4:40 PM - 4:55 PM
[ACG43-10] Air-water CO2 flux in a subtropical mangrove-seagrass-coral continuum: A comparative study
Keywords:air-water CO2 flux, partial pressure of CO2, mangrove, seagrass, coral, Iriomote Islands
The present study has been carried out off the Iriomote Islands, Japan where mangroves, seagrasses and coral reefs coexist in close vicinity to each other. The variation in the partial pressure of CO2 [pCO2(water)] in the aquatic bodies lying adjacent to these three ecosystems along with the air-water CO2 fluxes were estimated for two 24-h cycles (during July, 2017) respectively in the mangrove, seagrass and coral waters. Other carbonate chemistry parameters like pH, total alkalinity (TAlk) and dissolved inorganic carbon (DIC) were also monitored.
Though these three distinct types of ecosystems were located within 1 km distance from each other, stark differences in the mean pCO2 (water) was observed during the study. The mean pCO2(water) was found highest in the mangrove waters (906 ± 572 µatm) followed by the seagrass waters (480 ± 88 µatm) (which lies intermediate between the mangroves in the coastal periphery and the corals towards the shelf) and the least was observed in the coral waters (416 ± 82 µatm). It can be seen that the difference between the seagrass waters and the coral waters was much less compared to that observed with the mangroves. The air-water CO2 fluxes also mirrored the variability of pCO2(water). Considering the average of the entire diurnal dataset, all the waters acted as a source of CO2, however, the magnitude was highest in the mangroves (405 ± 464 µmol m-2 h-1) followed by seagrasses (57 ± 71 µmol m-2 h-1) and corals (5 ± 66 µmol m-2 h-1). It is worth mentioning that the mangrove surrounding waters scarcely showed negative flux values (i.e. acted as a sink for CO2) during the entire diurnal cycle, whereas, the seagrass waters acted as sinks for a substantial time period (maximum sink magnitude: -183 µmol m-2 h-1). Coral waters acted as a sink for CO2 for much more time than the seagrass waters and the maximum sink magnitude was also much higher than the seagrass waters (-206 µmol m-2 h-1). Apart from pCO2(water), TAlk and DIC also showed a similar trend. The highest mean values for both the parameters were observed in the mangroves (TAlk: 2291 µmol kg-1; DIC: 2074 µmol kg-1), followed by seagrass (TAlk: 2219 µmol kg-1; DIC: 1933 µmol kg-1) and corals (TAlk: 2211 µmol kg-1; DIC: 1878 µmol kg-1). It can be seen that the decrease in DIC from mangroves to corals was much more prominent than the decrease in TAlk.
On the whole, it increased from the mangrove towards the coral site. Analyzing all the observations it can be concluded that, mangrove waters usually act as source of CO2 towards the atmosphere, and it has shown nothing contrary in this study site. Corals are mostly reported to exhibit near neutral character in terms of source/sink of CO2 which is exhibited here too. However, seagrass waters which mostly act as sink for CO2 has been also found to act as source of CO2 in his region which might be attributed to the influence of mangrove derived pCO2 rich water lying in close vicinity to the seagrass bed.