Overall effect of rice biomass and fish on the aquatic ecology of experimental rice plots

Abstract

The integration of rice and fish culture promises ecologically sound and economically successful management of floodwater ecosystems. Amongst other benefits, the stocking of fish in rice fields may contribute to the soil fertility of the rice field. We investigated the impact of the rice biomass and the stocking of fish (a polyculture of Nile tilapia and common carp) on ammonium, nitrate and ortho-phosphate levels in the field floodwater and the interstitial water of the oxidised soil layer. The collected data were submitted to principal component analysis (PCA) and ANOVA. The PCA solution contained three components, good for 66.21% of the total variability. Using the PCA components we were able to illustrate that the aquatic environment is largely determined by the growth of the rice crop. This is not surprising since the dry weight biomass per hectare for rice was more than 8000 kg as compared to an actual standing wet biomass of fish of 193.4 kg ha−1. We suggest that the large difference in biomass means that effects on nutrients caused by the fish are obscured by rice driven processes. Still, the stocking of fish had a significant effect on 5 out of 12 variables under research. Chlorophyll-a levels nearly doubled, while oxygen levels were lower. The presence of fish also decreased the concentration of ortho-phosphate in the water and in the interstitial soil water. The floodwater and soil interstitial water patterns for nutrients were very similar.