Assessment of the impact of agricultural by-products on the dynamics of phytoplankton functional groups on a rice-fish farm in Bonoufla, Côte d'Ivoire

Abstract

The objective of this study was to determine the influence of agricultural by-products on the dynamics of phytoplankton functional groups’ abundance on a rice-fish farm. To this end, experiments were conducted in rice-fish ponds by introducing three categories of agricultural by-products: maize bran (RSM), rice bran (RSR), mixed agricultural by-products (RPC) and a control with no exogenous feed (RC). Data were collected monthly between May 2019 and November 2020 during the Pre-Grow-out and Grow-out phases of Oreochromis niloticus. Phytoplankton were sampled by water filtration using a 20 µm mesh plankton net. Algal cells were identified and counted. The Reynolds Functional Groups (RFG) classification was used to form the Phytoplankton assemblages with the dominant microalgae. Physicochemical parameters likely to influence algal proliferation were measured using a HQ40D multi-parameter and nutrient salts were determined using spectrometric methods. It was found that the phytoplankton flora of the rice-fish ponds was composed of 235 algal taxa, with a higher diversity of Chlorophyta and Euglenophyta. The specific richness of phytoplankton was virtually identical from one feeding treatment to the next. The phytoplankton in these ponds were grouped into 08 functional groups. The group J, represented by species of the genus Coelastrum, Lacunastrum, Desmodesmus and Scenedesmus, and group W2, represented by species of the genus Trachelomonas and Euglena, were the most abundant in all rice-ponds and in all months. The relative density of these functional groups increased monthly and significantly during each rearing phase with the RPC treatment compared with the other treatments. The proliferation of taxa in these functional groups was positively correlated with the concentration of nutrient salts, pH and dissolved oxygen content of the water in the rice-fish ponds. These results show that the J and W2 phytoplankton assemblages are effective indicators for monitoring organic pollution of rice-fish pond water.