Composition and abundance of phytoplankton in the rearing tank of elver eels stadia with a bio-floc system

Bio-floc is a new technology used in fish farming systems, phytoplankton is one of the components in bio-floc so its existence is very important. The objective of this research is to study the composition and abundance of phytoplankton in the rearing container for the elver eel (Anguilla bicolor bicolor) bio-floc system. Observations were made on three eel rearing containers, as much as 1 liter of water was filtered using plankton net with 53 μm mesh size and preserved using Lugol’s solution. The parameters observed were the composition and abundance of phytoplankton, several environmental parameters were also analyzed to see their effect on phytoplankton. There were five classes of phytoplankton found, namely Bacillariophyceae (8 species), Chlorophyceae (9 species), Cyanophyceae (1 species), Dinophyceae (1 species), and Euglenophyceae (2 species), with the dominant species are Scenedesmus sp., Sphaerocystis sp., Oocystis sp., Gomphonema sp., Navicula sp., Rhopalodia sp., Synedra sp., and Aphanocapsa sp. The abundance values ranged from 1,785 to 925,250 individuals -1L. The composition and abundance of phytoplankton were influenced by phosphorus concentration.


Introduction
Bio-floc is a new technology used in fish farming systems.To control water quality, the abundance and active aerobic microbial communities were manipulated by immobilizing ammonium into microbial protein and recycling feed residues thereby increasing feed efficiency [1].Biota reared using a bio-floc system include shrimp, catfish, and tilapia [1][2][3][4][5][6].
Bio-flocs consist of various bacteria, fungi, microalgae (phytoplankton), zooplankton, Oligochaeta, detritus, and other suspended organisms [3,7,8].The addition of microalgae in the bio-floc system can increase the floc particle size [9,10].Phytoplankton utilizes nitrogen from water as part of their metabolism, to reduce the nitrogen content in the rearing pond [11].The basic component of phytoplankton cells is the protein which is synthesized from glucose using nitrogen adsorbed from water.Nitrogen constitutes about 1/6 of the carbon in algal cells [12].In addition to removing excess nutrients, these microorganisms can also be used as a food source for cultivated biota.
Phytoplankton is a component of bio-floc, so it is important to know the type and abundance of existing phytoplankton.In their research, Sukma et al and Tarkus et al [5,6] stated that the phytoplankton found in catfish (Clarias gariepinus) rearing containers with a bio-floc system was from the Chlorophyta, Cyanophyta, Bacillaryophyta, and Xanthophyta groups, with high abundance species.
Botryococcus sp, Chlorella vulgaris and Closterium sp.(Chlorophyta), Dactylococcopsis sp.(Cyanophyta), Thalassiotrix sp.Thalassionema sp. and Synedra sp.(Bacillaryophyta), and Tribonema sp.(Xanthophyta).While in the Vaname shrimp rearing pond with bio-floc system, the types of phytoplankton found were Oscillatoria sp., Protoperidinium sp., Chlorella sp., Cryptomonas sp., Ceratium sp., Diplosalis sp., Eutreptia sp., and Thallasionema sp.[13] Eel is a fishery commodity that has high economic value, its export demand continues to increase from year to year.Eel cultivation has been carried out in Indonesia, and various techniques have been developed to improve the quality of the eel cultivated.The current eel cultivation is using a green water system and clean water with a recirculation system.Cultivation of elver eels with bio-floc system has not been done.Observations of phytoplankton in the eel rearing container of the bio-floc system have never been carried out, even though phytoplankton have an important role in the bio-floc system.Therefore, observations were carried out to assess the composition and abundance of phytoplankton in the elver rear eel (Anguilla bicolor bicolor) bio-floc system.

Methods
Observations on the composition and abundance of phytoplankton were carried out in three rearing containers for elver eel (A.bicolor bicolor) using a bio-floc system (Figure 1).Elver eel rearing was carried out in a round plastic tarpaulin pond, the diameter of the container was 3 m, the height of the container was 1.20 m, and the water height was 1 m (Figure 1).Each container is filled with 10 kg of elver with an average weight of 96.6 g.Observations were made for 1.5 months (10 September 2020 -15 October 2020), and sampling was carried out four times during the observation.One litter of water sample was filtered using plankton net with 53 µm mesh size.Samples were preserved using Lugol's solution until the water turned brownish yellow or to a concentration of 1% [14].Identification was carried out under an inverted microscope (NIKON Diaphot 300), at 100x, 200x, and 400x magnifications based on [15,16,17,18,19].The abundance of phytoplankton was calculated using the Sedgwick Rafter method [14].Phytoplankton in the form of colonies and filaments are counted as one phytoplankton unit [20].
The water quality parameters measured were dissolved oxygen (DO), pH, temperature, and turbidity, measured using water quality checker (HORIBA U50).Parameters analyzed in the laboratory were total phosphorus (TP), nitrogen-nitrite (N-NO2), and total ammonia nitrogen (TAN) based on APHA [14].Measurement of water quality parameters was carried out simultaneously with water sampling for phytoplankton observations.This work did not require ethical approval under the research governance guidelines operating at the time of the research.

Water Quality
The total phosphorus content increased sharply at the third observation (1 October 2020), and at the end of the observation (15 October 2020), the value reached 1.4274 mg/L (Figure 2).The TAN content increased until the third observation and decreased at the fourth observation.The decrease in TAN was accompanied by an increase in the nitrite content, at the fourth observation it reached a value of 6.089 mg/L.This result shows that there is a nitrification process in the fourth observation, namely TAN is oxidized to nitrite, this process is carried out by autotrophs and heterotrophs bacteria which are bio-flocforming bacteria (Figure 2).As mentioned by Effendi [21] that nitrite reform or the nitrification process by nitrifying bacteria is carried out by oxidizing ammonia to nitrite and nitrate.The value of pH, dissolved oxygen (DO), and temperature are presented in Figure 3.The pH value during observations ranged from 7.24 to 7.58, this value indicates that supports the life of aquatic organisms, Effendi [21] states that aquatic organisms generally like the value of pH is around 7.0-8.5.The pH value in this observation is quite stable, changes in the pH value will affect the stability of the bio-floc [13].
In the bio-floc system, aeration is carried out to increase the dissolved oxygen concentration in the rearing tank.The dissolved oxygen content is not only important for the metabolic activity of the flock cells but also affects the floc structure.The high oxygen content can produce large flocs with high density [7].The dissolved oxygen value in this observation decreased from 9.98 mg/L to 3.02 mg/L at the third observation and started to rise again at the fourth observation (4.24 mg/L).the decrease in dissolved oxygen content along with the increasing value of nitrite indicates the occurrence of the nitrification process in the eel rearing container.The value of dissolved oxygen can still support the life of aquatic organisms.The dissolved oxygen content of up to 2 mg/L can still support the aquatic organisms' life, as long as there are no toxic materials in the waters [22].
The temperature ranges from 29.00ᵒC -30.61ᵒC, this value still shows that supports the life of aquatic organisms.The optimum temperature range for the phytoplankton growth in the waters is 20ᵒC-30ᵒC [21].Temperature is an important factor in microbial metabolism [7].At low temperatures (4°C) deflocculation can occur, while at high temperatures (30°C-35°C) will result in sludge accumulation, the appropriate temperature to obtain stable floc is at a temperature of 20°C-25°C [23].The value of turbidity is presented in Figure 4, the value increased from the first observation (20.4 NTU) to the fourth observation (183.3NTU).The high turbidity can be caused by the abundance of phytoplankton in the waters, Hargreaves [24] states that turbidity can be caused by various substances, including microalgae (phytoplankton), bacteria, dissolved organic matter, suspended clay particles, and colloidal solids.
The abundance of phytoplankton was higher until the end of the observation (Table 2, and Figure 5).The high abundance of phytoplankton indicates that water quality conditions strongly support the growth of phytoplankton.The abundance value ranges from 1,785 to 925,250 individuals -1 L, this abundance value is higher than the abundance value in the catfish rearing tank research results by [5,6], and in the Vaname shrimp rearing tank [13].The high abundance of phytoplankton during elver eel rearing in this bio-floc system can increase the floc particle size [9,10], reduce the nitrogen content in the rearing pond [11], and can be used as a source of food for reared fish.
The difference in the composition and abundance of phytoplankton in the elver eel rearing tank compared to the catfish and vaname shrimp rearing tank was due to differences in several water quality parameters.Dissolved oxygen and ammonia were lower and phosphate was higher in catfish rearing [5], dissolved oxygen was lower, ammonia and phosphate were higher in catfish rearing [6], and higher ammonia and phosphate in vaname shrimp rearing [13] than in eel rearing.The abundance of phytoplankton increased from the first observation (10 September 2020) to the fourth observation (15 Oct 2020), the abundance increased sharply at the third observation (1 Oct 2020) (Figure 5).The species with high abundance were Scenedesmus sp., Sphaerocystis sp., and Oocystis sp.(Chlorophyceae), Gomphonema sp., Navicula sp., Rhopalodia sp., and Synedra sp.(Bacillariophyceae), and Aphanocapsa sp.(Cyanophyceae) (Table 1).The high abundance of several types of phytoplankton shows that these species are able to adapt to environmental conditions.Navicula sp., Synedra sp., Scenedesmus sp., and Aphanocapsa sp. is a species that can live in waters with high nutrient content [15].The high abundance of phytoplankton in the third and fourth observations was mainly due to the high abundance of Navicula sp. and Sphaerocystis sp.(the third observation) and Sphaerocystis sp.(the fourth observation) (Table 1).
This observation shows that there has been a succession of phytoplankton in the elver eel rearing container, according to Wetzel [12], phytoplankton succession is a change in density, dominant species, and phytoplankton biomass in a community, changes in species composition are caused by differences in the growth rate of each species community in the controlled by environmental factors [25].The first to second observations show that Synedra sp. and Navicula sp. are the highest abundance, in the 3rd observations the dominant species were replaced by Navicula sp. and Sphaerocystis sp., and at the end of the observation (fourth observation) Sphaerocystis sp. is the most abundant species.On the fourth observation, it was also seen that Aphanocapsa sp.(Cyanophyceae) began to be high in abundance, the high type of blue algae in the waters must be considered especially if the dominant species is a toxic species because it will affect the fish being cultivated.The high Cyanophyceae at the end of the observation could be due to the high phosphorus in the waters, according to Widigdo and Wardiatno [26], the high Cyanophyceae in shrimp ponds due to the high phosphate content in these waters.Chlorophyceae and Bacillariophyceae are class that are expected to grow in fish rearing ponds, because they can be a natural food source for reared fish and being a source of oxygen.
The increase in TP concentration was in line with the increase in the abundance of phytoplankton (Figure 2 and Figure 5).TP is a nutrient that affects the growth of phytoplankton, its concentration is lower than other major nutrients but it is needed for the growth of phytoplankton in freshwater [12].Liu et al [27] also stated that TP affects the abundance of phytoplankton in the lake, the highest abundance of phytoplankton in Lake Cibuntu occurs when the concentration of TP is high [28].The higher abundance of phytoplankton is also accompanied by the higher value of turbidity (Figure 4 and Figure 5), as mentioned by Hargreaves [24] that the value of turbidity is caused by the abundance of phytoplankton.

Conclusions
At the beginning of the observation, Synedra sp. and Navicula sp. with high abundance, then on the third observation Navicula sp. and Sphaerocystis sp in high abundance, and at the end of the observation Sphaerocystis sp. had high abundance.At the end of the observation, it was also marked by the increase of Aphanocapsa sp. from the phylum Cyanophyta which is one of the most toxic phyla.The abundance of phytoplankton increased until the end of the observation along with the increasing total phosphorus content, it is suspected that phosphorus affects the abundance and composition of phytoplankton in the elver eel rearing container with a bio-floc system.

Figure 3 .
Figure 3. Value of dissolved oxygen, pH, and temperature during observation.

Table 1 .
Phytoplankton composition and abundance during observation.

Table 2 .
Phytoplankton abundance (individuals -1 L) for each class during observation.