Ecological state of tropical wetlands under different human disturbances (Lake Tana, Ethiopia)

Wetlands have become the world's most degraded ecosystems. Land use changes such as agricultural, urbanization, and industrialization have been identified as the primary wetland change drivers worldwide. Currently, these land use changes are particularly pervasive in tropical areas, given economic development dynamics in these often low- and middle-income regions. As a result, more than half of all known freshwater wetlands have been destroyed globally, with further losses of tropical wetlands projected. Despite this, information on the ecological state of the world's remaining wetlands, notably tropical wetlands, is scarce. When compared to temperate regions, our understanding of tropical wetland biodiversity is poor. More specifically, knowledge of small-size biota such as phytoplankton and zooplankton in tropical wetlands is patchy to non-existent, even though these assemblages form the foundation of the wetlands' food webs. In addition, the distribution and status of papyrus swamps, the typical tropical wetlands that support high biodiversity and human livelihoods, are currently unknown. The lack of a solid understanding of the ecological state of tropical wetlands limits our ability to sustainably manage and conserve these ecosystems. It is also true that local ecological phenomena in tropical wetlands are frequently too diverse to be predicted by general ecological theories, or that attempting to transfer knowledge from well-studied temperate wetlands to tropical wetlands is not recommended, implying that more ecological research in tropical wetlands is needed. Therefore, this study is conducted to better understand the ecological states of tropical wetlands under different human disturbances (Lake Tana, Ethiopia). We estimated how the spatial distribution and size of papyrus swamps in the Lake Tana region changed between 1985 and 2020. This is supplemented by an examination of the landscape metrics such as mean patch size, patch number, and patch density in papyrus swamps, which indicated the degree of fragmentation in papyrus swamps. This study shows that the last 35 years have seen a 55% decrease in the total surface area of papyrus swamps, primarily due to agricultural expansion. The small patches of papyrus swamps that existed in the northern and eastern parts of the study area before 1990 appear to have disappeared in recent years as well. This study also found that human disturbances at the wetlands level increased turbidity, decreased water levels, and reduced aquatic plant and plankton species diversity. Different wetland types with varying human disturbances and hydrological conditions in the area have different aquatic plants and plankton community compositions. In contrast to less degraded riverine papyrus swamps and lacustrine wetlands, which are dominated by sedge species, highly degraded river mouth wetlands are dominated by grass species, annual alien weeds, and highland species. It was also found that phytoplankton communities in river mouth wetlands were dominated by Bacillariophyta, whereas Chlorophyta dominated those of lacustrine wetlands. The zooplankton community in the wetlands surrounding Lake Tana are generally dominated by rotifers. In a mesocosm experiment, however, moderate sediment burial depth stimulated Cyperus papyrus, the foundation species in Lake Tana wetlands. Cyperus papyrus also has shown great potential in the treatment of municipal wastewater in artificial wetlands. Protecting Cyperus papyrus dominated swamps and (where possible) revegetating highly degraded wetlands with Cyperus papyrus should thus be a cornerstone of catchment management strategies in the Lake sub-basin.