Carbon dioxide and methane fluxes in ponds of Nile tilapia (Oreochromis niloticus) and integrated multi-trophic culture

INTRODUCTION
Aquaculture is an important sector of food production. Most fish and shrimp are produced in freshwater earthen ponds. Thus, there is growing interest in assessing the emissions or absorption of greenhouse gases (GHG) in such systems. The aim of this study is to determine the pathways, quantifying CO₂ and CH₄ fluxes between ponds and the atmosphere in Nile tilapia (Oreochromis niloticus) monoculture and in the integrated culture of this species with giant river prawn (Macrobrachium rosenbergii) and curimbatá (Prochilodus lineatus).

METHOD/ RESULTS
Four treatments were tested in 12 earthen ponds, with three replicates each: tilapia monoculture (TM); integrated cultivation of tilapia and curimbatá (TC); tilapia and prawn (TP); and tilapia, curimbatá, and shrimp (TCP). Ponds were harvested 6 months after stocking. Five samples of diffusive and ebullitive GHG were collected at the water–atmosphere interface during the culture, using diffusion chambers and funnels, respectively. Concentrations of CO₂ and CH₄ in the samples were measured in a gas chromatograph. 

Large variability was observed among ponds and over time for both diffusive and ebullitive flows of CO₂ and CH₄. As a result, no statistical differences were observed among treatments. Total diffusive CO₂ emissions during the entire culture were significantly higher than ebullitive fluxes, ranging from 1.9x10⁸±3.2x10⁸ gCO₂/ha/cycle in TM to 5.6x10⁸±8.7x10⁸ gCO₂/ha/cycle in TP. For diffusive CH₄, fluxes ranged from 2.1x10⁵±3.7x10⁵ gCH₄/ha/cycle in TCP to  5.0x10⁵ ± 8.4x10⁵ gCH₄/ha/cycle in TP. In the TCP treatment, diffusive absorption exceeded emissions, resulting in a negative total balance (-1.8x10⁵±3.6x10⁵ gCH4/ha/cycle). Thus, the culture combining the three species (TCP) acted as a sink for methane.

CONCLUSION
The data indicated high complexity in CO₂ and CH₄ fluxes between the water surface and the atmosphere, resulting in large variability. Complementary studies are necessary to demonstrate which variables affect these processes.