A multitrophic model of a rice–fish agroecosystem: II. Linking the flooded rice–fishpond systems

Farmers in the tropics often husband rice and fish in linked rice floodwater–pond systems. The same aquatic food web occurs in both systems and is composed of species of bacteria, micro and macro phyto- and zooplankton, and fish (the phytophagous Nile tilapia (Oreochromis niloticus L. (Cichlidae (Cichlids) and the carnivorous North African catfish, Clarias gariepinus Burchell (Family: Clariidae (air breathing catfishes)). A vital function of their interaction is nutrient recycling. The mass dynamics of the pond food web species, including nutrient recycling, were modeled by dʹOultremont and Gutierrez [Ecol. Model. (2002), submitted for publication] and here are linked without modification to rice grown in a floodwater system. The physiologically based mass-age structured model of rice growth and development [Agric. Syst. 32 (1990a) 367; Agric. Syst. 32 (1990b) 341] was used to model the rice component. The linked system was used to explore the effects of fertilization and fish presence and feeding in ponds on nutrient recycling, phyto and zooplankton dynamics and rice and tilapia yields. In the linked system a net transfer of nutrients occurs from the pond to rice via the nutrients in fish feces that increase rice yield. Shading of the floodwater by the rice canopy has important affects on aquatic autotrophs that through bottom-up effects limit higher trophic levels. Last, a cautionary note is given on the potential effects of transgenic Bt rice on nutrient recycling.