The delta of the Red River plays an essential role in the economic development of North Vietnam (intensive agriculture, strong urbanization, industrial ports). This area also presents a major tourist interest due to the presence of a UNESCO Biosphere Conservation Zone (Island of Cat Ba) and the Bay of Halong , classified as a world heritage site by UNESCO (Fig. 1). Vietnam, since 1986, has been gradually opening to the world market economy within the framework of the policy of the “Doi Moi” (Revival) and now has a strong economic growth (8.5% in 2005). Concomitant with this is a significant increase in the anthropogenic pressure on the ecosystems, particularly in the industrial areas. The town of Haiphong is the 3rd largest in Vietnam with approximately 2 million inhabitants and has important industrial and port activities. A large economic development area is in the process of being installed to South East of Haiphong, at the mouth of the Bach Dang estuary (one of the principal arms of the Red River) and the anthropogenic impact, already elevated, will most probably increase further. The organic pollutants used in agriculture, the organic residues and inorganic components of urban effluents, and the metals derived from the urban and industrial activity, are all found in the deltaic system , many of which have undergone either none or only minimal treatment (Nhan et al. 1998, 1999, 2005, Viet et al. 2000, Hung & Thiemann 2002, Cao 2004). Environmental degradation resulting from these effluents can have major economic consequences, through negative impacts on public health, the natural resources, as well as on tourism in the affected areas and thus pose major challenges for sustainable economic development of the region.
Figure 1: Map of the Bach Dang Estuary. More details.. .
Scientific context and brief state of the art.
The estuarine zone is a zone of transition between the catchment area and the coastal zone, where the physical and trophic processes of transformation of the anthropogenic elements are particularly dynamic (Paerl et al. 2002). Thus, the passage of contaminants and other anthropogenic inputs in this transition zone will condition their fate (i.e., export to the ocean, sequestration in the sediments, integration in the trophic chain), and thus their future impact on the functioning of the coastal ecosystem.
The impact of anthropisation on these transitional ecosystems aroused much interest during the last few decades. The consensus which emerges today is that the anthropisation of these ecosystems, by disturbing the N/P/Si ratios of the nutrients, by bringing the xenobiotics, and particulate matter, has effects on the number, the diversity and the activity of microbial species (Crump et al. 1999, Riedel & Sanders 2003, Kritzberg et al. 2006). This leads to a modification of the structure of the microbial trophic networks (Kemp et al. 2005), which in return modifies the macroscopic properties of the ecosystems, in particular integration in the organisms, export, or the sequestration of these inputs (Wiegner et al. 2003). For example, while acting on the speciation of the anthropogenic compounds, the bacteria actively control their fate and their impact and thus their integration in the trophic chain (Jonnalagadda & Rao 1993, Gadd 1996). Similarly, nutrient inputs modify the structure of phytoplanktonic communities (Jacquet et al. 2006). Among modifications, changes of size spectrum are likely to modify the autotrophy/heterotrophy ratios by modifying the number of predation steps (and thus the losses of C), and the export/sequestration ratios related to aggregation processes and to the cell size (Richardson & Jackson 2007).
In order to understand the impact of anthropogenic inputs in coastal ecosystems on the use of these systems, it is necessary (1) to describe the fate of nutrients, xenobiotics and particles in the aquatic ecosystems concerned, and (2) to evaluate and compare their impact on the communities. This fate depends at the same time on the structure and the functioning of the trophic networks, but also of the nature of these inputs. This functional role of the trophic network is particularly important at the microbial level as it is located at the interface between detrital and living matter, mineral and organic matter and dissolved and particulate matter. The structure of the communities is thus subject to physical, anthropogenic and biological constraints which ultimately control the fate of these elements within the system, by favoring their integration in the organisms, their export, or their sequestration (Fig. 2).
Figure 2: Functional role of microbial networks