Levels of Available Nitrogen-Phosphorus Before and After Fish Mass Mortality in Maninjau Lake of Indonesia

MATERIALS AND METHODS

Sampling location: The present research was conducted in February, 2017, four months after mass mortality of O. niloticus and C. carpio occurred in Maninjau lake, West Sumatera, Indonesia. Fish mass mortality occurred in August and September, 2016. The N, P, TOM, Particulate Organic Matter (POM) and Dissolved Organic Matter (DOM) levels recorded in February, 2016, were compared with data take in February, 2017. The data were collected from four stations: Muko-Muko, Pasa, Pandan and Sungai Tampang. Water samples (250 mL) were taken at the surface (depth 0.10 m) and under floating net cages (depth 30 m) at each station using a Kemmerer Water Sampler (Wildco, USA) with diameter (D) 118 mm and sample volume 1200 mL. The sampler consists of a metal tube with stopper on each end that can be held open when the sampler is lowered by a line to a desired depth. After the stoppers close the ends of the tube, the sampler is retrieved with the desired samples of water being uncontaminated by other water. Then samples were placed in bottles and preserved with H₂SO₄.

Nitrogen and phosphorous analysis: The N and P levels were analyzed by (ultraviolet-visible) light spectrophotometry (UV 160 A. Japan) according to APHA¹⁹. The P was determined using the molybdovanadate method indicated by AOAC²⁰ at an absorbance of 400 nm. All samples were analyzed in triplicate. Redfield criteria were assessed by calculating the N/P ratio. An N/P<16 means that N is the limiting factor, while an N/P>16 means that P was the limiting factor, an N/P = 14-16 means that N and P were collectively at the limiting factor²¹.

Water quality analysis: Dissolved oxygen (O₂) levels were determined using a Yellow Spring oxygen meter model 52-Yellow Spring Instrument Co., Yellow Springs, OH, USA). The TOM, DOM and POM were analyzed from surface water samples using the titrimetric method²². The POM was calculated as the difference between TOM and DOM.

Statistical analysis: The N, P, TOM, POM and DOM levels before (February, 2016) and after (February, 2017) fish mass mortality were analyzed using a student t-test. The N, P, TOM, POM and DOM levels between stations were analyzed using one-way analysis of variance (ANOVA) followed by Tukey’s HDS post-hoc test using SPSS software (Version 16.0 for Windows, SPSS Inc, Chicago IL)²³. All samples were performed in triplicates and data presented as Means±Standard Deviation (SD). The treatment effects were considered to be significant at p<0.05.

RESULTS

Availability of N and P in surface water: The N and P levels in 0.10 m depth water before and after fish mortality occured are shown in Table 1.

Availability of N and P in water under floating net cages: The levels of N and P in 30 m depth water before and after fish mass mortality occurred are presented in Table 2.

Table 1:	Levels of nitrogen (N) and phosphorus (P) in Maninjau lake surface water
Surface water samples were taken at a depth of 0.10 m, data are presented as the Mean±SD of triplicate samples, *The difference between mean with different lower case letters in a columns and the difference between means with different upper case letters for each parameter are statically significant (p<0.05)

Table 2:	Levels of nitrogen (N) and phosphorus (P) in water under floating net cages in Maninjau lake
Surface water samples were taken at a depth of 30 m, data are presented as the Mean±SD of triplicate samples, *The difference between mean with different lower case letters in a columns and the difference between means with different upper case letters for each parameter are statically significant (p<0.05)

Table 3:	Levels of TOM, DOM and POM in Maninjau lake
Data are presented as the Mean±SD of triplicate samples, *The difference between mean with different lower case letters in a columns and the difference between means with different upper case letters for each parameter are statistically significant (p<0.05), *TOM: Total organic matter, DOM: Dissolved organic matter, POM: Particulate organic matter

Fig. 1:	Red field of nitrogen/phosphorus ratio (m mol–1) in Maninjau lake before and after fish mass mortality

Red field criteria of the N/P ratio: The N/P values reported in February, 2016 ranged from 2.13±0.68-5.31±0.83, whereas those calculated here in (February, 2017) ranged from 2.01±0.42-4.18±0.58 (Fig. 1). Interestingly all sampling stations had an N/P<16.

TOM, DOM and POM levels before and after fish mass mortality: The levels of TOM, DOM and POM before and after fish mass mortality in Maninjau lake are presented in Table 3.

DISCUSSION

The current research showed the levels of N and P at 0.10 and 30 m depths significantly increased (p<0.05) after fish mass mortality (February, 2017). This increase was the result of as much as 600 t of O. niloticus and C. carpio drowning and sinking to the bottom of Maninjau lake. Previously, Asir and Pulatsu²⁴ reported that each ton of dead Rainbow trout (Oncorhynchus mykiss) released as much as 56.00 kg of N and 10.66 kg of P into water bodies. Other researchers have found similar result regarding O. mykiss production reporting 43.9 and 8.8 kg t^–1 of N and P, respectively¹⁶.

The N and P are end-products of fish load that can affect water quality in fish farming areas⁹. While the concentration of N and P in water can be estimated from the total amount of feed given to the fish²⁴, it is possible that increases in N and P may also result from fish mass mortality.

For example, the main end-product of protein metabolism in teleost fish is ammonia (NH₃) and most nitrogenous wastes from same fish species are excreted as urea²⁵. Riche and Brown²⁶ also reported that the end-products of protein catabolism in fish were dissolved NH₃ and urea. Meanwhile, the amount of N and P in fish feed and changing feed conversion ratios have been influenced the amount and types of nutrients excreted into the aquatic environment²⁷. Furthermore, Mallekh et al.²⁸ found that the fish size, species and feed type affect the amount of digestive residue.

The levels of N and P at 0.10 and 30 m depths after fish mass mortality in Maninjau lake was significantly different between water sampling stations (p<0.05). The level of N ranged from 1.83-2.30 mg L^–1 at 0.10 m and 2.11-2.60 mg L^–1 at 30 m, while, P levels ranged from 0.50-0.91 mg L^–1 at 0.10 m and 0.81-0.92 mg L^–1 at 30 m. Fish mass mortality have an effect to N and P levels in Maninjau lake. The dead fish was decomposed by bacteria which can increase the level of N and P. The level of N and P at the surface and 30 m depth of Maninjau lake tend to higher than before fish mass mortality. In Toba lake, Indonesia, the N and P levels were 0,013-0,457 mg L^–1 and 0,005-0.116 mg L^–1, lower compared than Maninjau lake. This condition was due to fish mass mortality occurred in this place¹. The N and P levels at Maninjau lake based on the regulations of the Ministry of Environment of the Republic of Indonesia Number 28/2009²⁹. The water status of Maninjau lake refer to hypereutrophic. Morever, the levels of N and P at the bottom of Maninjau lake (under floating net cages, 30 m depth) was higher than at the surface (0.10 m depth). The P containing nutrients settle and accumulated in deeper parts of the lake most likely when orthophosphate in the epilimnion zone undergoes co-precipitation and absorption with particulates or macro metals such as iron, manganese, aluminum and/or other organic compounds¹.

The amount of fish mass mortality observed in Maninjau lake was correlated with the number floating net cages at each station. The number of floating net cages was highest at Pasa (133 units/10,000 m²) and the lowest at Muko-Muko (69 units/10,000 m²). The N levels at 0.10 and 30 m depths at Pasa in February, 2017 were 2.15±0.025 and 2.30±0.020 mg L^–1, respectively. While, the level of P was 0.72±0.01 and 0.97±0.01 mg L^–1, respectively. Furthermore, the productivity of Maninjau lake is relatively high as indicated by the brightness of water, which ranges from 1.30-1.80 m. Compared to that of Toba lake which is lower water brightness 7.0-15.0 m¹.

In general, orthophosphate is a P containing nutrient whose availability for growth of phytoplankton. The orthophosphate level at the surface of Maninjau lake was relatively high, ranging from 0.055-0.125 mg L^–1. The nutrient element of P is highly dependent on the dissolved O₂ concentration. The levels of dissolved O₂ at 0.10 m at each station after fish mass mortality were 5.89, 5.93, 6.17 and 5.85 mg L^–1, respectively. However, the dissolved O₂ levels at 30 m were 4.70, 4.92, 5.01 and 4.82 mg L^–1, respectively. Largely an aerobic conditions are may be fount at depths greater than 50 m and the average depth of Maninjau lake is 105 m¹⁰. More orthophosphate is found in particle form of dissolved O₂ decreases below 50 m. According to Nomosatryo and Lukman¹ the accumulation of P at the bottom of Toba lake is related to higher levels of orthophosphate at the water’s surface. Morever, Kelly³⁰ stated that orthophosphate compounds liberated from sediment diffuse at the water’s surface. The levels of N tended to be a limiting factor for algae growth in Maninjau lake because all stations had an N/P<16 (Fig. 1). This result was similar to that of Lake Sentani-Indonesia N/P 10.24³¹ but different from that of other Indonesia lakes. Both N and P levels were found to limit algae growth in Toba lake (N/P1.9-46.3)¹, while P was the limiting factor in Panglima Besar Soedirman Reservoir (N/P = 19.13-65.82)⁸.

Analysis of different types of organic matter in Maninjau lake revealed that although DOM, POM and TOM levels increased. The DOM levels were dominant (Table 3). The level of TOM was higher after fish mortality, the TOM was highest at Pandan and Sungai Tampang stations. Similarly, Lukman and Hidayat³² reported the level of DOM was higher than the POM in Toba lake. Analysis of the accumulation of DOM in aquaculture activity is very important³³, because the process of sedimentation and mineralization may contribute to the decline of organic values, especially in Maninjau lake and both processes are easily to occur in lentic waters.

Interestingly the level of TOM at Pasa, Pandan and Sungai Tampang stations after fish mortality were relatively similar mean TOM 15.63 mg L^–1 or (0.01563 kg m^–3), whereas, that at Muko-Muko was 6.97 mg L^–1 (00.00697 kg m^–3). These difference in the level TOM also confirm that the increase in TOM found in February, 2017 were largely the result of fish mass mortality. This condition may be correlated with water retention time of lake. According to Syandri et al.¹⁰, Maninjau lake has an area 99.5 km² (75.38%) from the catchment area, has water volume 10,226,001,629.2 m³, average depth was 105 m, the average of outflow water was 12.86 m³/sec and the role of groundwater is quite large as a source of water at Maninjau lake. So Maninjau lake has water retention time during 25 years.

CONCLUSION

The current research found clear evidence that mass mortality of O. niloticus and C. carpio related to floating net cages in Maninjau Lake is a significant source of N, P and TOM. The level of these parameters was significantly higher in deeper water (30 m depths) compared to that at the surface (0.10 m depths) after fish mortality. It is concluded that a major side effect of fish mass mortality is that it decreased the water quality. Fish mass mortality can be reduced by adjusting the timing and stocking density of aquaculture fish. This will help to reduce the downstream negative effects on the lake and surrounding and in turn positively effect to many local people, as improving labor conditions lead to increased economic welfare.

SIGNIFICANCE STATEMENTS

This study discovered the increasing levels of N, P and TOM after fish mass mortality in Maninjau lake. This condition has negative effect on water quality which cause hypereutrophic condition. While N level tended to as a limiting factor for algae growth at Maninjau lake. This study is an emerging issue now days and require urgent action from the concerned authorities and awareness from fish farmers by adjusting rearing time and stocking density of aquaculture fish for the future.

ACKNOWLEDGMENT

This study was supported by a study grant (Riset Unggulan Perguruan Tinggi) from the Directorate of Research and Community Service of the Ministry of Research Technology and Higher Education, Republic of Indonesia, (Grant No. SP.DIPA-042.06-0/2017).