Characterization of the CO₂ System in a Coral Reef, a Seagrass Meadow, and a Mangrove Forest in the Central Red Sea

Abstract

The Red Sea is characterized by its high seawater temperature and salinity, and the resilience of its coastal ecosystems to global warming is of growing interest. This high salinity and temperature might also render the Red Sea a favorable ecosystem for calcification and therefore resistant to ocean acidification. However, there is a lack of survey data on the CO 2 system of Red Sea coastal ecosystems. A 1-year survey of the CO 2 system was performed in a seagrass lagoon, a mangrove forest, and a coral reef in the central Red Sea, including fortnight seawater sampling and high-frequency pH T monitoring. In the coral reef, the CO 2 system mean and variability over the measurement period are within the range of other world's reefs with pH T , dissolved inorganic carbon (DIC), total alkalinity (TA), pCO 2, and Ω arag of 8.016±0.077, 2061±58 μmol/kg, 2415±34 μmol/kg, 461±39 μatm, and 3.9±0.4, respectively. Here, comparisons with an offshore site highlight dominance of calcification and photosynthesis in summer-autumn, and dissolution and heterotrophy in winter-spring. In the seagrass meadow, the pH T , DIC, TA, pCO 2 , and Ω arag were 8.00±0.09, 1986±68 μmol/kg, 2352±49 μmol/kg, 411±66 μatm, and 4.0±0.3, respectively. The seagrass meadow TA and DIC were consistently lower than offshore water. The mangrove forest showed the highest amplitudes of variation, with pH T , DIC, TA, pCO 2 , and Ω arag , were 7.95±0.26, 2069±132 μmol/kg, 2438±91 μmol/kg, 493 ±178 μatm, and 4.1±0.6, respectively. We highlight the need for more research on sources and sinks of DIC and TA in coastal ecosystems.Plain Language Summary Global warming and ocean acidification are consequences of increased CO 2 emissions to the atmosphere by humankind and are major threats to marine ecosystems. The Red Sea waters are naturally warm and saline. The resilience of its coral reefs, mangrove forests, and seagrass meadows is of growing interest for the scientific community in the context of global warming. The high temperature and salinity might render the Red Sea quite resistant to ocean acidification as well as an environment chemically very favorable for calcification, notably by corals. Calcification is a process dampened by the acidity (pH) of water, which depends on the chemistry of CO 2 in seawater. Warm and saline water naturally tend to have a more basic pH and then be less corrosive to calcareous skeletons. However, the chemistry of the CO 2 and acidity baselines and variability in the Red Sea are poorly documented. We conducted a year-round survey of the CO 2 chemistry of seawater in a seagrass meadow, mangrove forest, and coral reef ecosystem, involving discrete water sampling and highfrequency measurements.

Stunted Mangrove Trees in the Oligotrophic Central Red Sea Relate to Nitrogen Limitation

“…Therefore, nutrient limitation of mangrove stands is expected to increase from south to north along the Red Sea. However, salinity increases from the south to the hypersaline conditions in the north and temperature declines along the latitudinal gradient (Sawall et al, 2015;Saderne et al, 2019a;Anton et al, 2020). High salinity has been shown to also contribute to dwarfing of A. marina in South Africa (Naidoo, 2006) and to influence growth and leaf turnover of the mangrove Avicennia germinans in the Caribbean (Suárez and Medina, 2005).…”

Role of vegetated coastal ecosystems as nitrogen and phosphorous filters and sinks in the coasts of Saudi Arabia

“…This ratio is, however, consistent with the low ratio found in the water column above seagrass beds of the Saudi coast of the Gulf, at 1.6±2.1 (mean±SD) (KFUPM 2011). A higher ratio has been reported in a seagrass and mangrove ecosystem of the central Red Sea with 8.1±9.6 and 4.9±5.6, respectively (mean±SD; calculated from Saderne et al 2019).…”

Anomalies in the carbonate system of Red Sea coastal habitats

“…6b; Papaud and Poisson, 1986). However, this effect is relatively small compared to the contribution of ecosystem processes when large ecosystem anomalies are present in seagrass meadows and mangrove forests -a result of high, localised metabolic rates (Burkholz et al, 2019;Ho et al, 2014;Sea et al, 2018). Linked measurements of CO 2 gas exchange with metabolic fluxes are required to resolve the magnitude of this effect for Red Sea coastal habitats.…”

“…This implies that the carbonate system in stagnant water columns over mangrove forests could be different to what is observed in those with variable water exchanges, and the flushing of mangrove forests from surrounding waters can vastly reduce their contribution to air-sea CO 2 exchanges. As such, the control of surrounding water exchanges and water residence times should be considered further in these ecosystems, as studies often quantify the influence of mangrove forests on air-sea CO 2 exchange using stagnant water columns (Borges et al, 2003;Bouillon et al, 2007Bouillon et al, , 2008Macklin et al, 2019;Sea et al, 2018).…”