Nutrient removal effectivity of seaweeds as biofilter and water pollution control in hybrid grouper aquaculture wastewater at low salinity


 This research aims to determine the performance and nutrient removal effectivity of seaweeds species, namely Ulva fasciata., Sargassum illicifolium, Gelidium sp., and Dictyota sp. as a biofilter in hybrid grouper aquaculture wastewater at low salinity (14-17 ppt). This study used a Completely Randomized Design with 3 replications. The statistical analysis was carried out by Analysis of Variance continued with the Least Significance Difference test, and the Kruskal-Walis test with the Mann-Whitney analysis, and T-test with a confidence level of 95%. The concentrations of ammonia, nitrate, phosphate, and water quality parameters were observed every 4 days. The seaweeds uptake of N and P in water, and Specific Growth Rate were also observed. The results showed that U. fasciata was able to reduce ammonia by 75.95% and nitrate by 79.53%, which were the highest compared to other treatments, while Dictyota sp. was able to reduce the highest phosphate by 87.5% for 20 days. The highest SGR was achieved by U. fasciata at 1.91 %day-1. The highest N content uptake by seaweeds was 104.4%, and the total P of 182.3% occurred in U. fasciata. Overall, U. fasciata has the highest performance and effectiveness as a biofilter that is able to reduce nutrient waste in low-salinity from grouper aquaculture, for re-circulation or before being discharged into the environment to reduce eutrophication and harmful Algal Blooms (HABs) in aquatic environment. The highest growth rate of U. fasciata can be used as food with highly nutritional and economical value.


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The need for animal protein derived from fish is increasing, FAO (2020) reported that the 30 world fish consumption was 156 million tonnes in 2018. Therefore, in order to meet these needs, 31 cultivation efforts are carried out including the groupers' species. The Indonesian government 32 increases grouper cultivation to meet the domestic and export demand, as well as reduce natural  However, the increasing grouper culture without proper waste management before being 41 discharged into open water cause eutrophication, due to the accumulation of leftover feed and 42 faeces. The use of "pellet" feed in intensive modern cultivation has created new problems in the 43 waters (Edwards 2015), such as triggering eutrophication, due to the input of excessive nitogen 44 and phosphorus into the water, which leads to harmful algal blooms (HABs). HABs threaten 45 biodiversity, habitat loss, and increase the harmful algal blooms' frequency (Fan et al. 2020), as 46 well as having impact on the social, economic, and health of coastal communities. The ingrated 47 multi-trophic aquaculture (IMTA) is an effort to overcome this problem through the use of 48 seaweeds as an inorganic extractive species (Troell et al. 2009;Laramore et al. 2018

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Seaweeds are reported to have the ability to reduce nutrients in water. However, their activity in 59 IMTA as an inorganic extractive species must be considered its suitability between the main 60 commodity (finfish), and the environmental conditions of the aquaculture. Besides, the effectivity 61 as biofilter of each species is different capability (Kang et al. 2011), dependent on volume and 62 circulation of water, initial biomass, water quality and the physiology of each seaweeds (Hayashi 63 et al. 2008). This is because each seaweeds has different tolerance for life suitability, therefore, it 64 affecting its ability to carry out nutrient removal, life survival, and growth process. Therefore, 65 the hybrid grouper aquaculture which cultivated in the highlands at a salinity of 14-17 ppt, also requires appropriate seaweeds as a biofilter that able to tolerate these water conditions, and 67 works efficiently. This research aims to determine the most effective species of U. fasciata, S. 68 illicifolium, Gelidium sp., and Dictyota sp. As biofilter in hybrid grouper cultivation at low   Wastewater from grouper culture was obtained from hybrid grouper farm which is 84 cultivated in mountainous areas (393m height above sea level), and then soaked in 15 aquariums 85 which each volume was 15 L. Every treatments used 3 g L -1 of each seaweeds that has been 86 acclimated before was tested as biofilter in grouper culture wastewater, in a closed system with 87 aeration to determine its performance and effectiveness to remove nutrients wastewater for 20 88 days. The treatments consisted of P0 (Control), P1 (U. fasciata), P2 (S. illicifolium), P3 89 (Gelidium sp.), and P4 (Dictyota sp.). Water quality parameters were observed using water 90 quality checker including temperature, pH, salinity, TDS (5 in 1 meter, PH-9909) and dissolved 91 oxygen (DO Meter Lutron-5519). Total Suspended Solids was calculated by filtration method 92 using Whatman GF / C filter paper, and then compared between initial and final weight.

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Ammonia, Nitrate, and Phosphate contents were tested using the Visicolour ECO test kit method 94 and a Compact photometer PF-12. Nutrient uptake efficiency was calculated using the following 95 formula : Total N content in seaweed was tested using the Kjeldahl method, while for total P with 98 the Spectrophotometry method.

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The absorption of N and P by seaweed is assumed by the percentage of N and P added in homogeneity test to determine the parametric or non-parametric statistical assessment. 111 Furthermore, the ANOVA test was used to observe the significant difference between the 112 treatments in each water quality parameter, for the data that was normally distributed and 113 homogeneous was tested using the LSD (Least Significant Difference) analysis. While, the data 114 that were not normally distributed and not homogeneous was tested using the Kruskal-Walis   increasing on days 12 th and 16 th , the DO decreased to an average of 6.1 mgL -1 on day 20 th (Fig.   134 1a.).

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The concentration of ammonia in wastewater with U. fasciata treatment on day 4 th was 138 successfully reduced to 0.57 ± 0.04 mgL -1 (63.92%), which was the lowest compared to others.

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In all treatments, there was an increase in ammonia on the day 16 th , and then decreasing on the 140 day 20 th until 73.88% (Fig. 2a).

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The concentration of nitrate in water in the U. fasciata treatment increased until day 4 th , 142 and then consistently decreased until it reached 1.47 ± 0.3 mgL -1 , or reduced the nitrate by 143 79.49% on day 20 th . Meanwhile, other treatments showed that the increase in nitrate 144 concentration occurred up to day 8 th (Fig. 2b) (Fig. 2c).

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The largest percentage of ammonia reduction in water was shown by the treatment of U.

Content of N total and P total in thallus
163 Figure 4 showed that the percentage of the total P content addition in thallus seaweed 164 tends to be higher than that of total N content. Overall, U. fasciata has the most effective 165 absorption of N and P in wastewater because it has the highest addition of total N and P contents 166 in thallus. Meanwhile, Gelidium sp. has the lowest N and P absorption compared to U. fasciata, 167 Dictyota sp., and S. illicifolium. 170 U. fasciata has the best growth rate at 1.91% day -1 , followed by Dictyota sp at 0.36% 171 day -1 , and Gelidium sp at 0.25% day -1 . While S. illicifolium has the lowest SGR -1.71% day -1 172 ( Figure 5) because some thalli were fragmented, therefore effect on the final weight of seaweed .

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Treatments that used U. fasciata and Dictyota sp. has a relatively higher DO than the 175 control, because the these species have a better metabolism, survival, and photosynthesis 176 process, which then increased DO and reduced the dissolved carbon dioxide in aquaculture 177 wastewater. Meanwhile, the DO concentration with S. illicifolium and Gelidium sp. Treatments 178 were lower than the control, because the metabolic process was disrupted to adapt to the 179 conditions of wastewater quality, therefore the photosynthesis process was not optimal. U. 180 fasciata and Dictyota sp., which were able to increase DO than other treatments, also showed a 181 tendency to have higher in pH (Fig. 1b). This was happened because these seaweed was able to 182 produce DO so that increase water pH, while concentration of dissoved CO 2 which causes 183 acidity tend to decrease in water . Therefore, U. fasciata and Dictyota sp. 184 have capability to balance the pH in acidic water conditions, which are less favorable for aquatic 185 organisms.

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The increasing of ammonia occured until day 16 th , it might be caused that on the 16 th day 187 the process of converting ammonia to ammonium and nitrate was disrupted due to a decrease in 188 water temperature which reduces the ability of nitrifying bacteria to decompose ammonia. This The addition of the total N and P content in seaweed thallus was assumed as its ability to 202 absorb nutrients in the water. Therefore, the greater percentation of the total N and P addition, addition, tends to be higher than that of total N content addition to thallus seaweed. This is 211 occurred because the concentration of phosphate in water was higher than that of ammonia.

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Therefore, the phosphate absorbtion was higher than the nitrate. The highest percentage of total P 213 content addition of the thallus was found in U. fasciata (182.3%), while the lowest was in the  Availability of data and materials 256 The authors declared that the data and materials in this manuscripts are not publicly available 257 due to originality reasons but are available from the corresponding author on reasonable request.

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The authors declare that they have no competing interests