Ethiopia is a predominantly agrarian country where about 85% of the country’s population is engaged in the agricultural sector. The sector has enjoyed substantial growth during the last two decades. To increasing crop production and productivity to achieve high agricultural growth and alleviate food security problems in the growing population through, for example, intensive use of agricultural inputs such as fertilizers and pesticides are priorities for the Ethiopian government. As a result of this agricultural intensification policy of the Ethiopian government, the use of pesticides and fertilizers has increased year to year and will be expected to further increase in the years to come. The central Ethiopian rift valley region, particularly in the vicinity of Lake Ziway, is amongst the regions where agrochemicals (pesticides and fertilizers) are most intensively used by smallholder farmers producing vegetables and fruits (e.g., tomato, onion, cabbage, green bean and pepper) and by large-scale farms producing horticulture crops (e.g., cut-flowers and grape). Residual concentrations of pesticides and nutrients used by the small- and large-scale farmers may enter Lake Ziway through several routes such as agricultural land runoff, effluent discharge, drift during spraying, and inadequate handling of remnant pesticides and empty pesticide containers. Currently, there is high concern about the pollution of Lake Ziway by residuals of agrochemicals (e.g., pesticide) and their ecological effects. In addition, Lake Ziway is under threat of pollution by urban wastes (solid and liquid wastes) sourced from the fast-growing Batu and Meki towns found at the south-west and north-west side of the lake, respectively. Therefore, a systemic investigation that assesses the ecological impacts of pollutants to Lake Ziway (e.g., pesticides, trace metals and microplastics, and nutrients) due to agricultural and urbanization in the catchment area of the lake is needed to support its conservation and protection.The main objectives of our studies were; 1) to review the status, temporal and spatial variability of water quality and biological resources of Lake Ziway, 2) to assess the goods and services that local communities currently derive from the lake, 3) to investigate the current use and misuse of pesticides by small- and large-scale farmers in the vicinity of Lake Ziway, monitor pesticide concentrations in lake sediment and water compartments, and evaluate the associated ecological risks, 4) to assess the distribution of microplastics in the sediment and some fish species in Lake Ziway, and 5) to assess structural and functional effects of the pesticide imidacloprid to the aquatic ecosystem typical for the Ethiopian tropical climate .The thesis begins with a literature review (Chapter 2) on the biological resources, and spatio-temporal variation of water quality of the lake focusing on nutrients, metals and pesticides, and other stress factors such as sedimentation and water abstraction for irrigation use. The results of this study indicate the deteriorating trends of several water quality and ecological parameters. Several water concentration levels of nutrients and trace metals (e.g., PO43−, NO3−, NH4+, Ca2+, Cu and Ni) of the lake show increasing trends. For some parameters the water quality of the lake exceeded guideline values for safe drinking water (e.g., alkalinity, Fe, and pesticides like diazinon and spiroxamine) and for aquatic life (e.g., NH4+, Fe, Cr, Cu and Se). The literature review also showed that water samples from shoreline locations of the lake proximate to floriculture farms showed increased values for some physicochemical parameters (e.g., NO3−, NH4+, K, Na and electrical conductivity) and residual pesticides of various types (e.g., boscalid, methomyl, carbendazim and spiroxamine).In Chapter 3, the ecosystem goods and services (ES) that the Lake Ziway provides for the communities of the region were identified and prioritized by local people based on the relevance for their livelihood. Concurrently, the pesticide use and handling practices of the small- and large-scale farmers found in proximate to Lake Ziway was assessed. The potential impacts of pesticides on the ES of the lake was also assessed using a conceptual approach that links the effect of pesticides on organisms of the lake due to contamination to effect on ES provision by the lake ecosystem. The results of the study showed that Lake Ziway supplies a wide array of ES for the local communities including 15 classes of provisioning ES , 3 classes of regulating and maintenance ES, and 6 classes of cultural services. The study also indicated misuse and improper handling of pesticides by smallholder farmers. Malpractices of farmers included improper storage, over-dosage, too high application frequencies (up to 12 times/crop/season) in violation of recommended interval days, mixing pesticides near the water canal and dumping pesticide wastes into their surrounding environment. In addition, the wastewater effluent released from the floriculture farms into Lake Ziway is another concern as no evidence is presented that show its effective treatment before it released into the lake ecosystem. The study found that the pesticide use and handling practices of the farmers in the region were unsustainable and likely expose the Lake Ziway ecosystem to pesticide contamination at such levels potential to pose impact to the ES of the lake.The environmental levels of pesticides and physicochemical parameters, including nutrients in water and sediment compartments of Lake Ziway were investigated in Chapter 4. Variation in the distribution and composition of biological organisms (macroinvertebrates and fish) were also assessed by correlation with monitored environmental variables (pesticides and physicochemical variables). Ecological risks of the individual pesticide and risks due to the mixture of the pesticides were evaluated using risk quotients (RQ) and mixed-model approaches, respectively. The results showed contamination of water and sediment of Lake Ziway with different types of insecticides and fungicides, where malathion, dimethoate, metalaxyl, diazinon, chlorpyrifos, fenitrothion and endosulfan were detected in more than half of the water samples (> 50%), and diazinon, α-cypermethrin and endosulfan were observed frequently (> 25%) in sediment samples. Effects on physicochemical properties of the water of the lake and higher residual levels of the quantified pesticides were observed at locations proximate to floriculture, smallholder agriculture and urban settlements. The effects on structural and functional endpoints of the lake were also studied in relation to levels of the environmental variables (e.g., nutrients and pesticides). For most of the pesticides quantified in water and sediment the calculated SSD based acute RQ was > 1, indicating possible to very high ecological risks. Arthropods and fish are expected to be highly affected by the measured mixture of pesticides. The effect was high at locations of the lake that are proximate to smallholders’ farms, and receive largescale farms’ wastewater and sites where the inflow rivers join the lake.Spatio-temporal distribution of plastic particles in sediment and in the gastrointestinal tract of fish of Lake Ziway was studied, and discussed in Chapter 5. The effect of the contaminant, plastic particles, are also discussed by comparing its estimated concentration in the sediment of the lake to the threshold effect concentrations reported in the literature. The results of the study indicated that shoreline sediments of Lake Ziway are contaminated by plastic particles and the highest estimated sediment concentration was in exceedance of effect threshold values reported in the literature, thus it is likely to cause effect on benthic communities. More than one-third of the sampled fish individuals were also found with ingested plastic particles in their gastrointestinal tracts, which may also have human health risk implication. The particle size analysis result also demonstrated the benthopelagic transfer of plastic particles from sediment to fish.Chapter 6 presents and discusses the results of an effect study of imidacloprid pesticide, using a mesocosm experimental setup with tropical freshwater conditions typical for Ethiopia. Structural (e.g., macroinvertebrates, zooplanktons, phytoplankton and periphyton) and functional (e.g., decomposition of organic matter and physicochemical parameters) endpoints were studied. The recovery of the community from the effect of the pesticide was also studied. In addition, acute single species toxicity of imidacloprid to local freshwater arthropods was studied and discussed. Effect concentrations (L(E)C50 and L(E)C10) and no observed effect values (NOEC) were calculated for the experimental water quality parameters and biological endpoints. A direct effect of imidacloprid was observed on aquatic organisms, in which the macroinvertebrates: Cloeon dipterum and Caenis horaria, and the zooplankton: Brachionus sp. and Filinia sp. were the most negatively affected species compared to other species. Treatment-related significant increases in chlorophyll-a concentrations of periphyton and phytoplankton were also found, which are likely indirect effects as the primary producers are released from grazing pressure (e.g., by the grazers Cloeon dipterum, Brachionus sp. and Filinia sp. and scrapers Planorbidae sp. and Physidae sp.) as a direct effect of the imidacloprid insecticide. Higher sensitivity of tropical aquatic species to imidacloprid was also demonstrated relative to their temperate counterparts. Recovery was observed for zooplankton community (9 weeks), but no recovery was found for macroinvertebrates in 21 weeks of the recovery period of the experiment.In conclusion, key findings in our studies are discussed in Chapter 7. Agricultural and urbanization activities are affecting the ecology and water quality of Lake Ziway by discharging nutrients, trace metals, residual pesticides and plastic particles among others into the lake. Intervention measures and future research outlooks are pointed out, that can help the protection and conservation of Lake Ziway and other Ethiopian aquatic ecosystems experiencing similar anthropogenic pressures in their catchments. Accordingly, the thesis recommends: 1) training on pesticide safe use and handling for smallholder farmers to improve the skill and knowledge, 2) promotion and adoption of IPM technologies to reduce use and misuse of pesticide, 3) strengthening regulatory control on the registration, and purchase and use of pesticides, 4) implementation and improvement of urban waste management, 5) establishment of modern laboratory facilities to enable risk assessment of pesticides and emerging chemicals and 6) wide application of EPT richness index to monitor the water quality of Lake Ziway and other aquatic ecosystem with similar anthropogenic pressures, as it is quick, effective and cheap compared to monitoring of physical and chemical variables.