Intensive aquaculture has increased worldwide in recent decades coupled with growing awareness of sustainable development of this activity. One of the main concerns with the growth of the activity is the greenhouse gases (GHG) emissions. Methane (CH4), carbon dioxide (CO2), and nitrous oxide (N2O) produced in the sediment and water can be emitted to the atmosphere through diffusive or ebullitive transport across the water column. In this study, we measured the diffusive and ebullitive fluxes of CO₂, CH₄, and N₂O at the water-atmosphere boundary layer in both aquaculture and control areas in reservoirs and the ocean. The study area includes reservoirs with Nile tilapia (Oreochromis niloticus) farmed in net cages, as well as large-scale commercial production of brown mussels (Perna perna) and Pacific oysters (Crassostrea gigas) in marine aquaculture in Brazil.
Two different methods can be used to collect gas samples for each type of transport. The GHG diffusive emission at the water-atmosphere interface was measured using static polyvinyl chloride (PVC) chambers with an internal volume of 1 liter, equipped with floats and protection at the bottom to prevent any ebullitive influence. Four gas samples were taken from the inside of the chambers seven minutes apart (0, 7, 14, and 21 minutes) with plastic syringes. The GHG ebullitive emissions were measured using inverted collector funnels equipped with a volumetric bottle connected at its vertices to retain the bubbles released from the sediment.
The diffusive GHG flux in reservoirs with fish farming in net cages was 40.1 土 83.8 kg CO2eq m-2 year-1 (control = 9.8 土 16.5 kg CO2eq m-2 year-1) while the ebullitive GHG flux was 153.9 土 268.2 kg CO2eq m-2 year-1 (control = 30.6 土 221.9 kg CO2eq m-2 year-1). The diffusive GHG flux in marine aquaculture was 0.7 土 2.2 kg CO2eq m-2 year-1 (control = 1.0 土 2.3 kg CO2eq m-2 year-1) while the ebullitive GHG flux was 0.0 土 0.0 kg CO2eq m-2 year-1 (control = 0.0 土 0.0 kg CO2eq m-2 year-1). Emissions were higher in fish farming areas compared to their respective controls, while emissions from bivalve farming areas were similar to those of the corresponding controls. The GHG emission in reservoirs was higher than in marine areas, primarily due to the diffusive and ebullitive CH₄ emissions. The diffusive GHG flux in reservoirs was 33.7 土 78.4 kg CO2eq m-2 year-1 (control = 5.5 土 12.3 kg CO2eq m-2 year-1) while the ebullitive GHG flux was 153.6 土 265.4 kg CO2eq m-2 year-1 (control = 30.6 土 221.9 kg CO2eq m-2 year-1). The higher emissions in fish farming areas are primarily related to the organic matter content beneath the net cages. The comparable emissions in bivalve and control areas suggest that environmental factors exert a greater influence on emissions than the production of oysters and mussels.