The effects of two nutrient sources on the plankton community structure and C/N trophic shift in Ctenopharyngodon idella poly-culture ponds

To figure out the nutrient cycling process and energy conversion efficiency of major biogenic elements in grass carp (Ctenopharyngodon idella) poly-culture ponds, this study investigated the potential effects of two different foodstuff sources (Sudan grass S. Sudanense, and formula feed) on the plankton community structure, and C/N trophic shift in grass carp poly-culture ponds using analytical techniques of stable isotope (δ¹³C and δ¹⁵N) and C/N ratio. Our results showed that two feeding modes yielded no statistically significant impact on the community structure of plankton, but the feeding mode with Sudan grass helped promote the plankton growth in the water column. In the ponds fed with Sudan grass, Sudan grass contributed 84% and 6.8% of energy to herbivorous and filter-feeding fishes (silver carp H. michthysmolitrix and bighead carp A. nobilis)), respectively. Moreover, Sudan grass contributed 7.4% of energy to particulate organic matter. In the ponds fed with artificial feed, herbivorous fish derived 70.4% of energy source from formula feed. Formula feed contributed 5.1%, 8.1% and 12% of energy source to bighead carp, POM and sediment, respectively. Our findings suggested that, first of all, the carbon and nitrogen elements of two foodstuff sources were utilized by herbivorous fish (grass carp and blunt-snout bream M. amblycephala), and then transferred the nutrients to filter-feeding fishes through the pathways of plankton, residual feeds and fish feces. Ultimately, the biogenic elements were dedicated to the sediment. In contrast with formula feed, the nutrient of Sudan grass could be more utilized by cultured fish than that deposited in the sediment. The trophic transfer was slower in ponds fed by Sudan grass than that in ponds fed by formula feed. Energy transfer of cultured fish was extremely relevant to foodstuff sources. In particular, the conversion efficiency in Sudan grass ponds was higher than that in formula feed ponds.