DETERMINATION OF THE TROPICAL STATUS OF FLOATING NET CAGE WATER BASED ON THE DISTRIBUTION OF NITROGEN, PHOSPHORUS AND CHLOROPHYLL-A

The dominant source of pollutants for floating net cages are fish feed and feces. They cause phosphorus and nitrogen in water increase, trigger eutrophication, marked by the appearance of algae. Algae are green plants, contain chlorophyll-a. The content of phosphorus, nitrogen and chlorophyll-a can be used to determine the tropical status of water. The objective of this research is to determine the tropical status of marine cage water. The research method was descriptive laboratory. Nitrogen content is measured as nitrite according to SNI 6989.9-2004, nitrate according to APHA Section 4500-NO3, ammonia according to SNI 19-1655-1989. Phosphorus analysis according to SNI 06-6989-31: 2005. Chlorophyll-a analysis used the Strickland & Parson method by spectrophotometry. Laboratory data were analyzed for tropical status based on nitrogen, phosphorus and chlorophyll-a content. The results showed that the tropical status of the KJA water of Gajah Mungkur Reservoir Wonogiri in the rainy season had eutrophic status, containing high levels of phosphorus and nitrogen elements. The eutrophic status indicated that the water had been polluted by an enhancement of nitrogen levels by 18.345 μg/L and phosphorus by 420.65 μg/L. These nitrogen and phosphorus pollutants increased the growth of chlorophyll-a by 12.70 μg/L.


INTRODUCTION
Gajah Mungkur Wonogiri Reservoir (WGM) is used for the cultivation of red tilapia in floating net cages (KJA), developed by fish farmers and PT Aquafarm Nusantara. Feed form of pellets, 70% of the feed is eaten by fish, the remaining 30% is released into the water as pollutants (Marganof et al., 2007), a source of nitrogen and phosphorus pollutants from indigenous activities (Pujiastuti et al., 2010). KJA activities as a contributor to the dominant Nitrogen and Phosphorus pollutants enter the WGM waters (Pujiastuti et al., 2016). Nitrogen in water dissolves as ammonia, and undergoes biotransformation into nitrite and nitrate (Saha et al., 2013). In KJA WGM, ammonia undergoes biotransformation naturally by Nitrosomonas sp. Into nitrite (N-NO2), which is then transformed into nitrate by Nitrobacter sp. (Pujiastuti et al., 2018). Nitrite compounds are toxic to fish, while nitrate compounds can stimulate the algae growth.
Chlorophyll-a is a green plant pigment, required for photosynthesis.
Chlorophyll-a parameter indicates the level of algal biomass. estimated that the average weight is 1% of the biomass (Permen LH, 2009). Algae is green plant, so its presence can be measured by the chlorophyll-a parameter. Chlorophyll-a analysis can be carried out using Strickland & Parson method (Parson et al., 1984). Chlorophyll-a content has a positive correlation to nitrite (N-NO2) and nitrate (N-NO3) in water (Polan and Terbiyik, 2013). The high content of chlorophyll-a indicates the fertility of the waters, so that it can cause a reduction in dissolved oxygen in water (Wiryanto et al., 2012). Reservoir water sediments at a depth of 25 m contain chlorophyll-a concentrations of 0.2-1.14 mg/m3 (Nuchsin, 2007). As a limiting factor, phosphorus will limit the eutrophication process when the nitrogen content in water is more than 8 times the phosphorus level. Likewise, nitrogen is a limiting factor for eutrophication if the water content is less than eight times the phosphorus content (UNEP-IETC/ILEC, 2001). Eutrophication is classified below four categories of tropic status.
Tropical status is defined as the status of reservoir water based on nutrient content and phytoplankton biomass content or productivity (Permen,L.H, 2009  Chlorophyll-a analysis procedure (Parson et al., 1984): 1) Take     3) The optimum pH for the nitrification process was between 7.5-8.5, although nitrifying bacteria was sensitive to pH, these bacteria can adjust to pH values beyond the optimum range. The optimum pH of nitrification was 8.4, at pH 7 the efficiency can still be achieved at 80% and 90% of the maximum rate of the nitrification process at 7.8 -8.9, while at pH outside 7.0 -9.8 the process rate which occured in less than 50%. Nitrite is unstable at acidic pH (Shen et al, 2003). The rate of the nitrification process will decrease at pH 6.3 -6.7 and at pH 5 -5.5 the nitrification process will stop (Sykes et al., 2006). WGM KJA water had a pH of 7.8 -7.9. It is the optimal pH of the  In Figure 2, it can be seen that PT Corophyll-a to the trophic status of WGM waters of KJA zone is presented in  Chlorophyll-a content became smaller at the sampling point WGM outlet water compared to KJA water, which ranged from 5.60 -7.38 µg/L, with an average content of 6.32 µg/L. The tropical status in WGM outlet waters was eutrophic, indicating that the water had been polluted by increased nitrogen and phosphorus levels. It is due to mixing of Keduang river water that entered the reservoir.

CONCLUSION
The WGM KJA water had eutrophic status during the rainy season. It shows that WGM KJA water had been polluted by the enhancement of nitrogen as 18,345 µg/L and phosphorus as 420,65 µg/L.