Nitrogen And Phosphorus Absorption By Ulva Lactuca And Eucheuma Denticulatum In Aquaculture Tail Water

China is the largest aquaculture country in the word,the mariculture industry plays a vital role in the development of fishery production.In recent years,with the continuous expansion of the scale of offshore and land-based aquaculture,the aquaculture model has developed towards intensive,large-scale and high-density.Excessive aquafeed input would produce large amounts of tail water during the aquaculture process.The tail water of aquaculture has been discharged improperly,resulting in eutrophication in the waters and affecting the sustainable and high-quality development of aquaculture.Therefore,the removal of phosphorus and nitrogen before the discharge of aquaculture tail water has become an urgent problem to be solved in the aquaculture of economical aquatic animals.As an important primary marine producer,macroalgae can synthesize organic matter through photosynthesis,rapidly absorb nutrients such as nitrogen and phosphorus in seawater,effectively reduce the eutrophication of water and improve water quality.This method of in-situ remediationhas the advantages of low cost and high ecological and economic benefits.In this paper,two large economical seaweeds,Ulva lactuca and Eucheuma denticulatum,were used as experimental materials to studythe absorption capacity of nitrogen and phosphorus and the growth capacity of algae,the growth of algae under the condition of constant high nitrogen and phosphorus,the absorption potential of nitrogen and phosphorus nutrients and the repair effect of algae in aquaculture tail water,under eutrophic conditions through orthogonal experiments.The main results are as follows:1.Optimal combination of nitrogen and phosphorus absorption by U.lactuca and E.denticulatum in eutrophic seawaterIn the orthogonal experiment,the daily growth rates of U.lactuca and E.denticulatum were 24.46±0.53%·d^-1 and 8.11±0.15%·d^-1,respectively.The removal rate of NH₃-N,NO₃-N and PO₄-P reached 99.83%,95.66%and 97.49%.Under the experimental conditions,the absorption rates of NH₃-N,NO₃-N and PO₄-P by U.lactuca and E.denticulatum showed an increasing trend with the increase of the initial nutrient concentration.When the nitrogen sources were NH₃-N and NO₃-N with the same concentration,the absorption rate of NH₃-N by algae was significantly higher than that of NO₃-N（P<0.01）.E.denticulatum preferentially absorbs NH₃-N,and then absorbs NO₃-N.The range analysis showed that U.lactuca and E.denticulatum were cultured in layers in eutrophic seawater with a nitrogen-phosphorus ratio of 6:1 and an NH₃-N concentration of 200μmol·L^-1,and U.lactuca had the highest daily growth rate.E.denticulatum were cultured in eutrophic seawater with nitrogen-phosphorus ratio of3:1 and NH₃-N concentration of 200μmol·L^-1,and the daily growth rate of E.denticulatum was the highest.U.lactuca and E.denticulatum were cultured in layers in seawater with a nitrogen-phosphorus ratio of 6:1 and an NH₃-N concentration of200μmol·L^-1,and macroalgae had the highest rate of NH₃-N uptake.U.lactuca were cultured in seawater with a nitrogen-phosphorus ratio of 12:1 and a NO₃-N concentration of 150μmol·L^-1,and macroalgae had the highest absorption rate of NO₃-N.E.denticulatum were cultured in eutrophic seawater with a nitrogen-phosphorus ratio of 3:1,a nitrogen concentration of 200μmol·L^-1,the coexistence of nitrogen sources of NH₃-N and NO₃-N,and a concentration ratio of 1:1,and macroalgae had the highest rate of PO₄-P uptake.Through the analysis of variance,when the NH₃-N absorption rate was used as the index,the effect of nitrogen source type was extremely significant（P<0.01）,and the effect of inorganic nitrogen concentration and seaweed combination was significant（P<0.05）.When the NO₃-N absorption rate was used as the index,the effect of nitrogen source type was extremely significant（P<0.01）,and the effect of seaweed combination was significant（P<0.05）.When the PO₄-P absorption rate was used as the index,the nitrogen-to-phosphorus ratio and the inorganic nitrogen concentration had a very significant effect（P<0.01）,and the seaweed combination had a significant effect（P<0.05）.2.Growth and absorption potential of U.lactuca and E.denticulatum under continuous nutrient supplementationThe daily growth rate of E.denticulatum in water with high nitrogen concentration for a long time was 4.83±0.82%·d^-1,which was significantly lower than that of the control group of 8.33±0.89%·d^-1（P<0.05）.Under the condition of continuous nutrient supplementation,the daily growth rate of U.lactuca was 25.14±2.06%·d^-1,which was significantly higher than that of the control group 17.20±0.44%·d^-1,（P<0.05）.Under the conditions of continuous supplementation of NH₃-N and NO₃-N,the daily removal rate of NH₃-N and NO₃-N from seawater in the 1g treatment group was over 85.81%,and the absorption rate ranged from 4.48±0.34-7.82±0.46μmol·g^-1·h^-1,2.86±0.38-6.06±0.19μmol·g^-1·h^-1.The seawater NH₃-N and NO₃-N uptake rates of macroalgae with a culture density of 0.1 g·L^-1 were 24.84μmol·g^-1·h^-1 and 37.70μmol·g^-1·h^-1,respectively.Under the conditions of continuous supplementation of PO₄-P,the daily removal rate of PO₄-P by macroalgae in the treatment group was25.80%-33.31%,and the absorption rate ranged from 0.68±0.13-0.83±0.12μmol·g^-1·h^-1.3.The effect of seaweed on nitrogen and phosphorus absorption in aquaculture tail waterDuring the 15-day experiment,except for the E.denticulatum group with the culture density of 0.1 g·L^-1,the NH₃-N removal rate of macroalgae in other treatment groups was above 96.59%,and the NO₃-N removal rate was above 84.15%.When the culture density was 0.1 g·L^-1,the absorption rates of NH₃-N and NO₃-N by U.lactuca-E.denticulatum（layered）group were the highest,9.76±0.27μmol·g^-1·h^-1and 1.03±0.14μmol·g^-1·h^-1,respectively.After the aquaculture tail water is treated,the NH₃-N concentration is 3.43μmol·L^-1,the NO₃-N concentration is 3.65±2.06μmol·L^-1,and the inorganic nitrogen concentration meets Discharge requirement of Aquaculture tailwater of Hainan province,China（local standards DB46/T 475-2019）first-level discharge standards.The removal rate of PO₄-P in the aquaculture tail water by macroalgae in the treatment group with the culture density of 0.4-1.0 g·L^-1 was more than 96.20%.The rate of PO₄-P uptake by the macroalgae in the treatment group with a culture density of 0.4 g·L^-1 was significantly higher than that in the treatment groups with a culture density of 0.7 g·L^-1 and 1.0 g·L^-1.After the aquaculture tail water is treated,the PO₄-P concentration meets the secondary discharge standard of Discharge requirement of Aquaculture tailwater of Hainan province,China（local standards DB46/T 475-2019）.In monoculture,the daily growth rate of macroalgae in the U.lactuca group with a culture density of 0.1 g·L^-1 was 12.40±0.89%·d^-1,which was significantly higher than the other culture densities.The daily growth rate of macroalgae in the E.denticulatum group with a culture density of 0.4 g·L^-1 was 5.84±0.41%·d^-1,which was significantly higher than the other culture densities.In layered culture,when the culture density was 0.1 g·L^-1,the daily growth rate of U.lactuca and E.denticulatum was significantly higher than that of other culture densities.U.lactuca and E.denticulatum have strong ability to absord NH₃-N,NO₃-N and PO₄-P in eutrophic water.When U.lactuca and E.denticulatum are cultured in the tail water treatment area,macroalgae can not only absorb a large amount of nitrogen and phosphorus,but also increase the yield and obtain economic benefits.The research results can provide reference for the cultivation of U.lactuca and E.denticulatum and the restoration of eutrophic water.Meanwhile,the synergistic purification of aquaculture tail water by macroalgae can provide support for the construction of intensive and efficient tail water treatment technology in the future.