Divergent impacts of animal bioturbation on methane and nitrous oxide emissions from mariculture ponds.

Aquaculture systems are of increasing concern as an important source of atmospheric methane (CH) and nitrous oxide (NO). However, the role of animals in regulating CH and NO emissions from aquaculture systems remains unclear. Here, we established mesocosm trials to investigate impacts of bioturbation of different aquaculture species (i.e., clam, shrimp, and crab) on CH and NO fluxes in a mariculture pond. Across the initial, middle, and final culturing stages, mean CH flux in mesocosm without animals was 4.81 ± 0.09 µg CH m h, while the existence of clam, shrimp, and crab significantly increased CH flux by 35.3 %, 80.6 %, and 138 %, respectively. Bioturbation significantly decreased dissolved oxygen (DO) concentration by 5.19‒44.8 % but increased porewater CH concentration by 14.1‒59.9 %, indicating that lowered DO caused by animal respiration promoted CH production in sediment. Moreover, bioturbation of animals significantly increased ebullitive CH fluxes by 41.0‒216 %, contributing 57.4‒77.2 % of the increased CH emission in mesocosms with animals. However, shrimp and crab significantly reduced NO flux by 30.3 % and 42.5 %, respectively, primarily due to lowered DO conditions suppressing nitrification and limiting NO supply for denitrification. By contrast, clam significantly increased NO emission by 181 % because its filter-feeding behavior excreted more NH and NO into overlying water and thereby facilitating NO production. The NO concentration in overlying water was 1.72‒2.83-fold of that in porewater, and the calculated diffusive NO flux was 1.80‒37.5 % greater than chamber-measured NO efflux. This implied that NO might be primarily produced in overlying water rather than sediments, and the produced NO can either evade as water-air fluxes or diffuse downwards into sediments to be consumed. Overall, our study advocates that aquaculture-related climate mitigation strategies should place more attention on the divergent impacts of animal bioturbation on CH and NO emissions.