NITROGEN EXCHANGE FLUX OF SEDIMENT-WATER INTERFACE IN LITOPENAEUS VANNAMEI SALT ALKALI POND

Coastal saline-alkali land aquaculture is increasingly rising, with the nutrient exchange at the sediment-water interface playing an important role in the ecosystem. It serves as an indicator of endogenous pollution levels in pond sediments. In order to explore the nitrogen transformation dynamics at the sediment-water interface in Litopenaeus vannamei culture ponds. Measurements were taken from May to July 2019, analyzing various nitrogen forms in overlying water and sediment interstitial water across three ponds in Binzhou City, Shandong Province, with different salinity (28, 45, 55). Using Fick's first law, nitrogen exchange fluxes at the sediment-water interface were estimated, and correlations between environmental factors and exchange fluxes were examined. The results show that, (1) Nitrogen forms, including DIN, DON, and TN, predominantly diffuse from sediment to water, indicating sediment as the source. Conversely, \rmNO^-_3 -N moves from water to sediment, with sediment acting as the sink. During the breeding period, total exchange fluxes of DIN were 1.69, 23.07, and 19.36 mg/m², DON were 36.60, 27.90, and 19.98 mg/m², and TN were 38.09, 43.66, and 32.56 mg/m², respectively, for salinity levels of 28, 45, and 55. (2) Seasonally, exchange fluxes of DIN, DON, and TN were significantly higher in May compared to July. For instance, the average DIN exchange fluxes for salinity levels 28, 45, and 55 were 2.08, 6.37, and 12.47 mg/(m²·d) in May respectively, decreasing to –0.48, 0.06, and 1.47 mg/(m²·d) in July respectively. Notably, the DIN exchange flux in salinity 55 group was significantly higher than that in the other two groups (P<0.05). In May, the DON exchange fluxes were 13.91, 5.32, and 6.79 mg/(m²·d), respectively. Salinity 28 group exhibited significantly higher values compared to the other two groups (P<0.05). Conversely, in July, the fluxes were 5.82, 10.94, and 5 mg/(m²·d), respectively, with the salinity 45 group significantly surpassing the others (P<0.05). Additionally, in May, the average TN exchange fluxes were 15.9, 8.79, and 19.16 mg/(m²·d) respectively. Significantly, the salinity 45 group exhibited lower fluxes than that in the other two groups (P<0.05). However, in July, the fluxes were 5.31, 9.1, and 3.28 mg/(m²·d) respectively, with the salinity 45 group significantly surpassing the salinity 55 group (P<0.05). (3) Redundancy analysis showed positively correlations between salinity and exchange fluxes of \rmNO^-_2 -N, \rmNO^-_3 -N, and \rmNH^+_4 -N. Organic matter exhibited positive correlations with \rmNH^+_4 -N and TN fluxes. There was a significant negative correlation between temperature and \rmNH^+_4 -N exchange flux, moreover, water content and porosity demonstrated positive correlations with \rmNO^-_3 -N and DON exchange fluxes. In conclusion, sediments act as potential sources of nitrogen, with pollution potential significantly reduced towards the end of aquaculture compared to the early stage. The salinity 55 group facilitates DIN release, while the salinity 28 and 45 groups facilitates promote DON release. The results enhance understanding of nitrogen exchange fluxes in large surface aquaculture ponds and provide valuable data to support the scientific management of this aquaculture model.