De ecologie van de belangrijkste Ethiopische Rift Valley lakes Abaya en Chamo, met speciale aandacht voor de kwaliteit van het water en voedselweb structuur

Ethiopia is characterized by diverse geological formations and climatic conditions, as well as by the occurrence of a variety of water bodies, including lakes, rivers, reservoirs and floodplains. With the exception of Lake Tana, the majority of the Ethiopian lakes are confined within the Ethiopian Rift Valley. Although these Rift Valley lakes are recognized for their outstanding biodiversity, they are extremely fragile inland waters that experience major threats, which primarily arise from human activities, such as large scale conversion of natural vegetation into cropland and massive human population growth in the settlements around the lakes. A profound understanding of the ecology of these lakes in relation to human activities in their watershed is the cornerstone for sustainable management of the region. The present PhD research aims to provide a better understanding of the ecology of two major Ethiopian Rift Valley lakes, Abaya and Chamo. We specifically aim to understand the structure and the functioning of the two study lakes and aim to explore to which extent long-term changes in the ecology of the lakes are related to changes of human activities in the region. Our research used a combination of historical data, satellite images and detailed data collected during intensive field sampling campaigns in multiple consecutive years (2012, 2013, 2014 and 2015). The findings from this study can contribute to the development of effective management programs that aim at the sustainable conservation of the ecological integrity of the region. In Chapter 1, we explore a comparative limnology of the two study lakes that includes both physico-chemical and biological variables. Chapter 2 reports on a detailed analysis of the trophic structure of the fish community in Lake Chamo and Lake Abaya using stable isotopes. Chapter 3 focusses on the relationship between changes in vegetation characteristics in the catchment area, rainfall erosivity and accumulation of sediments in the study lakes. Finally, Chapter 4 reports the long-term changes in major environmental variables and relate these to changes in fertilizer use and vegetation cover in the catchment area of both lakes. In the general discussion, we try to integrate the major findings of the different chapters. Our comparative limnological study (chapter 1) revealed that both lakes have similar total nitrogen and total phosphorus concentrations, whereas they strongly differ in water transparency and the amount suspended solids in the water column. The dominance of inorganic suspended solids in both lakes suggests that turbidity of the two lakes is caused by erosion in the watersheds. We observed a seasonal effect (dry and wet season), with higher nutrient and suspended solid concentrations in the wet season than in the dry season. Although the fish community composition in both lakes is very similar, the total fish abundance is considerably lower in Abaya compared to Chamo. Phytoplankton productivity in Abaya was significantly lower than in Lake Chamo. Likewise Lake Abaya has a lower ratio of phytoplankton productivity to total phosphorus and zooplankton to phytoplankton productivity than Lake Chamo. The higher fish to zooplankton ratio in Lake Abaya compared to Lake Chamo suggests allochthonous terrestrial energy subsidies in Lake Abaya. Investigation of the trophic structure of the fish community in Abaya and Chamo (chapter 2) shows that the isotopic niche of the fish community in Lake Abaya is larger and more diversified compared to that in Lake Chamo. In contrast, the fish community in Lake Chamo has a larger trophic food web length. In Lake Abaya, the energy source for fish originates from sediment organic matter while in Lake Chamo pelagic primary production is the major source for fish. Our study demonstrates that differences in the trophic structure of the fish community in the two lakes results from intraspecific niche variation between lakes rather than from compositional variation between fish communities. We argue that these differences are largely the result of differences in productivity between both lakes. Forest cover in the catchment area of Abaya and Chamo basin was already low in 2000 (0.1% and 3.4%, respectively), and further reduced during the period 2000 – 2012 (chapter 3). Rainfall erosivity in the area is high but did not change over time during the period 1974-2012. The high sediment loads in the lakes are likely the results from the low vegetation cover in the catchments and is expected to further increase with the continuing conversion of natural vegetation to crop lands. In Chapter 4, we clearly demonstrate that nutrient concentrations and electric conductivity of water in both lakes, as well as fertilizer use rate in agricultural lands increased strongly in the period 1961-2015, whereas water transparency, phytoplankton biomass and forest cover in the catchment areas significantly declined. Our data suggest that the dramatic increase in nutrient concentrations in both lakes is probably associated with increased fertilizer use, whereas the decline in water transparency seems to be associated with deforestation in the catchments of the lakes. The excessive nutrient enrichment in both lakes might, however, also be linked to other aspects of intensification of agriculture, such as increasing crop farming close to the rivers and lake shore, or increased sediment erosion. The excessive sediment loads and the profound nutrient enrichment in Lake Abaya and Lake Chamo are important anthropogenic disturbances that severely affect the ecological integrity of these lakes, and also undermine the delivery of vital ecosystem services to the region. Our results stress the urgent need for measures that prevent further environmental deterioration of these two iconic Ethiopian Rift Valley lakes.status: publishe