Dynamics of O2 and pCO2 in a Southeast Asia seagrass meadow: Metabolic rates and carbon sink capacity

Dissolved oxygen (DO) and partial pressure of CO2 (pCO(2)) were measured at half-hourly intervals from June 29 to September 9, 2019, in a seagrass meadow in the Southeast Asia archipelagos region. The open water mass balance of the O-2 approach was used to calculate metabolic rates (i.e., gross primary production (GPP), community respiration (CR), and net community production (NCP). The calculations show that GPP and CR rates in the seagrass meadow of Dongsha Island were approximately 2.5 times higher than the global means (GPP, 507 +/- 173 vs. 225 +/- 11 mmol O-2 m(-2) d(-1); CR, 497 +/- 171 vs. 188 +/- 10 mmol O-2 m(-2) d(-1)), while NCP was similar to the global mean (8 +/- 61 vs. 27 +/- 6 mmol O-2 m(-2) d(-1)), suggesting that seagrass meadows with high GPP may not necessarily hold high potential for carbon sequestration. The current data set also reveal that NCP tended to increase with GPP only at lower GPP levels, while NCP did not increase with GPP anymore at higher GPP levels. Moreover, the autotrophic/heterotrophic status did not correspond well to the sink/source behavior of CO2, suggesting that organic carbon metabolism could not be the only dominant factor in determining the sink/source status in a typical seagrass meadow underlain by carbonate sediments, which was further supported by the observed decrease in the trend of pCO(2) with a relatively stable NCP level over the study period. These results demonstrate that the metabolism and the relationship between NCP and pCO(2) in the seagrass meadows of Dongsha Island may deviate greatly from the global mean condition. To obtain a better assessment of the global potential of seagrass meadows as a nature-based solution for carbon sequestration, more regional-specific studies are still needed in the key regions, such as Indonesia and the Pacific archipelagos, that support extensive seagrass meadows but have not been charted.

Automated summary

Dissolved oxygen (DO) and partial pressure of CO2 (pCO(2)) were measured in a seagrass meadow in Southeast Asia, showing that the rates of gross primary production (GPP) and community respiration (CR) were higher than global means, while net community production (NCP) was similar. However, the relationship between NCP and GPP differed at lower and higher GPP levels. The autotrophic/heterotrophic status did not correspond well to the sink/source behavior of CO2, suggesting that other factors besides organic carbon metabolism influence carbon sequestration in seagrass meadows.