Tropical reservoirs have been identified as important methane (CH(4)) sources to the atmosphere, primarily through turbine and downstream degassing. However, the importance of ebullition (gas bubbling) remains unclear. We hypothesized that ebullition is a disproportionately large CH(4) source from reservoirs with dendritic littoral zones because of ebullition hot spots occurring where rivers supply allochthonous organic material. We explored this hypothesis in Lake Kariba (Zambia/Zimbabwe; surface area >5000 km(2)) by surveying ebullition in bays with and without river inputs using an echosounder and traditional surface chambers. The two techniques yielded similar results, and revealed substantially higher fluxes in river deltas (∼10(3) mg CH(4) m(-2) d(-1)) compared to nonriver bays (<100 mg CH(4) m(-2) d(-1)). Hydroacoustic measurements resolved at 5 m intervals showed that flux events varied over several orders of magnitude (up to 10(5) mg CH(4) m(-2) d(-1)), and also identified strong differences in ebullition frequency. Both factors contributed to emission differences between all sites. A CH(4) mass balance for the deepest basin of Lake Kariba indicated that hot spot ebullition was the largest atmospheric emission pathway, suggesting that future greenhouse gas budgets for tropical reservoirs should include a spatially well-resolved analysis of ebullition hot spots.

Environmental contextPlastic, and particularly microplastic, pollution is a growing environmental concern worldwide. Research regarding marine environments has led to a substantial increase in knowledge, yet little is known as regards the situation in freshwater environments. Although the occurrence of microplastics was demonstrated in Lake Geneva in 2012, the present research aims at confirming this pollution and expanding the data set for other lakes and environments of Switzerland. AbstractMarine microplastic (<5mm) water pollution has met growing public and scientific interest in the last few years. The situation in freshwater environments remains largely unknown, although it appears that they play an important role as part of the origin of marine pollution. Apart from the physical impacts on biota, chemical effects are to be expected as well, especially with smaller particles. This study aims at assessing plastic abundance in Lakes Geneva, Constance, Neuchâtel, Maggiore, Zurich and Brienz, and identifying the nature of the particles, potential ingestion by birds and fishes, and the associated pollutants. Lake surface transects and a few rivers were sampled using a floating manta net, and beach sediments were analysed. Plastics were sorted by type (fragments, pellets, cosmetic beads, lines, fibres, films, foams) and composition (polypropylene, polyethylene, polystyrene, etc.); fish and water birds were dissected to assess their potential exposure, and analyses were conducted on the hydrophobic micropollutants adsorbed to the microplastics as well as some potentially toxic additives they contained. Evidence of this pollution is shown for all lakes, microplastics of all types and diverse composition having been found in all samples. Birds and fish are prone to microplastic ingestion, and all the tested chemicals (both adsorbed micropollutants and contained additives) were found above the detection limit, and often the quantification limit. The sources and their respective contribution need to be confirmed and quantified, and the ecotoxicological effects need further investigation. Other questions remain open, including the transport and fate of plastic particles in the environment.

[1] Large dams can have major ecological and biogeochemical impacts on downstream ecosystems such as wetlands and riparian habitats. We examined sediment removal and carbon (C), nitrogen (N), and phosphorus (P) cycling in Itezhi-Tezhi Reservoir (ITT; area ¼ 364 km 2 , hydraulic residence time ¼ 0.7 yr), which is located directly upstream of a high ecological value floodplain ecosystem (Kafue Flats) in the Zambezi River Basin. Field investigations (sediment cores, sediment traps, water column samples), mass balance estimates, and a numerical biogeochemical reservoir model were combined to estimate N, P, C, and sediment removal, organic C mineralization, primary production, and N fixation. ), and significant N-fixation ($30% for the total primary production) was required to support primary production due to marginal inputs of inorganic N. Model simulations indicate that future hydropower development in the reservoir, involving the installation of turbines driven by hypolimnetic water, will likely result in the delivery of low-oxygen waters to downstream ecosystems and increased outputs of dissolved inorganic N and P by a factor of $4 and $2 compared to current dam management, respectively.

[1] Large dams affect the aquatic continuum from land to ocean by accumulating particles and nutrients in their reservoirs. We examined sediment cores to quantify sediment, organic carbon (OC), nitrogen (N), and phosphorous (P) accumulation, and to examine historic changes and spatial variability in the sedimentation pattern in Lake Kariba, the largest hydropower reservoir in the Zambezi River Basin (ZRB). Sediment characteristics (concentrations of OC, N, P; d 13 C and d 15 N; wet bulk density) showed large variability both with sediment depth and between cores. While organic matter (OM) in river deltas was primarily allochthonous in origin, OM characteristics (d 13 C, C:N) in lacustrine sediments suggest that autochthonous sources account for >45% of the OM that accumulates over large areas of the lake. At the same time, the relative contribution of allochthonous material within individual layers of lacustrine cores varied considerably with depth due to discrete flood deposits. The overall sediment accumulation rate in Lake Kariba is on the order of 4 × 10 6 t yr −1 , and the estimated OC accumulation of 120 × 10 3 t C yr −1 accounts for ∼1‰ of globally buried OC in reservoirs. In addition, mass balance calculations revealed that approximately 70% and 90% of incoming total N and P, respectively, are eliminated from the water column by sedimentation (N, P) and denitrification (N). Since Lake Kariba attenuates flow from ∼50% of the ZRB, these OC, N, and P removals represent a drastic reduction in nutrient loadings to downstream riparian ecosystems and to the coastal Indian Ocean.

Natural hazard assessments are always subject to uncertainties due to missing knowledge about the complexity of hazardous processes as well as their natural variability. Decision-makers in the field of natural hazard management need to understand the concept, components, sources, and implications of existing uncertainties in order to reach informed and transparent decisions. Until now, however, only few hazard maps include uncertainty visualizations which would be much needed for an enhanced communication among experts and decision-makers in order to make informed decisions possible. In this paper, an analysis of how uncertainty is currently treated and communicated by Swiss natural hazards experts is presented. The conducted expert survey confirmed that the communication of uncertainty has to be enhanced, possibly with the help of uncertainty visualizations. However, in order to visualize the spatial characteristics of uncertainty, existing uncertainties need to be quantified. This challenge is addressed by the exemplary simulation of a snow avalanche event using a deterministic model and quantified uncertainties with a sensitivity analysis. Suitable visualization methods for the resulting spatial variability of the uncertainties are suggested, and the advantages and disadvantages of their implementation in an interactive cartographic information system are discussed.

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[1] Large reservoirs in the tropics act as efficient nutrient traps and often develop hypoxic conditions in the hypolimnion. Both effects may have severe implications for aquatic ecosystems, such as limited primary production in downstream riparian agriculture and in natural wetlands due to reduced nutrient loads, and, if hypolimnetic waters are withdrawn, hypoxic conditions that pose toxic risks in downstream rivers. This study using Itezhi-Tezhi Reservoir (Zambia) as a model system aims at defining optimized turbine withdrawal to prevent hypoxia and to relieve low-nutrient conditions in the downstream Kafue Flats floodplain. A biogeochemical 1-D model simulating reservoir-internal processes and water quality in the outflow was used for estimating dissolved oxygen (DO) concentrations and inorganic nitrogen and phosphorus loads in the outflow. The water depth of turbine withdrawals was varied in a set of simulations to optimize outflow water quality. Releasing hypolimnetic water was shown to result in lower average outflow DO concentrations of 4.1-6.8 mg l À1 compared to the current 7.6 mg l À1 . More importantly, the outflow will remain hypoxic during up to 189 days. Meanwhile, withdrawing nutrient-rich hypolimnetic water compensated effectively for nutrient losses to the reservoir sediment. Both outflow DO concentrations and nutrient output could be optimized in the scenario with 50% epilimnetic turbine discharge originating from $13 m depth. In this optimal scenario, hypoxia was prevented permanently, and average DO concentrations decreased moderately to 5.2 mg l À1 . Additionally, five-times higher dissolved inorganic N and dissolved inorganic P loads resulted in comparison to the current dam operation.

The current boom of dam construction at low latitudes endangers the integrity and function of major tropical river systems. A deeper understanding of the physical and chemical functioning of tropical reservoirs is essential to mitigate dam-related impacts. However, the development of predictive tools is hampered by a lack of consistent data on physical mixing and biogeochemistry of tropical reservoirs. In this study, we focus on Lake Kariba (Southern Africa), the largest artificial lake in the world by volume. Kariba Dam forms a transboundary reservoir between Zambia and Zimbabwe, and therefore its management represents a socio-politically sensitive issue because the Kariba Dam operation completely changed the downstream hydrological regime. Although Lake Kariba represents a unique and scientifically interesting case study, there is no consistent dataset documenting its physical and chemical behaviour over time. This limits the scope for quantitative studies of this reservoir and its downstream impacts. To address this research gap, we aggregated a consistent database of in situ measurements of temperature and oxygen depth profiles for the entire 60 years of Lake Kariba’s lifetime and performed a detailed statistical analysis of the thermal and oxygen regime of the artificial lake to classify the different behaviours of the lake’s sub-basins. We demonstrate that the seasonal stratification strongly depends on the depth of the water column and on the distance from the lake inflow. Satellite data confirm these spatiotemporal variations in surface temperature, and reveal a consistent longitudinal warming trend of the lake surface water temperature of about 1.5°C from the inflow to the dam. Finally, our results suggest that the stratification dynamics of the lacustrine sub-basins have the potential to alter the downstream Zambezi water quality. Future research should focus on assessing such alterations and developing strategies to mitigate them.