Sampling mangrove peat in the southern Gulf of California
Mangrove forests, pantropical coastal wetland ecosystems, are severely threatened by human activities, on local scales due to development and worldwide by sea level rise. They provide many important ecosystem services, including nursery habitat for fishery species, protection of coastal communities from storm surge damage, timber, and ecotourism revenue. One mangrove service of global import is long-term carbon sequestration. Mangroves are highly productive, and store more carbon per area than other tropical forests. Much of this carbon is stored for long residence times in deep layers of organic-rich sediment, or peat. In some forests, mangrove peat deposits have been found to extend more than 10 m below ground and back in time more than 10,000 years. However, little effort has been made to use careful sampling and mapping of mangrove peat to quantify carbon stocks. Belowground carbon storage may be spatially variable, and so quantifying the amount of carbon in mangrove forests requires some understanding of the geological and ecological processes that control fluxes of carbon into and out of the sediment. Of the many factors that may control carbon storage, I set out to examine two in this study: forest size and ecological zone. I hypothesized that large forests contain more carbon per unit area as they are likely to have a longer and more stable history of sediment deposition in any given location. I also hypothesized that the waterfront fringe zone, dominated by the mangrove species Rhizophora mangle, will store less carbon per unit area than the landward hinterland zone, dominated by Avicennia germinans, because hinterland areas usually represent the older parts of the forest where a relatively large amount of sediment build-up has occurred. In the summer of 2014, I sampled mangrove sediments in forests near the city of La Paz, BCS, in the southern Gulf of California.
The results demonstrate the deep penetration and richness of peat deposits, even in the mangrove forests of this arid region. I found that forest area does not predict peat storage, and that hinterland zones possess more and deeper peat than fringe zones. This observation, along with x-ray fluorescence scans of the mangrove sediment cores, will allow me to reconstruct the history of organic matter deposition in these forests and to construct better understanding of how carbon cycling will behave here under global change.