Temporal and spatial variations of air-sea CO2 fluxes and their key influence factors in seagrass meadows of Hainan Island, South China Sea

Abstract

Seagrass ecosystems have received a great deal of attention for contributing to uptake of atmospheric CO₂, and thereby helping to mitigate global climate change ('blue carbon'). Carbon budgets for seagrass ecosystems are developed by estimating air-sea CO₂ fluxes. Data for air-sea CO₂ flux for tropical seagrass ecosystems are lacking, which is problematic for constraining global seagrass carbon budgets. Here, we sought to address this important data gap for tropical seagrass ecosystems (dominated by Thalassia hemprichii and Enhalus acoroides) from the Hainan Island of South China Sea, while also testing what the main factors driving the variations of air-sea CO₂ fluxes are. We found that air-sea CO₂ fluxes exhibited a U-shape diurnal variability from 6 a.m. to 6 a.m. of the next day, with the highest and lowest air-sea CO₂ fluxes values at early morning and afternoon, respectively. Biological processes were the driving force for mediating diurnal variations of seawater pCO₂. The pCO_{2, sea} in different seasons displayed a trend of increasing from spring, reaching maximum in summer and then a decreasing trend after summer, where water temperature, wind speed and seagrass growth mainly drove the variations. This resulted in net uptake of CO₂ in all seasons except during summer in our study seagrass ecosystems, with greater negative values found in autumn (-3.63 ± 0.76 mmol m^-2 d^-1) than those in winter (-2.84 ± 0.60 mmol m^-2 d^-1). While the nutrient loading induced seagrass biomass changes (especially the seagrass T. hemprichii), which mediated the air-sea CO₂ fluxes changes among different seagrass meadows. Net annual CO₂ uptake potential under low nutrient loading (-0.77 ± 0.16 mol m^-2 yr^-1) was 23-54 % greater than high nutrient loading seagrass meadows, with the average annual air-sea CO₂ flux of the three seagrass meadows as -0.64 ± 0.13 mol m^-2 yr^-1. These results suggest that tropical seagrass meadows of Hainan Island are a significant CO₂ sink of atmospheric CO₂, but this capacity can be diminished by nutrient loading. Scaling up, we estimate the annual atmospheric CO₂ uptake by seagrass meadows of Hainan Island (total area 55.28 km²) was 1544 t of CO₂ yr^-1, equivalent to the annual emissions from the wholesale, retail, accommodation and catering industries of 164,000 tourists in Hainan Island. With carbon neutrality becoming an important part of global climate governance, this study provides timely information for capitalising on the ability of seagrasses to contribute to natural climate solutions.

Keywords: Air-sea CO(2) flux; Blue carbon; Carbon budget; Carbon neutrality; Eutrophication; Seagrass ecosystems.