Low carbon sink capacity of Red Sea mangroves

Study Number

103

Author

Hanan Almahasheer, Oscar Serrano, Carlos M. Duarte, Ariane Arias-Ortiz,
Pere Masque, Xabier Irigoien

Abstract

Mangroves forests of Avicennia marina occupy about 135km2 in the Red Sea and represent one of the most important vegetated communities in this otherwise arid and oligotrophic region. We assessed the soil organic carbon (Corg) stocks, soil accretion rates (SAR; mm y−1) and soil Corg sequestration rates (g Corg m−2 yr−1) in 10 mangrove sites within four locations along the Saudi coast of the Central Red Sea. Soil Corg density and stock in Red Sea mangroves were among the lowest reported globally, with an average of 4±0.3mg Corg cm−3 and 43±5 Mg Corg ha−1 (in 1 m-thick soils), respectively. Sequestration rates of Corg, estimated at 3±1 and 15±1g Corg m−2 yr−1 for the long (millennia) and short (last century) temporal scales, respectively, were also relatively low compared to mangrove habitats from more humid bioregions. In contrast, the accretion rates of Central Red Sea mangroves soils were within the range reported for global mangrove forests. The relatively low Corg sink capacity of Red Sea mangroves could be due to the extreme environmental conditions such as low rainfall, nutrient limitation and high temperature, reducing the growth rates of the mangroves and increasing soil respiration rates.

Main Results and Conclusions

• The populations of mangroves in the Red Sea are among the only mangrove populations in the world that have not only remained stable but actually expanded.
  • “The Red Sea has recently been identified as possibly the only mangrove province where mangroves are not only stable, but have expanded by 12% over the last four decades.” (1)
• Nutrients are scarce in the Red Sea region, which likely explains why mangrove populations there are mostly dominated by dwarf trees.
  • “Lack of freshwater and soil inputs lead to acute nutrient limitation of Red Sea mangroves, resulting in mangrove forests being dominated by dwarf trees, similar to those in other arid areas (e.g. Gulf of California and areas lacking surface runoff such as the Indian River Lagoon in Florida).” (1)
• Interestingly, other dwarf mangrove trees in wet tropical climates have shown to have a large carbon sink capacity.
  • “… a recent assessment reports a Corg sequestration for dwarf mangrove forests in the arid shores of Baja California of, on average, 1000Mg Corg ha−1 in 1.5m-thick soils… similar to that found under some of the tallest tropical mangroves in the Mexican Pacific coast, which has been linked to the stability of these forests. These observations question the assumption that dwarf mangroves in arid shores support low carbon sequestration rates and stocks.” (2)
• Red Sea mangroves also have low organic carbon sequestration rates.
  • “The average soil organic carbon sequestration rates of 15 g Corg m−2 yr−1 for Central Red Sea mangroves soils is 10-fold lower than the average value of 163 g Corg m−2 yr−1 reported for mangroves globally, and are in the low range of values previously reported…” (4)
  • “The long-term (millenary time scale) soil organic carbon sequestration rates in Central Red Sea mangroves… was also well below the soil organic carbon sequestration rates derived from 14C chronologies reported for dwarf A. germinans forests in arid Baja California… and Pohnpei Island, Micronesia dominated by Rhizophora apiculata… despite SARs at the Micronesia sites being comparable to those reported here for Red Sea mangroves…” (5-6)
• Low carbon sequestration of the Red Sea mangroves likely has to do with lack of nutrition and soil quality as well as the arid conditions of the area.
  • “The relatively low organic carbon stocks and organic carbon sequestration rates in Red Sea mangroves are most likely due to the oligotrophic nature and low allochthonous inputs to the Red Sea.” (6)
  • “The lack of rivers and the extremely arid conditions result in nutrient-limited mangrove growth, reflected in low-biomass dwarfed trees, particularly within the study region in the Central Red Sea.” (6)
  • “Previous studies demonstrated that terrigenous Corg inputs can contribute up to 30% of Corg stocks in mangrove soil associated with riverine ecosystems, but the lack of rivers implies that there is no influx of riverine soil and organic matter in coastal areas and, therefore, Corg sequestration in Red Sea mangroves is limited to autochthonous production and fluxes from the ocean.” (6)
• Despite Red Sea mangroves remaining healthy while other mangrove populations have not, their contributions to blue carbon strategies to mitigate CO2 emissions is limited.