Cultivo de camarões em sistema de bioflocos: composição elementar (C, N e P) e influência das variáveis ambientais sobre as razões estequiométricas (C:N:P) do seston e bactérias livres

Abstract

Aquaculture has a promising contribution to the animal protein production. However, it can result in major environmental impacts such as wastes disposal with high organic matter content. Therefore, it is required to use techniques with lower effluent nutrient loading and reduced use of water. One of the alternatives developed in this way is biofloc technology (BFT), which promotes a high density of microorganisms, providing feed for shrimp (bioflocs) and water quality control through the assimilation of nitrogenous and phosphorus compounds (biofloc and free-living bacteria). Thus, the interaction between the assimilation and storage of nutrients (N and P) in the bacterial biomass is of extreme relevance for bioflocs efficiency. However, little is known about the stoichiometric elemental composition of the bioflocs and free-living bacteria and especially about which factors affect their composition. The aim of this study was to test for differences between elemental composition of seston (bioflocs) and free-living bacteria and to determine which environmental factors are responsible for the variation of their stoichiometric ratios (C:N:P) in shrimp culture systems using BFT. Samples were taken from 6 marine shrimp farms and the elemental composition of C, N and P and their stoichiometric ratios in seston and free-living bacteria were analyzed, as well as physical and chemical water quality parameters. The results indicated that the absolute elemental composition of C and P were higher in the seston. The C:N ratios, in turn, were lower in the free-living bacteria whereas the N: P ratios were lower in the seston. These differences were probably due to differences in the biotic composition of the two fractions and the ability of the seston to incorporate more nutrients than the free-living bacteria. The C:P and N:P ratios of the biofloc were reduced at higher temperature and total suspended solids (TSS), (P enrichment). This response is related to the organism metabolism and the growth rate of seston components, particularly attached bacteria. Other environmental variables, such as transparency, dissolved oxygen; pH and chlorophyll a were identified as covariates of temperature and TSS. We conclude that the elemental composition of the seston is richer in C and P and that systems with higher temperature and higher TSS have a greater ability to absorb and immobilize these elements in the seston, evidencing a greater potential to elimination of these elements through the removal of bioflocs and reuse of water.