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ON LOCATION 39 WINTER 2015 N° 36 actually grew faster than other plots, as the forests were able to capture and integrate the pollutants into their biomass. Researchers are now conducting microbiological research to determine how pollutants such as nitrogen are absorbed, and what effect increased levels will have on the sediment, water, and fauna living within the ecosystem. French Guiana’s endless cycle Fromard and his colleagues are also studying the mangroves of French Guiana, another overseas territory, but in South America. The mangroves here have experienced little human impact, but are subjected to a very active coastline that can advance or recede by two to three kilometers a year. This is due to the huge amounts of sediment deposited at the mouth of the Amazon River, which extend the coastline until erosion allows the sea to move inland. “This pattern of expansion and retraction works roughly in 30-year cycles,” notes Fromard. “We therefore created a database of coastal images, using aerial photography and satellite images going back to the 1950s, to determine how this cycle is being affected by climatic factors such as global warming, and how much carbon is released during periods of retraction.” The vitality of local mangroves also applies to the rich biodiversity that thrives in the ecosystem. Emma Michaud and her colleagues from the LEMAR,3 a partner lab in the ANR4 Biomango project5 which aims to study and preserve the mangroves of French Guiana, and observe the fauna that lives in sediments of mangrove forests, such as small shellfish, worms, and crabs. “Crabs are abundant, around 100- 500 per square meter, and play an important role through their bioturbation activities, boring into the mud on the floor of the mangrove forests,” points out Michaud. “Water passes through the holes they create, oxygenating the environment, which in turn allows other organisms to grow—and local biodiversity to expand.” The increased oxygen also gives rise to certain types of bacteria that can rapidly decompose organic elements such as fallen leaves. “We do not yet know what overall impact these bacteria have on the ecosystem’s dynamics, including carbon sequestration or mineralization,” concludes Michaud. “However, projects are underway to shed more light on this area, as well as to list the ecosystem’s various species, about which there is little information.” Carbon sinks of New Caledonia IRD and CNRS researchers based in New Caledonia have been studying local mangroves to improve understanding of their carbon sequestration capacity, and how this capacity is evolving in the face of climate change. “New Caledonia is an ideal location to study,” notes team leader Cyril Marchand,6 “because its mangroves are well-preserved— some enjoying UNESCO World Heritage status— while being subject to significant human pressure such as deforestation, aquaculture, and mining.” Marchand and his colleagues set up measurement equipment in mangrove forests, including ultrasound detectors mounted on tripods to gauge rising water levels and sediment flow, and an eddy covariance tower to record gas concentrations and 3D wind patterns. The measurements allowed them to calculate CO2 flux from the ecosystem to the atmosphere, and isolate the factors contributing to carbon sequestration, such as sediment water content, and temperature. The resulting data sets also helped researchers identify a new factor, that of sediment surface biofilm. Consisting of plant-like microorganisms, it reduces the flow of CO2 from sediment to the atmosphere, thereby increasing the mangrove’s carbon sink effect. Further studies of the impact of the biofilm could help establish more accurate carbon budgets. ii In Mayotte, the experimental plot of mangroves (Rhizophora mucronata) is criss-crossed by a network of pipes that release wastewater under controlled conditions. © H. BRETON/COLLÈGE VICTOR SCHOELCHER The Uca maracoani crab contributes to mangrove biodiversity by boring holes into the mud, thus oxygenating the ground. © F. FROMARD/ECOLAB


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