Nutrient fluxes across sediment-water interface in different grass carp polyculture enclosures

A range of polyculture systems including grass carp, silver carp and Litopenaeus vannamei was conducted by enclosure-experimental method from June to October, 2009. Seven treatments in triplicates were set as follows:grass carp only(G), grass carp + silver carp(GS), grass carp + Litopenaeus vannamei(GL) and triple group designed with 4 levels of stocking densities of grass carp +silver carp + Litopenaeus vannamei (that is GSL1, GSL2, GSL3, GSL4, respectively). Benthic fluxes of nutrients across the sediment-water interface were studied by laboratory culturing way, and their affected factors were analyzed. The results showed that nutrient fluxes were remarkably higher than those in lakes, cage culture area, green crab culture pond and sea cucumber culture pond. NH4+-N fluxes ranged from -87.87 mmol/(m2d) to 229.67 mmol/(m2d), while NOx--N fluxes and DIP fluxes ranged from -32.95 mmol/(m2d) to 27.13 mmol/(m2d), -500.2 mmol/(m2d) to 10.25 mmol/(m2d), respectively. Absolute values of fluxes of nutrients in August were higher than in other months. During our experiments, the fluxes of ammonium had alternative trends (positive values/negative values) from month to month. The exfluxes of nitrate + nitrite were observed in July and August but influxes of nitrate + nitrite were seen in other months. Dissolved inorganic phosphate (DIP) was uptaken by the sediments in August and September but released from the sediments in other months. In the mid to late cultivation time, benthic fluxes of NH4+-N, NOx--N and DIP were significantly different (P0.05) among different grass carp polyculture models. Among these groups, nutrients fluxes in the triple group were obviously higher than the monoculture group and the binary groups. In general, on the basis of water environment data, the treatment GSL2 in which stocking densities of grass carp, silver carp and Litopenaeus vannamei were 0.58 ind./m2, 0.23 ind./m2 and 48.9 ind./m2 was a relatively better culture model.