Study On Dynamics Of Nutrients Across Sediment-water Interface In Composite Culture Pond Of Grass Carp (Ctenopharyngodon Idellus) And Shrimp (Litopenaeus Vannamei)

Grass carp Ctenopharyngodon idellus is considered as an economically important aquaculture species in China with the largest production. Grass carp is susceptible to a wide variety of disease, and a lot of economic losses have been occurred in recent years. The polyculture of grass carp and other species can fully utilize plankton and organic debris from water colomn, reducing nutrient load, improving the material utilization, and producing higher yield. The polyculture model of grass carp and shrimp Litopenaeus vannamei has not seen in previous studies. By measuring the sediment-water interface nutrient fluxes, the denitrification rates, and the nutrient distribution in interstitial water, the objective of the present study is to assess the effects of different polyculture systems of grass carp on the ponds sediment-water interface (e.g. the dynamic changes of nutrients), and to provide a certain basis for the further optimize of polyculture models.Sediment samples were collected from grass carp experimental enclosures with different cultural models from June to October in 2009. In the laboratory incubation experiments, benthic fluxes of nutrients at the sediment-water interface were measured, and its related factors were analyzed. During our experiments, the fluxes of ammonium had alternative trends (positive values / negative values) from month to month. Dissolved inorganic phosphate (DIP) was uptaken by the sediments in August and September but released from the sediments in other months. The exfluxes of nitrate + nitrite were observed in July and August but influxes of nitrate + nitrite were seen in other months. In the mid to late farming time, benthic fluxes of NH₄⁺, NO₃^-+ NO₂^- and DIP were significantly different (p<0.05) among different grass carp polyculture models (the polyculture groups, the monoculture groups, the binary groups). Absolute values of fluxes of nutrients in August were higher then other months.AIT technology (acetylene inhibition) was used to study the denitrification, nitrification, ammonification rates in pond sediment of different grass carp polyculture system, The results showed that: 1, The rates of denitrification, nitrification and ammonification of different models ranged from 0⁷34.15μmol/(m²·d), 0¹209.20 mmol/(m²·d), and 0⁴1.25 mmol/(m²·d), respectively. 2, The denitrification rates were comparatively high, at the same level with that of many estuaries and pools; In the medium period of the culture, the nitrification rates were very little or even undetectable; The ammonification rates were increasing month by month for all the models, among which the triple polyculture model reached the highest. 3, Higher ratio of shrimp Litopenaeus vannamei to grass carp will lead to earlier denitrification, but it is difficult to maintain its initial level, while, with more portion of grass carp, the denitrification may appear later, but it will increase rapidly during the culture period; the scarcer of the culture varieties, the lower of the nitrification rates; the higher the proportion of silver carp Hypophthalmichthys molitrix stocking, the higher the nitrification rates. For the optimization of culture, the polyculture group(shrimp ,grass carp and Silver carp)was better than the monoculture group(shrimp only)and the binary group (grass carp & shrimp, grass carp & Silver carp),but for the polyculture group(grass, carp and Silver carp), as there are two kinds of stocking density, more studies is required to fix an optimal ratio of them.Sediment samples were collected from grass carp experimental enclosures with different cultural models from June to October in 2009. The temporal-spatial distribution characteristics and correlations of nutrients in the interstitial water and overlying water have been analyzed. The results showed that the concentrations of nutrients in the interstitial water were higher than that in the overlying water, and for the PO₄^3- extremely significant correlations were fond between interstitial water and overlying water. For the NO₂^--N and NO₃^--N, their distribution trends and changes during farming time were very similarly. The concentration of NH₄⁺-N were much higher than other DIN (>95%) in the sediment interstitial water. In the vertical scale, the concentration of NH₄⁺-N increased gradually with depths (0¹0 cm), while the concentration of NO₂^--N and NO₃^—N decreased with depths (0⁴ cm). In the interstitial water, the concentration of PO₄^3- increased significantly in the first layer (0-1 cm), while declined slowly after the third layer(2-3 cm),and decreased quickly with depths in the sediments (4-10 cm).Bottom soil cores were taken from the enclosures and sectioned to determine the redox condition. Several chemical species related to or indicators of reducing conditions, such as Eh, easily reduced organic matter (EOM), ferrous (Fe²⁺), total dissolved sulfide (TDS), and total ammonium nitrogen (TAN). The objective of this study was to determine the influence of different grass carp polyculture model on sediment-water interface, and the influence of different indicators of reducing condition on nitrification, denitrification, ammonification rates and nutrient fluxes. The results showed that, there were significant differences in indicators of reducing conditions among different grass carp polyculture model (P<0.05). Higher shrimp stoking may reduce the concentration of EOM. The disturb depths made by shrimp was mainly 0² cm. The concentration of Fe²⁺ and TAN in GSL was lower than other groups in the later period of culture. The dynamic changes of N were closely related to the concentration of Fe²⁺ and TAN, while the correlation with Eh and TDS was not significant.