The Spatial and Temporal Variability of Air-Sea CO2 Fluxes and the Effect of Net Coral Reef Calcification in the Indonesian Seas: A Numerical Sensitivity Study

Abstract

A numerical model system was developed and applied to simulate air-sea fluxes of CO 2 and coral reef calcification in the Indonesian Seas and adjacent ocean basin for the period 1960-2014 on a fine resolution grid (ca. 11 km) in order to study their response to rising sea water temperatures and CO 2 concentrations in the atmosphere. Results were analyzed for different sub-regions on the Sunda Shelf (Gulf of Thailand, Malacca Strait, Java Sea) and show realistic and different levels, signs and pronounced temporal variability in air-sea CO 2 flux. The Gulf of Thailand changes from an atmospheric CO 2 sink during the boreal winter to a CO 2 source in summer due to higher water temperatures, while other sub-regions as well as the entire averaged Sunda Shelf act as a continuous source of CO 2 for the atmosphere. However, increasing atmospheric CO 2 concentrations weakened this source function during the simulation period. In 2007, the model simulations showed even a first flux inversion, in course of which the Java Sea took up CO 2 . The simulated trends suggest that the entire Sunda Shelf will turn into a permanent sink for atmospheric CO 2 within the next 30-35 years if current trends remain constant. Considering the period between 2010 and 2014, coral reef calcification enhanced the average CO 2 emission of the Sunda Shelf by more than 10% from 15 to 17 Tg C yr −1 due to lowering the pH and increasing the partial pressure of CO 2 in surface water. During the entire period of simulation, net reef calcification decreased although increasing seawater temperature mitigated effects of reduced CO 2 emission and the resulting decrease of the pH values on reef calcification. Our realistic simulation results already without consideration of any biological processes suggest that biological processes taking up and releasing CO 2 are currently well balanced in these tropical regions. However, the counteracting effects of climate change on the reef calcification, on other biological processes and the carbonate system need to be investigated in more detail. SST increased by about 0.6 • C during the last 55 years, while SSS decreased by about 0.7 psu.

Dissolved organic matter from tropical peatlands reduces shelf sea light availability in the Singapore Strait, Southeast Asia

“…1). The annual mean flow is westwards from the South China Sea through the Singapore Strait and northward through the Malacca Strait to the Indian Ocean, with water residence times of 1− 2 yr over most of the shelf (Mayer et al 2015(Mayer et al , 2018. The central Sunda Shelf (southern Malacca Strait, Singapore Strait, and Karimata Strait) also experiences strong tidal currents that mix the water column all the way to the seafloor and prevent stratification (Mayer & Pohlmann 2014, Hamzah et al 2020.…”

Extensive remineralization of peatland-derived dissolved organic carbon and acidification in the Sunda Shelf Sea, Southeast Asia

“…The Singapore Strait is located in the center of the Sunda Shelf Sea, and is close to the coastal peatlands on Sumatra (Figure 1), with water depth mostly less than 40 m. The annual average circulation runs from the South China Sea into the Indian Ocean through the Java Sea and the Malacca Strait (Gordon et al, 2012). However, the monsoon system causes the main ocean currents to reverse direction seasonally (Figure 1) (Mayer & Pohlmann, 2014;Mayer et al, 2018). During the Northeast Monsoon (November to February), water flows from the South China Sea westwards through the Singapore Strait and along the coast of Sumatra.…”

“…Our additional experiments then show that of the remaining 30%-40% of this tDOC, <25% is still photo-remineralizable, which means that at most additional remineralization of 7%-10% of the initial tDOC flux might occur during its transport from the Singapore Strait to the exits of the shelf to the open ocean, and that any biodegradation proceeds at most slowly. Depending on how much of the remaining tDOC really is biodegradable, these results imply that possibly around 20%-30% of the total peatland tDOC from Sumatra is sufficiently refractory to resist remineralization on the Sunda Shelf and be exported to the Indian Ocean, given that the total water residence time in the Malacca Strait is approximately 2 years (Mayer et al, 2018).…”

Dissolved organic matter from tropical peatlands impacts shelf sea light availability on coral reefs in the Singapore Strait, Southeast Asia

“…1). The annual mean flow is westwards from the South China Sea through the Singapore Strait and northward through the Malacca Strait to the Indian Ocean, with water residence times of 1-2 years for most parts of the shelf (Mayer et al 2015, Mayer et al 2018. This region of the Sunda Shelf (southern Malacca Strait, Singapore Strait, and Karimata Strait) experiences strong tidal currents that mix the water column all the way to the seafloor and prevent stratification (Mayer & Pohlmann 2014, Hamzah et al 2020.…”