Detection of Eutrophication in Benoa Bay-Bali

Eutrophication is an increase of organic material caused by the increase of nutrients in aquatic ecosystem by human activities. One effect of eutrophication is Harmful Algae Blooms (HABs) that are very harmful to the environment and humans. The Purpose of this study were determine spatial distribution of nutrient based on total nitrogen (total N) and total phosphate (total P), ratio of total N, and trophic level in Benoa Bay. Nutrient measurements were using spectrophotometric method. The Spatial distribution of total N and P show a generally pattern concentration increases in waters near the mainland and estuary. The highest concentrations of total N and P were found in Loloan River estuary closed to the floating net cages (FNC). Ratio of total N showed generally that the waters in Benoa Bay were dominated by Nitrate. High ammonia concentration was found in all rivers and Loloan River estuary. Benoa Bay is mostly in Oligotrophic conditions based on total N, while eutrophic conditions were only found in the Loloan River estuary. Based on total P, Benoa Bay was generally in


Introduction
Eutrophication is one of the problems in the world, especially in developing countries (Yang et al., 2008).Eutrophication is an increasing of organic material caused by the addition of nutrients in an aquatic ecosystem by human activities (Nixon, 1995).Human activities such as urbanization, industrialization, waste disposal, aquaculture and intensive agriculture are activities that accelerate nutrient concentration in waters (Anderson et al., 2002;Liu and Qiu, 2007;Diaz and Rosenberg, 2008).One of the harmful effects of eutrophication is Harmful Algae Blooms (HABs) or better known as red tide (Anderson et al., 2002).
HABs is an increase in the abundance of phytoplankton (algae) in aquatic ecosystems that can cause death due to toxicity or anoxia conditions in water (GEOHAB, 2001).HABs are not only harmful to marine organisms, but also harmful to humans who consume fish that contain toxins from HABs.Toxins usually accumulate in body tissues of organisms (Wardianto et al., 2004;Anderson et al., 2008).Some diseases caused by HABs such as paralytic shellfish poisoning (PSP), neurotoxic shellfish poisoning (NSP), amnesic shellfish poisoning (ASP), and ciguatera fish poisoning (CFP) (Anderson et al., 2008).In some cases, HABs cause of humans death (Wardianto et al., 2004).According to Widiarti and Pratiwi (2003), several cases of poisoning from consuming shells infected by HABs have occurred in Lewotobi and Lewouran (East Nusa Tenggara), Sebatik Island (East Borneo), Makassar Waters and Ambon Bay.
In Indonesia, HABs cases characterized by eutrophic or hypertrophic conditions occur in Jakarta Bay and Lampung Bay (Damar 2003;Wardianto et al., 2004;Irawan, 2015).The conditions that support the occurrence of eutrophication in bay waters are calm currents, small waves, high nutrient input, and environmental factors (such as temperature, salinity, pH etc.) that support the phytoplankton life of (Zhao, 2004;Cai et al., 2007;Roy et al., 2013).Turner and Rabalais (1991) explained that the bay is area that gets the most nutrient input from the mainland.Nutrient is a major factor that limiting the phytoplankton growth in aquatic ecosystem under normal conditions.
Early detection of eutrophication characterized by nutrient enhancement is the first step to detect HABs.
Benoa Bay is an estuary of six rivers in Bali such as Buaji, Sama, Mati, Badung, Bualu and Loloan Rivers.Previous research conducted by Suteja and Purwiyanto (2018) showed nutrient input (nitrate and phosphate) from six rivers to the Benoa Bay is very high and potential to trigger eutrophication.Besides that, Benoa Bay is also characterized by a weak current (Hendrawan and Asai, 2014) thereby increasing the potential for eutrophication.In addition, Benoa Bay is utilized by fishermen as fishing zone (for fish and shellfish) and cultivation zone.If HABs occur in Benoa Bay and people consume fish caught from this area, it will endanger human health.The similarities characteristic between Benoa Bay with Lampung and Jakarta bay have potential for eutrophication and HABs occurring.The purpose of this study were determine spatial distribution of nutrient based on Total N and P, ratio of total N, and trophic level in Benoa Bay.

Study site
This study was conducted for 4 months with sampling in July 2017 (dry season).The study sites were conducted in Benoa Bay with 30 sampling points (Figure 1).6 points in this study were in the river and 26 points in Benoa Bay.The location of this study is adjacent to the I Gusti Ngurah Rai Bali international airport and Ngurah Rai forest park (Taman Hutan Raya Ngurah Rai).

Data analysis
Total N in this study was calculated from the summation between nitrate, nitrite and ammonia (Total Inorganic Nitrogen/TIN) without calculating Total Organic Nitrogen (TON).The data that have been obtained was processed by using Microsoft Excel and mapped spatially.The ratio of total N was obtained by using the percentage of nitrate, nitrite and ammonia content in each station.The trophic level in Benoa Bay refers to Richardson et al. (2007) (Table 1).

Total N
The Total N measurement results in Benoa Bay show that the concentration varies on each station (Figure 2).The lowest of total N (0.089 mg.L -1 ) was found in station 10 located in the middle of Benoa Bay, while the highest concentration (16.25 mg.L -1 ) was found at station 25 located in Loloan River.The highest concentration of Total N in Loloan River is thought to be due to intense waste disposal.This can be seen from the visual observations made in the Loloan River during sampling process.The similar result in January also obtained by Rahayu et al. (2018) in Loloan River, but different from that obtained by Suteja and Purwiyanto (2018) who get the highest nitrate concentration in Buaji River.2013) that 1 unit of FNC can produce 243.9 kg of nitrogen in six months of fish cultivation.It was generally seen that the total N concentration was high around the land or river estuary and decreases towards the bay's mouth.This pattern was caused by runoff and river flow carrying nitrate, nitrite and ammonia from the land.Inorganic nitrogen in the waters mainly comes from agricultural systems that use fertilizer and carried by the river flow to water ecosystem (Savci, 2012a;Savci, 2012b).The total N spatial distribution has a similar pattern with nitrate that obtained by Rahayu et al. (2018) in Benoa Bay.

Ratio of Total N
The percentage ratio between Nitrate, Nitrite and Ammonia in Benoa Bay were differs between stations (Figure 3).Overall it was found that nitrate concentrations dominated on each station in Benoa Bay.According to Ruckart et al. (2008) and Purwiyanto et al. ( 2012) that nitrate was the most stable of nitrogen over time in aerobic water condition.Ammonia concentrations were found more dominant at stations 1, 26 and 30 than nitrate and nitrite.Stations 1 and 30 were stations that closed to fish cultivation with FNC.Feeding by using pellets and trash fish that were not consumed by the cultivation fish were strongly suspected of contributing highly ammonia concentration.Gorlach-Lira et al. (2013) explained that FNC activities were increase the ammonia content in the water.Nitrogen waste from FNC can be derived from cultivated feed residues and fish feces (Mansur et al., 2013).The high concentration of ammonia at stations 1 and 30 was due to the low dissolved oxygen (DO) at the station (Figure 4).This condition makes denitrification reactions to be dominant.The results of this study were in line with those obtained by Purwiyanto et al. ( 2012  The ratio of total N in rivers showed a different pattern than in Benoa Bay (Figure 5).Ammonia concentrations in rivers were more dominant than nitrates.The high concentration of ammonia in the river was suspected by the low DO (3.25 -5.15 mg.L -1 ).In addition, the high ammonia was due to the disposal of domestic waste along the watershed to the river.The Ratio of total N ratio showed only in the Badung River which has more dominant nitrate than ammonia.This is due to the high water discharge that was able to neutralize the incoming waste.Described by Suteja and Purwiyanto (2018) that Badung River was the greatest water discharge compared to other rivers, but this river gives the second largest water discharge to the Benoa Bay due to dam made by Regional Water Company (Perusahaan Daerah Air Minum/PDAM).The water from Badung River is only channeled to the Benoa Bay when the volume of water in the dam has exceeded the capacity of the dam.The total P concentration in Benoa Bay varies between stations (Figure 6).The average concentration of Total P in Benoa Bay was 0.5715 + 0.3768 mg.L -1 ranging from 0.0360 mg.L -1 (station 28) to 1,2153 mg.L -1 (station 13).Spatial distribution of Total P in the Benoa Bay showed that its concentration increases in the coastal area of Benoa Cape and decreases progressively towards the middle of the bay.This is assumed by the waste disposal derived from community activities (hotels, settlements, etc.) located around Benoa Cape.Spatial distribution of Total P in the west side of Benoa Bay (estuaries of the Dead River, Sama River and Badung River) showed an increase concentration towards the middle of the bay.This is assumed by the presence of phosphate dilution by the high freshwater input from Mati and Badung Rivers.Badung and Mati Rivers were the largest provide freshwater input to the Benoa Bay (Suteja and Purwiyanto, 2016).The total P distribution has similar pattern with previous result conducted by Rahayu et al. (2018) in February.

Trophic level
The Trophic level based on total N and P in Benoa Bay showed that the waters were divided into 4 categories: oligotrophic, mesotrophic, eutrophic and hypertrophic (Figure 7).Benoa Bay generally was in oligotrophic (poor nutrient content) and mesotrophic (moderate nutrient content) based on total N. Eutrophic and hypertrophic was found around Loloan River estuary which was allegedly caused by the high activity of effluent from Loloan River and FNC.hypertrophic conditions were found almost throughout the Benoa Bay based on total P. Mesotrophic and eutrophic conditions were found in front of the Mati and Badung Rivers, this allegedly caused by the Total P dilution by fresh water entering Benoa Bay through the rivers.Further research is needed to measure trophic levels based on plankton abundance and chlorophyll concentrations in the Benoa Bay.

Conclusion
The Spatial distribution of total N and P show a generally pattern concentration increases in waters near the mainland and estuary.The highest concentrations of total N and P were found in Loloan River estuary closed to the FNC.Ratio of total N showed generally that the waters in Benoa Bay were dominated by Nitrate.High ammonia concentration was found in all rivers and Loloan River estuary.Benoa Bay is mostly in Oligotrophic conditions based on total N, while eutrophic conditions were only found in the Loloan River estuary.Based on total P, Benoa Bay was generally in hypertrophic level.

Figure 1 .
Figure 1.Map of sampling location in Benoa Bay.

Tabel 1 .
Category of trophic level based onTotal N

Figure 2 .
Figure 2. The spatial distribution of total N in Benoa Bay.
) who get high ammonium concentrations when DO is low in Muara Angke.The high ammonia at station 26 was suspected by the waste disposal carried out by the various activities at the Benoa Harbor.The increasing of ammonia concentration towards the harbor was obtained by Puspitasari and Natsir (2016) at Lembar Harbor in West Nusa Tenggara.Described by Huhta et al. (2007) that waste from toilets and medical facilities on ships can produce 12-15 g nitrogen per person per day.Wiweska-Bien et al. (2016) added that 182 tons of nitrogen annually only from vessel food waste.

Figure 4 .
Figure 4. Spatial distribution of DO in Benoa Bay.

Figure 6 .
Figure 6.The spatial distribution of total P in Benoa Bay.