rain result in drastic salinity oscillation in Perak estuary. Rainfall can be
a major source of variation in surface-water quality (Matias-Peralta,
2010). In this study, high precipitation during NE monsoon season might
be a causative factor.
Based on the Interim Malaysia Water Quality Standard (IMWQS), Perak River can
be characterised as Class II A (Water supply II: conventional treatment required,
Fishery II: Sensitive aquatic species). In this study, the concentrations of
all nutrients measured in the study area were below the maximum levels standard
of water quality limits, where the maximum concentration limit of phosphorus,
ammoniacal-nitrogen and nitrate-nitrogen were 200, 300 and 400 μg L-1,
respectively (DOE, 2010).
Zooplankton abundance and distribution in Perak River showed that the zooplankton
density fluctuated with the influence of the salinity. Rezai
et al. (2009) stated that higher zooplankton abundances may be partly
due to the intrusion of more saline waters, as the mixing of these waters may
provide favourable conditions for zooplankton development. In this study, the
lowest zooplankton density was recorded in the estuarine station and highest
in the marine station. In the contrary, Jayasinghe (2005)
found that the highest zooplankton densities occurred in the estuarine area
rather than in the river or the sea. In this study, low zooplankton density
in the estuary was probably due to high predation pressure from the high abundance
of jellyfish populations. In addition, the estuarine water was more turbid compared
to the marine station, thus less phytoplankton was available as food item for
the zooplankton. Low zooplankton density in estuarine station might also be
due to the fact that the estuary area was exposed to more drastic salinity fluctuations
compared to the other stations. Thus, only species with high tolerance to salinity
changes can survive in this brackish environment. Rezai
et al. (2004) and Wickstead (1958) stated
that a variety of physical, chemical and biological factors, including salinity
might affect the density of some zooplankton. The zooplankton species of Perak
river were mainly comprised a combination of marine and freshwater tolerant
species. According to Champalbert et al. (2007),
the changes in salinity will cause changes in zooplankton composition.
On the other hand, zooplankton density in Perak estuary showed low density
in the beginning of NE monsoon and increased gradually with the peak appearing
in the middle of the monsoon. Contrary to the study by Yoshida
et al. (2006), zooplankton abundance normally peaked in the beginning
and decreased gradually towards the end of the monsoon. The differences could
be due to the differences in the environmental quality associated with a particular
estuary, which, could affect to the zooplankton temporal and spatial fluctuations.
High precipitation in the beginning of NE monsoon caused high water turbulence
which led to turbidity increment. Low light penetration due to high turbidity
level was probably a limiting factor for phytoplankton growth (Cloern,
1987), thus resulted in less food concentration for zooplankton.
Multi-dimensional scaling analysis based on zooplankton abundance in Perak
river showed that there was a gradient of changes of zooplankton community with
three distinct patterns. In the upstream station, the zooplankton community
was dominated by the freshwater species and a few estuarine types, whilst the
marine area consisted mainly of marine species and a few estuarine species.
In the estuarine area, there was a mixture of estuarine and marine species with
small overlap with the river species. Rundle et al.
(1998) stated that estuarine ecosystems work as a continuum of overlapping
communities along the salinity gradient. The direction of estuarine colonization,
as evidenced by the species preferences, is from the sea rather than from fresh
water. Factors such as river flow and tidal movement drive the marine communities
towards estuaries (Waniek, 2003) thus form the estuarine
communities (Froneman, 2004). In the present study,
cladocerans were only found in the upstream station, while copepods were found
in all stations, even though only copepod nauplii and copepodite stages were
found in the upstream station. A number of studies stated that crustaceans,
especially copepods form a major portion of the zooplankton community in most
tropical estuarine systems (Day et al., 1989; Mishra
and Panigrahy, 1999). Likewise, in this study, Perak river-estuarine ecosystem
was dominated by copepods where they contributed more than 90% of the total
zooplankton count. The huge freshwater input from the large Perak river might
have caused the marine station to have a similar zooplankton composition as
the estuary. The highest copepod diversity index of copepods was observed in
the marine station and decreased as the salinity decreased. Diversity index,
H in this study ranged from 1.59 to 2.20 which was comparable to the study
done by Jayasinghe (2005) in Langat river estuary.
Zooplankton distribution is greatly related to various environmental, physical
and chemical factors. In this study, salinity played important roles to control
diversity. On the other hand, turbidity and predation pressure play important
roles in controlling zooplankton abundance in Perak river.
The study was supported by E-science fund (04-01-04-SF0418) and Research University
Grant Scheme (RUGS), UPM (05-01-09-0727) and the grants from the Japan Society
for the Promotion of Science (JSPS): Grant-in-Aid for Scientific Research (B)
23405031 awarded to JN. The authors would like to acknowledge Prof. Shuhei Nishida
for his professional assistance in confirming copepod species. The authors wish
to thank Perumal K., Azmi A., Nursuhayati, A.S., Safura, S. and Marinni, K.
from the Laboratory of Marine Biotechnology, Institute of Bioscience, UPM for
their technical assistance.
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