Ecological succession of plankton in a biofloc system with molasses as carbon source

Abstract

The objective of this research was to establish the ecological succession of the planktonic groups developed in a biofloc system for the cultivation of tilapia. For this purpose, 20 fish of 7±2 cm long and an average weight of 8.6 g were introduced in 200 liters’ cylinders. Every day they were fed commercial food with 40% protein. The diet was calculated considering 10% of the body mass and molasses was added to promote the development of heterotrophic bacteria maintaining a C/N ratio of 15:1. Diversity and abundance of the organisms associated with the bioflocs was analyzed during 10 weeks by direct observation under an optical microscope (Olympus CBX50) connected to an image processing software. At the same time, physicochemical parameters (pH, nitrite, nitrates and ammonium) were measured using a HANNA Aquaculture Photometer auto analyzer (HI83203). Results showed a marked succession of planktonic organisms as the system matured, with the occurrence of facilitation, tolerance and inhibition mechanisms among the groups. Chlorophytes and cyanobacteria were among the facilitator species responsible for the transformation of nitrogen compounds into assimilable forms available for the development of other organisms with more complex structures, so that -throughout the experiment- ciliates and other protozoa appeared quickly, but with variations both in diversity and abundance among the groups. Later on, rotifers, one of the most conspicuous groups, increased represented by the genera Lepadella , Phillodina , Lecane and Habrotrocha . The last group to appear was that of nematodes (sixth week), which indicates that the system was already mature by providing sufficient carbohydrates for the formation of the collagen structures of this group.