Processes of organic accumulation in sediment and pore water along the mangrove fringed coast of French Guiana
Résumé
From the Amapa coast (Brazil) to the Orinoco river (Venezuela), a series of migrating mudbanks, resulting from the huge mud discharge of the river Amazon, are colonized by mangrove forests, constituting one of the longest mangrove belt. Guiana's mangroves are predominantly composed of Avicennia germinans, Rhizophora mangle and very scarce Laguncularia racemosa.The highly dynamic feature of this coastline limits mangrove lifetime to few decades. The purpose of that study is to determine the quantity and quality of organic carbon in sediments and in interstitial waters in that specific context. The organic content of the sediment rises up with the mangrove age, TOC ranges from 1.5% for the pioneer mangrove to 18% for the senescent forest. The sediment organic content of the younger mangrove swamp is mainly derived from algal mats with low C/N (6 to 8), a predominance of grey amorphous organic matter in optical observations and Δ13C values close to -22‰. On the opposite, the organic content of the senescent mangrove sediment mainly derives from higher-plant debris in the uppermost 30cm, as indicated by Δ13C values close to -28‰, relatively high C/N (30) and a predominance of ligno-cellulosic debris. Contrary to the vertical trend of TOC, DOC increases with depth and can reach more than 300 mgl-1 at 50cm deep in the older mangroves. This increase of DOC with depth is accompanied by increase of fluorescence/DOC ratio. This study suggests an accumulation of the refractory organic material in water beneath the level of enhanced microbial activity.