Relevance and magnitude of 'Blue Carbon' storage in mangrove sediments: Carbon accumulation rates vs. stocks, sources vs. sinks

Mangrove ecosystems store large amounts of 'Blue Carbon', in particular in the sediment. Research in the past decade has emphasized the quantitative significance of carbon storage in mangrove forests in climate change mitigation, mainly by determining carbon stocks and calculating potential CO2 emissions caused by mangrove degradation. However, while this approach focuses on the total amount of carbon that can be lost to degradation, it fails to capture the amount that is sequestered annually. Therefore, carbon accumulation in mangrove sediments also needs to be taken into account. This study (i) explains the differences between carbon stocks and carbon accumulation rates (CAR), (ii) it addresses the geographical variation of carbon storage and underlying factors and (iii) it assesses the global relevance of 'Blue Carbon' sequestration in mangrove sediments. Results indicate that reducing uncertainties in carbon storage estimates of individual systems requires a representative set of data that covers within-system variability. An example from Indonesia illustrates that a mangrove ecosystem with a high C stock can have a low CAR and vice versa. It is therefore conceivable that coastal environmental settings with high allochthonous supply of mineral sediment, organic matter and nutrients mostly have low carbon stocks, but high CARs. As these settings represent >80% of the global mangrove area they are most important in terms of long-term carbon storage. While a C stock is a measure of the vulnerability potential in the case of ecosystem degradation or total loss, a CAR is rather a measure of the mitigation potential of carbon storage in mangrove ecosystems. The global carbon storage in mangrove sediments of 32 Tg yr(-1) estimated from CARs in this study is at the upper end of the range of global budgets (14.6-31.1 Tg yr(-1), mean 22.9 Tg yr(-1)). It highlights that the mangrove carbon sink may be larger than previously thought, but the high variation in the global average CAR of 233 +/- 280 g C m(2) yr(-1) also indicates the need for further data.