Phytoplankton Community Dynamics in Sei Gesek Reservoir, Bintan Island

Bintan is a small island that has limited fresh water resources. Sei Gesek Reservoir is one of the drinking water sources in there. The aim of this research is to determine the dynamics of diversity and abundance phytoplankton in Sei Gesek Reservoir, Bintan Island, Riau Islands. Sampling was carried out at three area representing inlet, center, and outlet of Sei Gesek reservoir in periods 2016-2018. Phytoplankton samples were collected using plankton net by static method. Phytoplankton group were composed of 3 division: Chlorophyta, Cyanophyta, and Bacillariophyta. Each class of phytoplankton has dynamics of abundance and species each year. The highest average abundance was found in 2016 and then fluctuated to 2018. The highest species and abundance of the division were Chlorophyta, Cyanophyta, and Bacillariophyta, respectively. Phytoplankton community dynamics in Sei Gesek Reservoir is influenced by decreased water volume and acidic pH.


Introduction
Bintan Island was located in Riau Islands Province is a small island.This Island cover an area of about 1,167 km 2 [1].The fresh water resources on Bintan Island very limited.There are nine watersheds in Bintan Island [2], existing Sei Jang watershed becomes a priority watershed in providing fresh water on Bintan Island, with Sei Pulai and Sei Gesek reservoirs.
Since the establishment of the Riau Archipelago Province in 2002, development on Bintan Island has progressed very rapidly.However, environmental problems and conflicts of interest in resource management on this small island still occur.Characteristics of small islands cause various environmental problems, one of which is the guarantee of the availability of clean water in sufficient quantities and of good quality.To meet the needs of raw water on Bintan Island, the Sei Gesek Reservoir began to be built in 2011.Sei Gesek Reservoir has an inundation area of ± 6.82 ha with a storage volume of 277,888 m 3 .This reservoir is expected to be able to help meet the needs of raw water on Bintan Island which previously came from Sei Pulai, Kolong Enam, Lagoi Reservoir, and Jago Reservoir [3].
The increase of the concentration of nutrients in the Sei Gesek Reservoir is thought to be caused by the input of organic matter from the domestic waste of residents' settlements around the reservoir inlet.In addition, the existence of oil palm plantations in the upstream area of Gesek River which is the source of water for this reservoir is thought to have contributed to the addition of nutrients in the waters.Based on observations, Sei Gesek Reservoir also has problems water volume reduction, especially during the dry season.This decrease in water volume tends to cause the penetration of sunlight to reach the bottom of the water.This can also be the cause of abundant macrophytes in Sei Gesek Reservoir.Nutrient concentrations and high sunlight intensity are the main triggers for the rapid increase in the growth of autotrophic biota in stagnant waters [4].
Studies that have been carried out in Sei Gesek Reservoir are water quality [5] and fish diversity [6].Several studies regarding the structure of the phytoplankton community in the Sei Gesek Reservoir have been carried out.However, the dynamics of phytoplankton abundance has not been reported.The research hypothesis in this study is there any on phytoplankton dynamics temporally.The aim of this study is to determine the dynamics of diversity and abundance phytoplankton in Sei Gesek Reservoir, Bintan Island, Riau Islands.The final results of this study are expected to be useful as basic information for the sustainable management of the Sei Gesek Reservoir.

Material and Methods
Samplings were carried out at points representing the inlet of the Sei Gesek River flow, the center of the reservoir, and the outlet.Data were collected from 2016-2018.The sampling locations are shown in Figure 1.Water transparency was measured using a Secchi disk [7].Phytoplankton samples were taken by filtering one hundred L of water using plankton net no. 25 with a mesh size of 40 µm by the static method.Identification of phytoplankton was carried out at the Marine Biology Laboratory, FIKP UMRAH.The phytoplankton found were identified using the [8] and [9] identification books.The abundance of phytoplankton was calculated by the Sedgewick Rafter Counting Chamber Cell (SRC) with the census method based on the [7].

Phytoplankton Community
Based on the data that has been collected, the three phytoplankton found for three years consist of Bacillariophyta, Chlorophyta, and Cyanophyta groups.The abundance of phytoplankton fluctuates every year (Table 1).The highest abundance of phytoplankton was found in 2016, then decreased and fluctuated until 2018 (Figure 2).The decrease in the abundance of phytoplankton that occurred in 2017 is thought to be caused by a decrease in the volume of water in the Sei Gesek Reservoir.Phytoplankton from the Chlorophyta group had the highest percentage composition compared to Bacillariohyta and Cyanophyta, except in 2017 at the outlet of the Sei Gesek Reservoir where the highest abundance was from the Bacillariophyta group (Figure 2).

Environmental Condition
In general, the physical and chemical parameters of the waters in the Gesek Reservoir have met the phytoplankton requirements, except for the pH value (Table 2).The pH value in the Sei Gesek Reservoir is always below 6, this indicates that the Gesek Reservoir waters are quite acidic.According to [6], the pH value in the natural waters of Bintan Island ranges from 4.48-6.88,while the pH value in the waters of the former bauxite mining on Bintan Island ranges from 4.00 [10], 4.45-6.36[11], and 5.46-6.13[6].The low pH value is thought to be influenced by sediment conditions on Bintan Island which contain bauxite [12].The low pH value in the waters of Gesek Reservoir is thought to be influenced by organic matter decomposition that occurred since the beginning of the reservoir inundation.In addition, the macrophyte population that grows at high density in the Sei Gesek Reservoir can also be a contributor to autochthonous organic matter.

Conclusion
Phytoplankton in Sei Gesek Reservoir were found from 3 division: Chlorophyta, Cyanophyta, and Bacillariophyta.Each divison of phytoplankton has dynamics of abundance and species each year.The highest average abundance was found in 2016 and then fluctuated to 2018.The highest species and abundance of the division were Chlorophyta, Cyanophyta, and Bacillariophyta, respectively.

Figure 1 .
Figure 1.Sampling Site Three litters of water samples were collected using a Vandorn water sampler to determine water quality.A total of three replications were carried out in taking water samples at each sampling point.The water quality parameters observed consisted of water physical parameters (temperature, transparency), chemical parameters (pH, DO), and biological parameters (abundance of

Figure 2 .
Figure 2. Phytoplankton composition based on abundance in Sei Gesek Reservoir

Table 2 .
Average values of several aquatic environmental parameters in Sei Gesek Reservoir