Effects of organic carbon addition on water quality and phytoplankton assemblages in biofloc technology ponds

Abstract The aim of this study was to evaluate the effects of organic carbon source addition with heterotrophic bacteria (Bacillus sp.) on water quality and phytoplankton assemblages in biofloc technology (BFT) ponds. Water samples were collected biweekly from May to September 2015 to analyse the physico-chemical parameters of the study ponds (i.e. BFT vs. Control). The stoichiometry and composition of phytoplankton community were also monitored per pond. Initially, trophic status of the study ponds was similar (i.e. eutrophic). At the end of sampling period, total organic carbon (TOC), chemical oxygen demand (COD) and total nitrogen (TN) were much higher in BFT ponds, whereas nitrate (NO3 N) and total phosphorus (TP) were much higher in Control ponds. C, N and P contents of phytoplankton increased significantly through time. Phytoplankton C, N and P concentrations were significantly different between BFT and Control ponds. Moreover, C:P and N:P ratios of phytoplankton increased dramatically in BFT ponds during the sampling period. However, we observed a gradual decline in the C:N ratio of phytoplankton in BFT ponds, which is probably due to a greater proportionate increase in N than C concentrations. Redundancy Analysis showed that TP, NH4 N, NO3 N and TOC were the best explanatory variables to interpret phytoplankton composition. In Control ponds, cyanobacterial genera were dominant among the alga groups, particularly species of the genera Anabaena, Aphanizomenon, Microcystis and Oscillatoria. However, algae belonging to centric diatoms, Chlorophyceae and Cryptophyceae, tended to be more abundant in BFT ponds. Our results showed that the addition of organic carbon sources used in BFT ponds could change nutrient supplies in water, which in turn clearly impacts phytoplankton stoichiometry and composition.