4.2. Water Quality Characteristics
Water quality is a measure of the condition of water in terms of its physical, chemical, and biological characteristics. Water quality for aquaculture also shows a measure of water conditions relative to the needs of biota. Water quality is often a standard measure of the health condition of aquatic ecosystems [
61,
69,
70]. In general, the temperature of sea waters in Indonesia has a fairly wide range. Characteristics of different temperatures can be caused by topography or water depth associated with differences in sunlight penetration. The water temperature for marine biota and intensive technology whiteleg shrimp culture ranges from 28 to 30 °C [
44,
45].
Water temperature is influenced by season, latitude, altitude above sea level, time of day, air circulation, cloud cover, and the flow and depth of water bodies [
71]. A sudden change in the current pattern can also lower the water temperature [
72]. The water temperature in the coastal area of BB is 28.59 ± 0.18 °C in the rainy season and 28.14 ± 0.22 °C in the dry season. Water temperature in coastal areas is higher in the rainy season (June–September) than in the dry season (December–March) in Bangkok and Samut Prakam, Thailand [
28]. Other data, in the west season or rainy season in the waters of Manado Bay, North Sulawesi Province, Indonesia, is characterized by a powerful west wind accompanied by waves and currents causing the penetration of sunlight to the sea surface are not practical so that the water temperature is low [
70]. The average sea surface temperature in Indonesian waters, including in the Flores Sea, is generally higher in the west season (rainy season) than in the east season (dry season) [
73]. The research locations in BB and GT are in the Flores Sea, with lower water temperatures found in September measurements, namely in the dry season. The statistical analysis results between the water temperature in BB and GT showed significantly different results (
p < 0.05), with the water temperature in BB being lower than in GT in the rainy and dry seasons. The lower water temperature in BB is made possible by the higher water depth in the coastal area of BB compared to GT. The depth of the waters in the coastal area of BB reaches about 20 m, while in the GT, it only reaches about 10 m. Seawater temperature is influenced by sea depth, wind speed, sunlight conditions, and the strength of the vertical temperature [
74,
75,
76]. High water temperature was found in the water samples taken from the sampling point nearby the coastline (transect A) due to the shallow waters area. This is different from the water temperature on the transect perpendicular to the coastline, where there is no particular water temperature pattern in the BB and GT coastal areas.
Water salinity describes the total solids in the water after all the carbonates have been converted to oxides. Chlorides have replaced all the bromides and iodides, and all organic matter has been oxidized. One factor influencing salinity distribution in the waters is the amount of freshwater that enters marine waters [
69,
77]. In shallower waters, such as in the GT coastal area, freshwater intrusion can spread to the bottom of the water so that the water salinity becomes lower and is very significantly different (
p < 0.01) from salinity in the BB coastal area both during the rainy season (
Table 5) and during the dry season (
Table 6). Another cause is higher salinity in BB’s coastal areas than GT due to lower rainfall (1428 vs. 1936 mm/year) and deeper waters (20 vs. 10 m). Based on the results of the climate analysis, it was obtained that the value of Q = 0.80 or classified as climate type D or moderate climate in BB and the value of Q = 0.29 or classified as climate type B or wet climate in GT. From another analysis, based on the number of dry months, it was found that there were four dry months in BB and two dry months in GT. The three analyses carried out on rainfall data show that BB is drier than GT, so it impacts high salinity in the coastal area of BB, as previously described. A significantly negative correlation was found between mean monthly salinity and monthly rainfall [
78]. It has been reported that there is an increase in salinity in the waters around the BB and GT coasts, namely the southern region of Sulawesi, including the Flores Sea, during the dry season as a result of water masses from the Banda Sea with high water salinity entering these waters [
79]. Seasonal changes can result in temperature and water salinity [
70,
79,
80]. Intensive technology whiteleg shrimp culture activities do not influence the increase in water salinity in the dry season in the two subdistricts. However, it is more dominant due to the influence of the season, namely measurements made in the dry season. This is reinforced by the results of salinity measurements in the UL, which also increases during the dry season, even though there is no intensive technology whiteleg shrimp pond in this location.
In the rainy season, it is known as a mighty west wind accompanied by waves and currents that cause the evaporation rate to be hampered so that it is thought that it can withstand increased salinity [
70]. In addition, the low salinity of the water during the rainy season it is suspected that there is freshwater runoff from the land, which causes lower water salinity, because the measurement time in the rainy season is the peak of the rainy season in BB and GT (
Figure 5 and
Figure 6). The 32 rivers are flowing in the Bulukumba Regency area. The four large rivers have abundant water resources throughout the year, including Bialo, Binjawang, Balantiyeng, and Anyorang Rivers.
The low water salinity was found on transects close to the coastline. The salinity differences may have been caused by freshwater intrusion, which affects the salinity of water samples taken from sampling points nearby the land. Among the factors affecting salinity distribution in general waters is the freshwater flow that goes into the seawater [
81]. In shallower waters, the freshwater intrusion can reach the bottom of the waters, causing low salinity. On the contrary, salinity in open seawaters is not affected by freshwater intrusion from the land, which causes high salinity [
82].
The water pH in the coastal areas of the two subdistricts can be used as a water source for shrimp farming because the pH of good source water for intensive technology whiteleg shrimp culture is 7.5–8.5 [
45]. Suitable water pH for marine biota ranges between 7.0 and 8.5 [
44]. A small amount of organic material can cause the low pH of the water from the land carried through the river. It results from organic matter originating from whiteleg shrimp ponds and other acidic activities on land, which can decrease the water’s pH because the decomposition process of organic matter can produce acidic compounds. In addition, with the increase in the amount of waste, the pH of the water will decrease. This decrease is due to the increase in CO
2 concentration due to the respiration process carried out by various microorganisms [
22,
83,
84]. Generally, water pH increases in the rainy season and decreases in the dry season, which means that seasonal differences are an essential factor influencing water pH. As with water salinity, there is a tendency for higher water pH on transects farther from the coastline (
Table 7). pH values of waters increase as the waters become closer to the open seawaters. If not managed, it will harm aquatic organisms in coastal areas.
Dissolved oxygen is one of the most critical environmental variables directly affecting production and growth through metabolism and environmental conditions. Dissolved oxygen concentration in water can affect the growth, FCR, and carrying capacity of waters. Dissolved oxygen concentration in the rainy season was higher and significantly different (
p < 0.01) with dissolved oxygen in the dry season in the waters of BB and GT. The low concentration of dissolved oxygen in the dry season in both BB and GT coastal areas is thought to result from waste from intensive technology whiteleg shrimp ponds decomposed by microbes that use a lot of dissolved oxygen. Various processes decrease dissolved oxygen concentration when nutrients are high, including nitrate [
17,
70,
85,
86]. It was also found that the concentration of dissolved oxygen was lower in the nearshore transect (transect A) than other transects due to higher effluent near the coast that could require dissolved oxygen for its decomposition.
The low concentration of dissolved oxygen in the dry season measured in August and September can also be caused by low sea waves. Waves or ocean waves increase dissolved oxygen [
87,
88], so low waves in August and September also impact low dissolved oxygen concentrations. September, October, and November are the lowest sea waves in the Flores Sea [
89]. The dissolved oxygen concentration was lower in the dry season than in the rainy season in the coastal area of Lampung Bay, Lampung Province [
36]. Dissolved oxygen good for marine biota is more significant than 5.0 mg/L [
44]. Dissolved oxygen in both source and rearing water for intensive whiteleg shrimp culture is greater than 4 mg/L [
45].
Nutrients in the context of water quality are molecules in water that can be directly used by biota for cell growth [
90,
91]. Nutrients widely used by aquatic organisms, including seaweed, are ammonia, nitrate, nitrite, and phosphate [
92,
93]. In general, nitrate is the essential nutrient that determines the growth of plankton and higher plants in marine waters.
Besides supplementary feed and direct excretion of cultured aquatic organisms, other ammonia sources are thought to come from fertilizers used on land for agricultural activities carried to coastal areas. Sources of ammonia in waters come from fertilizers and feed [
94,
95]. Surface runoff from settlements generally contains higher nitrate and ammonia concentrations than forests, while fertilizer on agricultural land produces high nitrate and moderate ammonia [
96]. It is suspected that the increase in ammonia concentration in the coastal area of Bulukumba Regency comes from intensive technology whiteleg shrimp aquaculture ponds in the form of leftover feed and feces of whiteleg shrimp as well as other activities that use fertilizers.
The same study results were higher phosphate concentrations in the rainy season than in other seasons [
36,
70]. The low phosphate concentration in the coastal area of GT is thought to be utilized by seaweed cultured in the area. The area of seaweed
Kappaphycus alvarezii culture in the coastal area of Bulukumba Regency will reach 3766 ha in 2020 [
97]. It is known that besides N, other nutrients needed by seaweed are P. Phosphate is the P form that is absorbed by seaweed [
98,
99,
100]. Elemental P is an essential material for higher plants and aquatic algae, so this element is a limiting factor for aquatic plants and algae and significantly affects aquatic productivity [
101,
102]. Seaweed requires P for growth because it is the main constituent of Ribonucleic acid (RNA) and consequently is involved in protein synthesis [
103,
104]. In addition, the term “overconsumption” in aquatic algae is also known, namely when the water contains sufficient P, the aquatic algae accumulate P in the cells exceeding their needs [
103]. The excess P absorbed will be utilized when the waters lack P so that seaweed can still grow for some time during periods of P supply shortage. The average phosphate concentration is 0.06622 ± 0.01945 mg/L in coastal areas of BB and 0.09993 ± 0.10077 mg/L in the coastal area of the GT during the rainy season. An average of 0.03586 ± 0.04285 mg/L in the coastal area of BB and 0.09167 ± 0.06966 mg/L in the coastal area of GT during the dry season, when compared with the ideal concentration for seaweed growth habitat, which is 1.0–3.2 mg/L [
105], this situation is still far from optimal. Although there is no seaweed culture in BB, the decrease in phosphate concentration is thought to be due to the relatively small source of phosphate other than the aquaculture pond in BB in the dry season. In GT, there are agricultural activities such as rice fields that use many fertilizers, including SP-36 fertilizer, which contains phosphate, which can contribute phosphate compounds in the coastal area of GT. In the mainland part of BB, the land use is in the form of less productive forests, so the source of phosphate is thought to only come from intensive whiteleg shrimp pond waste. This assumption is reinforced by data on very low phosphate concentrations in UL (control) in the dry season (
Table 6) due to the absence of a phosphate source originating from intensive technology whiteleg shrimp ponds. The phosphate concentration in the source and rearing water of good whiteleg shrimp ranged between 0.01 and 5.0 mg/L [
45].
Excess or lack of phosphate concentration in aquaculture has an unfavorable impact on the cultured organisms. Excess or lack of a chemical substance in water will cause physiological disturbances in aquatic organisms [
106]. Although it is estimated that there is P waste of ±13.22 tons/year or 6.61 tons/cycle from intensive technology whiteleg shrimp culture wasted in the coastal area of Bulukumba Regency, the concentration is still relatively low and supports seaweed culture.
The chemical characteristics of the water govern the solubility of inorganic phosphorus in aquatic systems. There is a very close and positive relationship between phosphate and water pH, which means that the higher the pH, the higher the phosphate concentration. However, it has been reported that the solubility of inorganic phosphorus will decrease at a water pH greater than 9.0 [
107]. In this case, the pH of water in the coastal area of Bulukumba Regency is still in the range of water pH, which allows an increase in phosphate with an increase in water pH.
The total suspended solids concentration is less than 80 mg/L and the total organic matter concentration is less than 90 mg/L, supporting technology intensive whiteleg shrimp culture [
45]. The deeper water depths in the coastal area of BB impact and reduce the agitation of the seabed so that the concentration of total suspended solids will be lower [
108]. On the other hand, the relatively shallow water depth in the coastal area of GT results in more significant seabed agitation resulting in a higher total suspended solids concentration. Another factor that is thought to affect the total suspended solids in the coastal area of Bulukumba Regency is the sea waves, so that when the sea waves are low, namely measurements in the dry season, the concentration of total suspended solids is low. High sea speed and waves cause high total suspended solids [
108,
109]. Spatially and temporally, the presence and condition of total suspended solids in coastal areas can be influenced by several factors such as ocean currents, tides, river discharge, and land cover [
110,
111]. It is suspected that these various factors can cause a decrease in the concentration of total suspended solids and total organic matter in the coastal area of Bulukumba Regency during the dry season.
4.3. Water Quality Status
One of the causes of the decline in the environmental quality of coastal waters is the discharge of aquaculture waste during operations which contains high concentrations of organic matter and nutrients. This condition is a consequence of the input of pond production facilities in aquaculture, resulting in residual feed and feces dissolved into the surrounding waters [
19,
112,
113]. In commercial aquaculture, 30% of the total feed given is not consumed by fish/shrimp, and about 25–30% of the feed consumed will be excreted [
114]. The amount of N and P present in the feed will be retained in fish/shrimp meat between 25–30%, and the rest is wasted in the aquatic environment [
114]. Estimated nutrient load sourced from feed that is wasted into the coastal waters of Bulukumba Regency in the form of N reaches ± 56.01 tons/year and P reaches ± 13.22 tons/year in whiteleg shrimp culture with stocking densities of 100 up to 220 ind/m
2 in ponds with a total area of ±69.60 ha (
Table 2). Ten water quality variables are used to determine water quality status in coastal areas. Only four water quality variables cause water quality in the coastal area of Bulukumba Regency to be heavily polluted, namely high salinity, low dissolved oxygen, and high nitrate and ammonia concentrations in the dry season. At high salinity (hyperosmotic), as found in the coastal area of Bulukumba Regency during the dry season, shrimp growth will be disrupted due to the disturbed osmoregulation process. If the energy used for osmoregulation activity increases, the energy for growth decreases, thereby reducing the growth rate of aquatic organisms [
115,
116]. Meanwhile, dissolved oxygen in water can be reduced due to respiration and decomposition of organic matter on the bottom of the water [
117,
118], causing a decrease in dissolved oxygen in water, where a decrease in dissolved oxygen concentration to a critical point will cause hypoxia. Hypoxia (dissolved oxygen concentration <2.0 mg/L or about 30% saturation) is a phenomenon that occurs in the aquatic environment due to a decrease in dissolved oxygen concentration to a limit that can contribute greatly to a decrease in the growth and reproduction ability of aquatic organisms and even death of aquatic organisms in the water [
119,
120,
121].
A nitrate is a form of N that is less toxic than nitrite and ammonia, but it can be toxic to shrimp at high concentrations. Shrimp exposed to high nitrate concentrations for a long time showed shorter antennae length, gill abnormalities, and hepatopancreas blisters [
122,
123]. Short antennae and gill abnormalities are often considered early clinical signs of declining shrimp health [
122]. Several cases state that excessive ammonia concentrations can cause severe water problems [
55]. Such conditions can be caused by the contribution of N originating from the mainland. A significant N contribution comes from fisheries, including from intensive and super-intensive technologies whiteleg shrimp aquaculture ponds [
10,
11,
64]. N penetration also occurs in agriculture, agrochemical, forestry, animal husbandry, and other sectors. The effect of ammonia on shrimp is the narrowing of the gill surface; consequently, the rate of gas exchange in the gills is reduced [
124,
125]. In addition, other effects are a decrease in the number of blood cells, a decrease in the concentration of oxygen in the blood, a decrease in physical resistance and resistance to disease, and structural damage to various types of shrimp organs. Several research results state that the accumulation of ammonia in aquaculture water causes various kinds of damage to aquatic organisms, especially damage to the function and structure of body organs [
126,
127]. The concentration of ammonia strongly influences the toxicity of nitrite in the waters; if the concentration of the two variables is high, they are a stressor for shrimp and fish cultured so that shrimp and fish are more susceptible to disease [
128]. Ammonia can be reduced directly by assimilating heterotrophic bacteria, algae, and other aquatic plants [
129,
130]. Probiotics are heterotrophic bacteria commonly given to aquaculture in ponds because they can improve pond water quality by reducing total organic matter, ammonia, nitrate, and nitrite of water [
91,
131] or, in general, efforts are needed for proper water management. In addition, to obtain water for whiteleg shrimp culture with lower concentrations of ammonia and nitrate in BB and GT, it is advisable to take water from a location far from the coastline. It was mentioned earlier that the concentration of ammonia and nitrate becomes lower the farther from the coastline (
Table 7).
Nutrient enrichment can also reduce the brightness level by stimulating the growth of phytoplankton in water bodies. Increased nutrient concentrations can cause negative coral responses such as decreased reproductive success, calcification rate, and skeleton density [
38,
132]. These nutrients can also stimulate macroalgae growth, which can cause coral death due to algal overgrowth [
40,
133,
134]. Knowing the level and source of nitrate and ammonia pollution can be used to control nitrogen inputs and further determine more sustainable water management in coastal areas.
Changes in the status of water quality from complying with quality standards and being polluted in the rainy season to being heavily polluted in the dry season, apart from being suspected of having a load of waste originating from whiteleg shrimp culture, it is also suspected to be due to the influence of the season. Measurements and water sampling carried out in August and September coincided with the peak of the dry season at the site, which resulted in increased salinity and relatively small dilution of nutrients in coastal areas [
135]. The timing of stocking in whiteleg shrimp culture ponds in Bulukumba Regency needs to be carried out appropriately. Information has been obtained that one of the problems with whiteleg shrimp culture in Bulukumba Regency is climatic conditions that do not support intensive technology whiteleg shrimp culture [
136].
The research results are expected to provide basic information regarding the characteristics and status of water quality in the area. The findings of this study indicate that greater awareness of the environmental impact of intensive whiteleg shrimp ponds and climate is needed if the whiteleg shrimp aquaculture industry is to be developed and fulfills sustainable elements. Suppose the coastal area ecosystem must be protected for the future. It is not easy to present the best option for managing coastal area resources without monitoring water quality temporally and spatially [
137,
138]. The results of this assessment should be followed by regular monitoring of water quality. So that it is hoped a healthy aquatic ecosystem will be created that can impact the increase and sustainability of intensive technology whiteleg shrimp aquaculture production and coastal ecosystems where Taka Bonerate National Park is found, which has thus been designated as a UNESCO Biosphere Reserve; the bioecological balance of the elements that make up the ecosystem in coastal areas, including coral reefs, is an essential key. Therefore, maintaining a balanced level of biodiversity and density between all ecosystem components must be one of the targets and priorities in the coastal area management strategy to achieve a sustainable environment.