Influencing mechanism of farming clams on the CO2 flux from aquaculture ponds: Insights from ecosystem carbon metabolism

Abstract

Aquaculture ponds have garnered increasing attention for their potential roles in the global carbon cycle within the context of global warming. As a significant co-culture species in mariculture ponds, clams exert a notable impact on CO2 flux across the water-air interface. However, the mechanism underlying this influence is as yet unclear. This study measured the CO2 flux as well as the ecosystem carbon metabolism and environmental physiochemical parameters from four marine polyculture ponds, i.e., the pond containing swimming crabs and kuruma shrimp was referred to as CS, and the other three ponds containing three varying densities of razor clams additionally based on CS were noted as CSB1, CSB2, and CSB3 in order of increasing clam density. All the bi-species and tri-species polyculture ponds acted as atmospheric CO2 sinks during the farming season, with the peak in CSB1 and the lowest in CSB3. Net ecosystem production was a dominant regulator of CO2 uptake, and sediment respiration significantly increased CO2 production and then affected the CO2 flux, contributing 16.7–20.0 % to the ecosystem respiration (ER). Water pH could function as a potentially reliable CO2 flux predictor in the polyculture ponds, while Chl a was an internal driving factor. The polyculture of clams at low density may have exerted a “bottom-up” effect, accelerating nutrient cycling and thus stimulating the planktonic community, gross primary production (GPP), and ER, whereas a high stocking density of clams can have a “top-down” effect that would have the opposite ecological effects through increased predation pressure from clams. Although GPP and ER were simultaneously promoted or inhibited by clams at different stocking densities, the effect on GPP was always greater than that on ER, resulting in the CO2 sink function that the polyculture ponds performed through the water-air interface being strengthened at low clam densities and weakened at high clam densities. Given the good predictive ability of water pH on CO2 flux, monitoring and management of water pH could be an effective way to regulate the CO2 sink function of mariculture ponds.