Plankton is known as microscopic organisms that float and swim freely in oceanic currents and in other bodies of water. Plankton is made up of tiny plants which called phytoplankton and tiny animals called zooplankton. In addition, zooplankton is known as pelagic organism which cannot maintain their position in the water bodies or against the water flow (Idris, 1988). There are many species of zooplankton can be found in estuarine area such as copepod, isopod, bivalve and gastropod larvae.
Zooplankton can be divided into group such as holoplankton and meroplankton. Holoplankton is known as zooplankton that spend whole of their lifecycle in form of plankton while meroplankton is zooplankton that spend only a part of their lifecycle as plankton. Meroplanktons are usually spending their life as plankton during eggs or larvae stage (Idris, 1988). In addition, larvae of invertebrate organism such as mollusc and Crustacea can be classified as meroplankton. Holoplankton in estuarine is usually dominated by copepode which can adapt with marine and freshwater environment.
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The zooplankton study was carried out in Batang Kayan estuary, Lundu on 16th October 2010. Zooplankton communities are well known to be used as bio-indicator to determine the condition of water quality in Batang Kayan. Hence, the Batang Kayan can be considered as polluted river. The study is expected outcome is to document the zooplankton species and its relationship with water quality in Batang Kayan estuary.
There were few studies of zooplankton that has been carried out in Sarawak such as Kuching Bay in 2005 by Jane Francesca, Batang Lupar by Suhartina Bt Arbe (2007), Punang, Lawas and Limbang by Nur Atiqah Binti Mohamad Yusoff (2009). Based on these study, there 16 taxa found in Kuching Bay, 6 taxa in Batang Lupar and 10 taxa in Punang, Lawas and Limbang. There was no study or information on zooplankton community at Batang Kayan River. The main purposes of this study are:
To record the zooplankton community and its relation with the water quality.
To determine the species density, species diversity, species evenness and species composition of zooplankton
To produce the zooplankton community database for future biological monitoring in Batang Kayan River estuary.
2.1 Zooplankton composition
Plankton can be divided into 2 groups which are zooplankton and phytoplankton. Zooplanktons are also known as the heterotrophic plankton. The diversity of aquatic organism is increase towards the equator (Idris, 1983). Zooplankton can be divided into five different types of group which are microcrustacea, rotifers, coelenterates, ctenophores, annelids and mollusc (Idris, 1988). Some zooplankton species are known as single-celled animals such as foraminifera. Crustecea is the most common of zooplankton found in Malaysian freshwater (Idris, 1983). The number of zooplankton in tropical increasing, while at the temperate will remain unchanged (Idris, 1983).
In Peninsula Malaysia, the most common zooplankton that can be found is Cladocera. Based on Idris study, there are 6 families of common Cladocera found in Peninsula Malaysia. The six common families such as Family Sididae, Daphiniidae, Moinidae, Bosminidae, Macrothricidae and Family Chydoridae.
Zooplankton that is usually found in Sarawak can be classified in the lowest taxa such as Copepoda, Malacostraca, Ostracoda, Polychaeta, Appendicularia, Gastropoda, Bivalvia, Thecostraca, Hydrozoa, Amphipoda, Chaetognatha, Foraminifera, Chordata and Echinodermaia (Volin, 2005).
2.2 Adaptation of zooplankton
The distributions of zooplankton are usually based on the adaptation of zooplankton species. Salinity is one of physio-chemical the factors that can affect the distribution of zooplankton. Zooplankton in estuarine area can be divided into 4 different component based on the zooplankton adaptation. For example, stenohaline zooplankton such as Corycaeus sp. is usually found only at the river mouth of estuarine (Idris, 1988). Euryhaline zooplankton such as Paracalanus sp. can be found further into the mangrove area. Some species of zooplankton would not be able to tolerate with wide range of salinity changes. Zooplankton species such as Pseudodiaptomus sp. and Diaptomus sp. can only be found in estuarine and freshwater respectively.
The zooplankton community is abundance as compared to fitoplankton community in the estuarine (Idris, 1988). This is due to the zooplankton community is dominated by the benthic invertebrate, fish and crustacean larvae. The high population of zooplankton is recorded at continental area (Samolyk et al, 2003). The Abundance of zooplankton will increased with increasing temperature, salinity and chlorophyll a values (Vieira et al, 2000). Some species of zooplankton has ability to tolerate with wide range of salinity and pH changes. This species is usually found in the estuarine such as Acartia tonst (2ppt-36ppt), Acartia clause (14ppt-36ppt), Gammarus zaddachi zaddachi (1ppt-15ppt) and Gamrnanus locusta (25ppt-35ppt). (Wilson, 1994).Some species of copepods has the ability to adapt polluted sites (Bednarski & Ramirez, 2004).
2.3 Distribution and migration of zooplankton
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Zooplankton community is important in marine ecosystem food chain. Zooplankton can be classified based on their size such as picoplankton (< 2 Âµm), nanoplankton (2-20 Âµm), microplankton (20-200 Âµm), mesoplankton (0.2-20 mm), macroplankton (> 200 mm).
Zooplanktons are usually migrating into water bodies during day time and move to the surface during night time (Liu et al, 2003). This type of migration is known as vertically migration. However, the vertical distribution of zooplankton is influenced by physio-chemical parameter of the water such as temperature, salinity, dissolved oxygen and nutrient. Holocline is one of the factors that cause zooplankton change their vertical distribution and migration (Lougee et al., 2002). The main reason that zooplankton migrate deep into the water bodies during day time is to avoid from predator (Lampert, 1989). At lower temperature, the rate metabolism of zooplankton is lower than warm water (Pia, 2007). Hence, zooplankton can save energy by feeding in the cool water (Ellis, 2007). In addition, zooplankton will inhabit the habitat that rich source of nutrient or food. Zooplankton is more abundance at the coastal area. This is due to the amount of nutrient and chlorophyll a at coastal area is high which provides food for zooplankton (Rezai et al, 2000).
The total number of zooplankton species outside of the bay is higher than shoreline areas (Webber M., 2005). Hence, shallow nature of inshore areas may affect low species numbers as a result of absence of those species of common deep levels zooplankton (Webber M., 2005). In the estuary, abundance and biomass of mesozooplankton species is usually depending upon of changes in salinity. Hence, the abundance of zooplankton is depending on the amount of freshwater input from the river (Pia, 2007) . High tides occur due to a variety of physiological and physical processes which can cause the result has higher concentrations of larva and other mesozooplankton (Pia, 2007). Temperature and salinity may play a role in the distribution of zooplankton in estuaries.
Factor influence the zooplankton distribution
Zooplankton distribution in the estuaries is influenced by physico-chemical parameters of the water. In the estuaries, salinity is varied due to the large input of seawater during flood and large input of freshwater during ebb tides. The water salinity are also varied at different depth due to different in mixing in the rivers (Villate, 1997). The salinity range from 30 to 35 PSU is known as euhaline waters. Euhaline water is usually found at the mouth of the river where the freshwater meet the seawater (Pia, 2007). The large input of seawater during high tide will increase the marine zooplankton distribution and vice versa during low tide.
Temperature is also play an important role of the zooplankton distribution. The temperature is varied from the surface to the bottom of the water. The temperature has the effect on the zooplankton abundance in multiple levels (Pia, 2007). The growth of phytoplankton such as diatom is influenced by temperature. Diatom growth shows that positive correlated with the growth of zooplankton such as A. tonsa.
In addition, zooplankton community in aquatic ecosystems is decreased by increased of eutrophication (Lazzoro, 2006). The physico-chemical parameters and nutrient status of water body is important in governing the production of plankton such as zooplankton constitute important food source of many aquatic organism such as fishes (Basu, Roy, & Barik, 2010).
Zooplankton as bio-indicator of the river
Bio-indicator is defined as an organism that presence will indicates the quality of water environment condition (Wilson, 1994). Zooplankton is one of the aquatic organisms that could be used as a bio-indicator to determine the condition of the river. There are three component of estuarine system which can be used to assess the contamination. They are water, sediment and organism (Wilson, 1994). The changing of the water condition will stimulate the organism to adapt to the changes (Dulic et al, 2006). If the organisms fail to adapt with the changes, they cannot survive in the estuary. Then, condition of the water can be assessed by quantifying the degree of the organism adaptation (Wilson, 1994). Zooplankton responds quickly with the changes of water condition such as pH and nutrient. Therefore, zooplankton can be used as bio-indicators of the aquatic environment (Dulic et al, 2006). High amount of DO in water column is an indication of healthy system in a water body. Hence, this condition of the water suitable for aquatic organism to inhabit.
Zooplankton composition in Sarawak
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At least 58 families and 79 species of zooplankton is recorded (Volin, 2007). The example of species founds are Oithona spp., Acartia spp., Longipedia spp., Cyclops spp. and others. The study in razor clam area was carried out before and after the razor clam season. According to the study, they were 14 species commonly found before the razor clam season. The example of species found before razor clam season are Oithona spp., Pseudocalanus spp., Paracalanus spp. and Ameira spp. The common taxa found during razor clam season at Asajaya Laut is nauphli copepods, Thespesiopsyllus spp., spionid larvae and bivalve larvae. foraminifera, bivalve larvae, spionid larvae and gastropod larvae are the common species found at Pasir Puteh.
Based on the study done at Batang Lupar, there are 6 taxa were documented (Suhartina, 2007). Taxa of zooplankton that has been documented such as copepod, polycheate, cumacea, branchyura, gastropoda and mysidacea.
Material and method
3.1 Study Site
The study site was chosen at the Batang Kayan River. The field survey was carried out at Batang Kayan estuarine from 15th to 19th October 2010. The sampling site was divided into 5 stations. Coordinate of each station was recorded by using Global Positioning System. Based on the observation, the vegetation from station 1 to station 3 was dominated by Nypa sp. while station 4 and 5 dominated by Rhizophora sp. and Avicennia spp.. The water of Batang Kayan River was turbid possibly due to the erosion occur at the upper stream of the river. Land clearing for oil palm plantation occurred at the upper part of the river. In addition, the river is also affected by domestic waste effluent produced by the resident along the river. The sampling site is shown in figure 1.
Figure 1: Sampling site (Source: Google earth, 2010)
The zooplankton sampling was carried out during high tides which the entering of seawater to the river. The mesh size of plankton net used was 100 Âµm. Flow meter was placed at the mouth of the plankton net
3.2.1 Vertical distribution
For vertical distribution sampling, initial reading of the flow meter was recorded. Then, the plankton net was dropped down into the water up to 3.35 meters (11ft). Next, plankton net was pulled with moderate speed. The final reading of flow meter was recorded right after the plankton net was completely pulled out from the water. Then, the sample was put in the whirl-pak plastic bag and preserved with using Lugol's solution. Lastly, the sample was labeled according to each station. Two replicate of sample were collected at every station.
3.2.2 Horizontal distribution
For horizontal distribution sampling, initial reading of flow meter was recorded before towed. After that, plankton net was towed horizontally about 5 minute at every station. The time was recorded exactly right after the plankton net was towed. Then, plankton net was pulled out after 5 minute and the final reading of flow meter was recorded. The sample was put in whirl-pak plastic and preserved with using Lugol's solution. Two replicate of sample were collected at every station.
The water parameter such as dissolved oxygen (DO), pH and temperature were recorded in situ by using Eutech instrument (Model PCD 650) while the turbidity was measured using Eutech turbidity (Model TN-100) and salinity was measured using Hand refractometer (Model Atago S-10). The reading of water parameters were taken twice and the average value were recorded. The chlorophyll-a data of the water were adapted from Asmyrita Husna (2010)
3.4 Laboratory works
The sample was sieved using 100 Âµm mesh size sieve to drain out the water sample. Any organism that retained on the sieve was transfer to specimen container which filled by distilled water.
3.4.2 Sub-sample, sorting and counting
After sieving process, the sample was subsampled by using Folsom's Splitter. The samples were divided into equal half. A half of sample was poured onto petri dish and observed under Stereo Microscope (RaxVision). The different shape of zooplankton were separated and enumerated.
3.4.3 Species identification of Zooplankton
For identification process, a sample from each taxa is sorted out and placed on the prepared slide. Then, the sample was observed under compound microscope (LEICA CME).
3.5 Data analysis
Species richness (D), species diversity (H'), species evenness (J), species density and species percentage (%) were calculated for each station using the following equation:-
Margalef index (Margalef, 1958)
Where S is the total number of species in a sample and N is the total number of individual in a sample.
Shannon-Weiner Index (H') (Poole, 1974)
Where s is the number of species found in a sample, n is the total of individual one species in a sample and N is total number of all species in the sample.
Pielou Index (Poole, 1974)
Where H' is species diversity and S is the number of species.
Species density (ind/l)
Where n is the total individual one species in a sample and V is the volume of water (L).
Species percentage (%)
Species percentage (%)= X 100
Where n is the number of individual species in a sample and N is the total species in a sample
Flow meter formula:-
Distance in meter(m)=
Difference in count X Rotor constant
Where rotor constant is 26,873
Volume in cubic meter (m3)=
3.14 X (Net diameter)2