The Congo River is one of the most understudied basins in the world. Second only to the Amazon river basin in terms of drainage area and water discharge, the Congo River harbors the richest known fish species diversity on the African continent, which also represents a resource of critical importance to riparian human population. In shrill contrast with its importance for local, regional and global biodiversity, its ecology, dynamics and ecosystem functioning are poorly understood simply because it has only seldom been studied, and only for very specific aspects. So far no integrated interdisciplinary studies on the Congo River have been carried out, such an approach is the pre-requisite to fully understand the biodiversity and functioning of an aquatic ecosystem.
A large number of conceptual models have been developed over the past decades to describe the overall ecological functioning of large river systems. The importance of longitudinal and lateral gradients were underlined, with contrasting importance of autochtonous and allochtonous primary production for fueling the food web in function of spatial and temporal scales. Unfortunately, all these concepts were validated through observations in temperate or subtropical riverine systems, except for Orinoco floodplains. However, these concepts help us to delineate factors driving species and trophic biodiversity of consumers, including fish. Therefore, in this proposal, we aim to link terrestrial inputs, primary producers (algae and aquatic macrophytes), invertebrate and fish biodiversity to ecosystem dynamics and functioning in the Congo river.
In short, the proposal will address the following four key questions:
1/ How diverse are fish communities in two sub-catchments of the Congo River (Lubilu and Lomami) in terms of biodiversity and functional/trophic diversity?
2/ Which factors can be identified in regulating this diversity? In order to detect underlying mechanisms, we will provide comprehensive measurements of possible regulating factors, including aquatic primary producer abundance, diversity and production, physico-chemical conditions, nutrient status (linked to aquatic productivity), and organic matter availability and origin. These data will not only be used to support the work on higher trophic levels, but will also be analyzed in the context of furthering our understanding on carbon and nutrient cycling in the Congo basin.
3/ To which extent do fish communities in these river systems depend on autochtonous (aquatic) primary production or on lateral allochtonous (terrestrial) production? Stable isotope analyses of representative fish communities and potential food sources will be used to determine the degree of allochtonous support and to describe the trophic structure.
4/ What is the importance of seasonal flood events on global ecosystem functioning? In order to better capture seasonal variability and to have information on carbon sources and basal food sources signatures preceding the larger-scale sampling campaigns, regular monitoring for a selected number of parameters and involving local scientists will be set up at one site on both subcatchments studied.
In achieving the above objectives, we will not only provide the first comprehensive and interdisciplinary dataset on the aquatic biogeochemistry and ecology for any part of the Congo River basin, but also provide important data to address the underlying fundamental ecological questions needed for the management of the main goods and services provided by the River (fish).
Two catchments on opposite sides of the Congo river mainstem will be sampled, both join the main Congo River downstream of Kisangani. One catchment is the Lubilu river which drains water from the UNESCO Man and Biosphere (MAB) Yangambi Biosphere Reserve. The watershed is still preserved from human activities is covered by primary tropical humid forests. Data obtained on this site will then characterize near-pristine conditions and serve as a reference baseline for comparisons made within the COBAFISH project and by future other projects. The ecosystem dynamics and functioning observed in the Lubilu basin will be considered as representative of natural forested area in the Congo basin. The second catchment is situated on the Lower Lomami near its confluence with the Congo River and encompasses several smaller tributaries. The basin has a distinct set of land-use patterns and vegetation, which enable stream water characteristics to be linked to features of the surrounding land. Previous observations on the Lower Lomami highlighted an important fish biodiversity.
Contrasting conditions between the Lubilu and Lomami basins and between Lomami subcatchments will offer us the opportunity to generate a unique dataset with a large range of biogeochemical, physico-chemical and ecological conditions allowing to test conceptual models of tropical river functioning and to answer the above-mentioned project key questions.
In this project, we will exploit the historical (late 1950s) fish database acquired in the Lubilu river and the more recent observations made in the Lomami river by the KMMA and during the Congo 2009 and 2010 expeditions. Supplementary new data on biogeochemistry, hydrobiology and nutrient (including Si) cycles will be acquired in both basins during both seasonal campaigns and routine monitoring (WP1). Primary producers, aquatic macrofauna and fish biodiversity will be investigated in several stations in both basins (WP2). Finally, comprehensive understanding of river ecology and food web functioning will be acquired by means of gut content analysis and tracing of various isotopic trophic markers (WP3). In particular, the origin of the organic matter fueling the upper levels of the food web will be highlighted.