The Community Structure of Plankton at Floating Net Cages Aquaculture Area in Sendang Biru Water Indonesia

. The leftover food and feces from fish aquaculture in the floating net cages (FNC) may contribute to the nutrient enrichment at surrounding FNC water. The phytoplankton community structure is the one of important indicator for water quality evaluation. This research aims to determine the plankton community structure and water quality around FNC, Sendang Biru, Malang. Plankton samples and physico-chemical parameters were taken at 3 locations at surrounding FNC area. The results show that phytoplankton from the Bacillariophyceae class and zooplankton from the Hexanauplia class (copepods) dominate in the community structure. Phytoplankton abundance ranges from 795-11405 ind/L, diversity index (H') ranges from 1.47-2.43, evenness index (E) ranges from 0.76-0.95, and dominance index (D) ranges from 0.1-0.3. Meanwhile, zooplankton abundance ranges from 15-634 ind/L, H' value in range of 2.05-2.58, E value in the range of 0.72-0.84, D value in the range of 0.11 -0.17. Nitrate and phosphate have values that exceed the specified quality standards.


Introduction
Malang Regency is one of the regions in East Java with a coastline of 77 km and is in the waters of the Indian Ocean.One of the coastal areas in Malang Regency is Sendang Biru [1].Sendang Biru has large fishery resource potential in East Java [2].Utilization of fishery resources can be used to meet community protein needs [3].One effort to fulfil the need for protein derived from fish can be marine cultivation using a floating net cage (FNC) system [4].FNC, as one of the activities for developing marine aquaculture, have the potential to have negative effects on the aquatic environment if excessive exploitation occurs [5].The activity of FNC produces solid waste (in the form of inedible feed and mucus) and dissolved waste (nitrogen and phosphate compounds) which enter directly into the waters and can cause the risk of eutrophication [6].Solid waste, such as leftover food and fish faeces can affect water quality, mainly nitrogen and phosphate compounds [7 8].The quality of waters can be analysed based on physical, chemical and biological parameters.Measurement of physical and chemical parameters tends to describe the quality of the environment at a certain time, while monitoring using biological indicators can provide clues about changes in these waters [9].Rapid changes in nutrients in waters can affect phytoplankton communities and can gradually increase their growth [10].
Phytoplankton population explosions (blooms) have different effects, depending on the type and response of the phytoplankton species.Explosions of phytoplankton populations that have the potential to be dangerous are known as Harmful Algal Blooms (HABs) [11].Due to the increase in nutrients, an increase in phytoplankton biomass followed by an increase in zooplankton will also occur [6].Therefore, based on its nature which tends to be sensitive to changes in the environment in which it lives, plankton can be used as a biological indicator refer to its abundance, diversity and dominance of species [12].Environmental monitoring based on community structure, such as fluctuations in plankton diversity, can be an indicator of changes in water quality [13].This is also supported by previous research [5], that changes in plankton community structure occurred after the existence of FNC due to the increase in nutrients and phytoplankton.The evaluation of ecological indexes of plankton community structure are importance to determine the condition of the waters around the FNC.Therefore, the adverse effect of water quality toward the aquaculture activities as well as other organism in Sendang Biru water is avoided.The samples of plankton and water for physicochemical parameters were taken in September and October 2019 at surrounding area of FNC (Figure 1).The sampling location and coordinates were namely T1 (8°26'14.90"S;112°40'47.80"E,inside the FNC water); T2 (8°26'14.93"S;112°40'48.74"E,east-side of FNC) and T3 (8°26'15.14"S;112°40'46.80"E,west-side of FNC)

Water Quality Measurement
There were several water quality that measured on site included temperature, salinity, pH, dissolved oxygen (DO) using a thermometer, refractometer, pH meter, and DO meter respectively.While the concentration of nitrate and phosphate in the water samples were analyzed in laboratory using spectrophotometric

Plankton Sampling and Analysis
Phytoplankton samples were taken using a plankton net of Kitahara type which has a diameter of 0.3 m and a mesh size of 80 µm.Meanwhile zooplankton samples were taken with a NORPAC type plankton net with a diameter of 0.45 m and a mesh size of 0.15 mm.The filtered samples were transferred into sample bottles and preserved with 2.5% of formalin and 0.7% of Lugol solution [14].The phytoplankton and zooplankton were observed using Sedgewick rafter under compound and stereo microscope respectively [15].Furthermore, plankton samples were counted for analysis of abundance (N), Shannon Wiener Diversity Index (H'), Pielou's Evenness Index (E).and Simpson dominance index (D) [16,17].

Water quality in surrounding area of Sendang Biru FNC
The dynamics of the abundance and structure of plankton communities are influenced by physicochemical factors, especially the availability of nutrients and the quality of light as well as the ability of phytoplankton to utilize it Therefore, it is necessary to provide an overview of the environmental conditions around FNC, Sendang Biru, Malang.This current research found that water quality in surrounding area of Sendang Biru FNC (Table 1) showed value range of temperature (22,0±0,3°C), salinity (35,3±1,2‰), pH (7,5±0,1) and DO (7,7±0,3) which were in the normal range and meet quality standards that stipulated by The Indonesian Minister of Environment Decree [18].Meanwhile, the value of nitrate and phosphate were exceeded the quality standards.In general, the observed water quality was in optimal level for plankton growth, such as temperature were in level of 20° C to 30° C [19] Temperature plays a role in regulating life processes and the distribution of organisms, so that temperature is an important factor for the life of organisms [20].The salinity content is above 20‰ allows phytoplankton to carry out photosynthesis and growth.[21].The pH can be influenced by biological activities (photosynthesis and respiration of organisms), temperature, and the presence of ions in the waters [22].A good pH to support biota life is in the range of 6.5-8 [23].The optimum pH for phytoplankton to grow is in the range of 6-8, while the optimum pH for zooplankton is in the range of 5-8 [24].DO plays an important role in determining the number of organisms in waters [24].The minimum concentration of dissolved oxygen is 2 mg/L in normal, unpolluted conditions [25].However, the range of nitrate content in the waters around the Sendang Biru FNC can be tolerated by phytoplankton seen from the presence of phytoplankton.This was explained in previous research by [26], that the minimum requirement for nitrate absorbed by phytoplankton (diatoms) ranges from 0.001-0.007mg/L.Meanwhile the optimum nitrate level in waters for phytoplankton ranges from 0.9-3.5 mg/L [26].The same thing also happens to the phosphate content which can still be tolerated by phytoplankton.This was explained by previous research [17] that the optimum phosphate content in waters for phytoplankton growth ranges from 0.27 to 5.51 mg/L.The trend of nitrate and phosphate concentration tend to higher in October (Table 1).This possibly related to the lifetime of aquaculture activities.In October, there was an increasing of accumulated organic wastes from leftover food that rich of protein and faeces as consequence of the increasing age and size of cultured fishes.The research by [7 8] reported that fish feed and fish faeces contribute to the nitrate and phosphate pollution in water.

Plankton Composition
The water quality in surrounding area of Sendang Biru FNC supported to the growth of phytoplankton.Based on Figure 2, showed that the phytoplankton from the three sampling locations consisted of 43 genera included 32 genera from the class Bacillariophyceae or Diatom (labelled with *) at Figure 2); 7 genera from the class Dinophyceae or Dinoflagellata (labelled with #) at Figure 2); 3 genera from the class Cyanophyceae (labelled with ##) at Figure 2) and 1 genus from the class Chrysophyceae (labelled with ###) at Figure 2).The large number of genera from the Bacillariophyceae class at the sampling location is due to its adaptation ability toward the environment, is cosmopolitan, resistant to extreme conditions, and has high reproductive ability [27].Bacillariophyceae also have a heteromorphic type, that is, each species can have several different morphologies [28].Thus, if there is a change in the environment, Bacillariophyceae can adapt by changing their morphological shape, especially in the valve section [28].In waters, more than 100,000 species of diatoms can be found in fresh and marine waters [29].Apart from diatoms, many genera from the Dinoflagellate class were also found at the sampling locations.Dinoflagellates are organisms that can live in all aquatic conditions, especially marine waters [30].The large composition of dinoflagellates around FNC is thought to be caused by adequate environmental conditions.This is in accordance with previous research [31], that Dinoflagellates can multiply if environmental factors such as light, temperature, salinity and nutrients are adequate.[32] and have an efficient body shape for swimming and are equipped with sensory antennae to determine the position of prey and predators [33].

Plankton Abundance (N)
Refer to the Table 1, exhibited that the abundance (N) of phytoplankton range of 105 -11405 ind./L.The highest abundance of phytoplankton was found at T1, in period of September (11405 ind./L) as well as in October (8870 ind/L).In overall, the abundance of phytoplankton at all sampling locations were categorized as low abundance since they were lower than 12000 ind./L [34].
The abundance of zooplankton ranges from 15 to 634 ind./L.Similar with phytoplankton, the highest abundance of zooplankton was found in T1 where September was observed 189 ind./L and October was found (634 ind/L).The abundance of zooplankton from the three-sampling location also categorized low abundance since they lower than 1000-4000 ind./L [35].T1 was located inside of FNC, that water quality contained higher nutrient due to the aquaculture activities such as feeding and faeces from cultured fishes.The nutrient availability are factors that can influence phytoplankton abundance [36], which the abundance of phytoplankton plays a role as food for zooplankton [37].In general, the phytoplankton abundance was higher in September than October.Contrary, the zooplankton abundance was found higher in October.This condition might be influenced by increasing rainfall intensity during October that led the change of zooplankton distribution pattern and feeding activities [38].These grassing zooplankton in all sampling locations would be followed by phytoplankton abundance.

Shannon Wiener Diversity Index (H') of Plankton
The diversity index (H') of phytoplankton ranges from 1.47 to 2.36 and zooplankton were in ranges of 2.05 to 2.58.The highest value of diversity for phytoplankton (H' =2.43) and zooplankton (H'=2.58)were found at T2, in period of October.Based on the observed H' value of phytoplankton and zooplankton, indicated that condition of community structure in Sendang Biru FNC water area were fluctuated from stable to very stable category [39].A stable community structure is characterized by good interactions between species so that it is characterized by high diversity and evenness [40].Stability in a community tends to maintain equilibrium from disturbances such as natural events or human activities that can change the community [ 41 33].Meanwhile, diversity can be interpreted as meaning that the more species found, the greater the diversity, depending on the number of individuals of each species [42].

Pielou Evenness Index (E)
The Pielou evenness index (E) of phytoplankton ranges from 0.76 to 0.95 and zooplankton ranges from 0.72 to 0.84, hence it can be categorized as high evenness since the value of E>0.5 [17].The high phytoplankton evenness index might be correlated with the low category of abundance.The previous research [43] reported that a decrease in community evenness could be due to nutrient enrichment followed by a high abundance of phytoplankton, and some species could become dominant.From this explanation, it can be assumed that the evenness index has a negative correlation with phytoplankton abundance.Meanwhile, the high zooplankton evenness index is thought to be due to the availability of phytoplankton as zooplankton food.

Dominance Index (D)
The phytoplankton dominance index (D) had ranged of 0.1 to 0.3 and zooplankton was in range of 0.11-0.17.Those dominance index were categorized as low category since the value of D<0.5 [17].The dominance index at T2 and T3 during October period were in level of D=0.10 for phytoplankton and D=0.11.The dominance index during September tends to be higher than October, which intercorrelated with the dominant species namely Melosira sp.Refer to the data of plankton composition at Figure 2, showed that Melosira sp was observed as the most abundance species in all sampling sites during period of September with percentage tend to dominate i.e.T1=34.3%;T2=46.6%;T3=28.3% while during October, it was significantly declined (T1=6.5%;T2=2.2%;T3=7.9%).In overall, during October the number of species in all sampling locations were increasing and no dominant species (Figure 2), hence the dominance index decreased (Table 2).This condition accordance with the reference by [43].The references of [44 45], explained that dominance index are inversely proportional to the evenness and diversity index.Therefore, it can be stated that the increasing of evenness and diversity index in October as shown at Table 2, might be generated by the declining of phytoplankton dominance index.
Mostly, the temporal fluctuating of dominance, evenness, and diversity indexes of zooplankton relatively similar with the phytoplankton.The exception was found in T1 (located inside of FNC), which zooplankton community might be affected by the predation of carnivorous fishes that cultured in FNC namely Lates calcarifer.The predation is the one factor of low dominance [45].

Conclusion
The research found that plankton community structure in Sendang Biru FNC have low abundance (N phytoplankton=105-11405 ind./L;N zooplankton= 15-634 ind./L), the diversity index was in range of 1.47 -2.58; the evenness index (E) ranges from 0.72-0.95and the dominance index (D) in range of 0.1 to 0.3.

Figure 2 .
Figure 2. The composition of phytoplankton species from the classes of *) Bacillariophyceae; #) Dinophyceae;##) Cyanophyceae and ###) Chrysophyceae.Melosira sp (red text) is the most abundance species that found in all sampling sites during period of September and declining during October.

Table 1 .
The water quality the in surrounding area of Sendang Biru FNC [18]: * Quality Standards: The Indonesian Minister of Environment Decree No. 51 Yr. 2004[18]

Table 2 .
The ecological indexes of Phytoplankton and Zooplankton