Impacts Of Two Benthic Animals’ Bioturbation On The Migration And Transformation Of Nutrients At The Sediment-water Interface In Pond Polyculture

Cyprinus carpio and Litopenaeus vannamei are two common cash species in pond polyculture, they can not only make great profits but also play a very important role in pond ecological system. As C. carpio and L. vannamei possess benthic behavior, they would make bioturbation to sediment-water interface and affect the biogeochemistry processes at the interface. The objective of the present study is to assess the effects of C. carpio and L. vannamei on the migration and transformation of nutrients at the sediment-water interface. The primary results were listed below.1. Effects of Cyprinus carpio on benthic nutrient fluxes with special reference to nitrification, denitrification and nitrate ammonification in pond polycultureThe influence of C. carpio on nutrient fluxes and nitrification, denitrification and nitrate ammonification rates at the sediment-water interface in pond polyculture was investigated through field experiments and ex situ incubation. C. carpio enhanced the SOC (sediment oxygen consumption) from April to September (except in July). C. carpio showed no influence on the NH4+-N flux, but it promoted NOX--N (NO2--N and NO3--N) releasing from the sediment to the water from May to September and it also promoted soluble reactive phosphorus (SRP) releasing from July to September. The nitrification, denitrification, and nitrate ammonification were also significantly promoted by C. carpio. The results suggested that C. carpio played a very important role in the migration and transformation of nitrogen and phosphorus at sediment-water interface, and it could also help nitrogen removal in aquaculture water.2. Effect of carps’ bioturbation in different density on denitrification, nitrification and nitrate ammonification rates on sediment-water interfaceTo explore the effect of different density’s carps’ bioturbation on denitrification, nitrification and nitrate ammonification rates,six treatments were set in this research:treatment with no fish(CO);treatments with 2、4、6、8 and 10 carps in each flume(C2、C4、C6、C8、C10).Sediment samples were collected regularly with a sediment sampler with no boundary perturbation;denitrification, nitrification and nitrate ammonification rates were measured using AIT (acetylene inhibition);tracers were used to determine the depth of the carps’ physical disturbance.The results showed that:(1)In the five treatments with carps,the depth of the carps’ physical disturbance was mainly in 1-5cm;(2)Nitrification rate in CO was significantly faster than that in the other treatments(P<0.05),while the rates in C8 and C10 which had a higher density of carps were significantly faster than that in C2、C4 and C6(P <0.05);(3)Denitrification rate was almost to 0 in CO during the research,but in C2、 C4、C6、C8 and C10,the rate got faster in the treatment with carps of higher density;(4) Nitrate ammonification rate changed a little in each treatment,but the rate in C8 and C10 was relatively faster than that in the other treatments.The present study suggested that carps’ bioturbation could evidently promote denitrification, nitrification and nitrate ammonification,when there was enough organic carbon in the sediment,carps’ bioturbation would have a positive affect on the procedure of removing nitrogen in eutrophic ponds.3. Impact of Litopenaeus vannamei bioturbation on nitrogen dynamics and benthic fluxes at the sediment-water interface in pond aquacultureTo investigate the effects of the bioturbation of Litopenaeus vannamei at the sediment-water interface through field experiments and ex situ incubation. Sediment oxygen consumption (SOC), benthic nitrogen dynamics (nitrification, denitrification, and nitrate ammonification), and the benthic fluxes of NH4+-N, NO3--N, NO2--N, and soluble reactive phosphorus (SRP) were measured monthly from April to September. The results showed that L. vannamei promoted SOC and decreased the concentration of dissolved oxygen (DO) in the bottom water. Enhanced nutrient release from the sediments to the overlying water due to L. vannamei was observed, which consequently enriched the bottom water. The presence of L. vannamei inhibited nitrification later in the cultivation period and hardly had any significant effects on denitrification and nitrate ammonification throughout the study. The results suggest that farmers should more thoroughly consider changes in the benthic fluxes of oxygen and nutrients, as these aspects could be influenced by shrimp bioturbation, whereas benthic nitrogen dynamics in aquaculture ponds were comparatively less influenced.4. Impact of Litopenaeus vannamei bioturbation on benthic fluxes at the sediment-water interface under different densityTo explore the effect of different density’s shrimps’ bioturbation on benthic fluxes at the sediment-water interface, five treatments were set in this research:treatment with no shrimp (CO); treatments with 2、4、6、8 shrimps in each flume (C2、C4、C6、C8); Sediment oxygen consumption (SOC), the benthic fluxes of NH4+-N, NO3--N, NO2--N,and soluble reactive phosphorus (SRP) were measured every 7 days. The results showed that:(1)The depth of the shrimps’ physical disturbance was mainly in 1-3cm; (2)compared with the control, L. vannamei promoted SOC under each density, but the effect became significant in C2 on Day 15, while it became significant in C4, C6, and C8 on Day 1. (3)NH4+-N released from sediment in each treatment, there was no significant difference between the NH4+-N flux in CO and C2, and the fluxes in C4, C6, and C8 were higher than the other two. (4)The NO3--N releasing was inhibited in C6 and C8 before Day 8, but it turned to be promoted on Day 22. (5)The NO2--N releasing was promoted in C4, C6, and C8, and the effect grow stronger with the increase of the density. (6)There was no significant difference between the SRP flux in CO and C2, the SRP releasing was promoted in C4 since Day 8, while it was inhibited in C6 and C8 on Day 1 and Day 8. The results suggest that L. vannamei could promote SOC and nutrient releasing, and the promoting effect grows with the increase of the density, but when the density reaches 132.75 ind/m2, the effect won’t become stronger any more.5. Impact of Litopenaeus vannamei bioturbation on benthic fluxes at the sediment-water interface in different salinitiesTo explore the effect of shrimps’ bioturbation on benthic fluxes at the sediment-water interface in different salinities, three treatments were set in this research:S5, S20, and S35(the water salinity were 5,20 and 35 ppt); Sediment oxygen consumption (SOC), the benthic fluxes of NH4+-N, NO3+-N, NO2--N, and soluble reactive phosphorus (SRP) were measured every 7 days. The results showed that L. vannamei could promote SOC in each treatment, but the effect was strongest in S5, and weakest in S20. L. vannamei promoted NH4+-N releasing in all treatments, and the effect grew stronger with the increase in water salinity. In S5, L. vannamei had the strongest promoting effect on NO2--N releasing, but the weakest promoting effect on NO3--N releasing. While in S20, L.vannamei had the strongest promoting effect on NO3--N releasing. There was no significant difference in the SRP flux among the treatments before Day 8, while the SRP releasing was more significantly stimulated in higher salinity since Day 15.