An analysis of primary and secondary production in lake Kariba in a changing climate

Doctor EducationisAnalysis of temperature, rainfall and evaporation records over a 44-year period spanning the years 1964 to 2008 indicates changes in the climate around Lake Kariba. Mean annual temperatures have increased by approximately 1.5oC, and pan evaporation rates by about 25%, with rainfall having declined by an average of 27.1 mm since 1964 at an average rate of 6.3 mm per decade. At the same time, lake water temperatures, evaporation rates, and water loss from the lake have increased, which have adversely affected lake water levels, nutrient and thermal dynamics. The most prominent influence of the changing climate on Lake Kariba has been a reduction in the lake water levels, averaging 9.5 m over the past two decades. These are associated with increased warming, reduced rainfall and diminished water and therefore nutrient inflow into the lake. The warmer climate has increased temperatures in the upper layers of lake water, the epilimnion, by an overall average of 1.9°C between 1965 and 2009. The warmer epilimnion has led to a more stable thermocline in the lake, and its upward migration from a previously reported 20 m depth to the current 2 to 5 m depth reported in the lake’s Sanyati Basin. A consequence of the more stable thermocline has been the trapping of greater amounts of nutrients in the deep, cold bottom waters of the lake, the hypolimnion, and this coupled with a shorter mixing (turnover) period is leading to reduced nutrient availability within the epilimnion. This is evident from a measured 50% reduction in nitrogen levels within the epilimnion, with phosphorus levels displaying a much smaller net decline due to localised sources of pollution inflows into the lake. These changes in lake thermal dynamics and density stratification have reduced the volume of the lake epilimnion by ~50%, which includes the well mixed, oxygenated euphotic zone leading to more acidic waters (lower pH) and increased water ionic concentrations (conductivity), and decreased dissolved oxygen levels, which have resulted in a 95% reduction in phytoplankton biomass and a 57% decline in primary production rates since the 1980s. The reduced nitrogen levels especially have contributed to a proliferation of nitrogen-fixing Cyanophyceae, the dominant Cylindrospermopsis raciborskii comprising up to 66% of the total phytoplankton biomass and 45.6% of the measured total phytoplankton cellular concentrations. Also, shifts in seasonal dominance of different phytoplankton groups have been observed in the lake during turnover, the Cyanophyceae having increased in dominance from 60% of the total phytoplankton biomass in the early 1980’s to the current 75%. In contrast, the Bacillariophyceae have declined substantially, from 18% of the total phytoplankton biomass in the early 1980’s to the current 1.7%. The diminished phytoplankton biomass of more palatable phytoplankton, and the proliferation of smaller, less palatable phytoplankton taxa, has resulted in reduced zooplankton biomass and species richness and altered zooplankton species composition. Concentrations of large Cladocera and Copepoda especially have declined substantially in the lake by up to 93.3% since the mid 1970s, with small Rotifera currently comprising 64% of the total zooplankton biomass. The reductions in zooplankton biomass correspond with recorded decreases in catches per unit effort for the sardine Limnothrissa miodon (Kapenta), which have been steadily declining in the lake since 1986

Implications of a changing climate on the Kapenta fish stocks of Lake Kariba, Zimbabwe

The study investigated the influence of climatic variables (rainfall, temperature, and evaporation rates) and lake water levels on stocks of the sardine species commonly known as Kapenta (in Lake Kariba, Zimbabwe). Of concern is that climate changes may continue to adversely affect water levels, the stratification cycle, nutrient fluxes and fish production. Results suggest that both climate (maximum temperature in particular) and nutrients, which are influenced by water levels, are the primary determinants of Lake Kariba’s Kapenta production. Analyses showed that rainfall is decreasing at a rate of 0.63 mm per year, while evaporation rates have increased by 31% since 1963

Review of phytoplankton dynamics in tropical African lakes

Article #64This paper provides a synthesis of current knowledge on phytoplankton production, seasonality, and stratification in tropical African lakes and considers the effects of nutrient enrichment and the potential impacts of climate warming on phytoplankton production and composition. Tropical African lakes are especially sensitive to climate warming as they experience wide fluctuations in the thermocline over a narrow range of high water temperatures. Recent climate warming has reduced phytoplankton biomass and production in the lakes. A decline in the production of palatable chlorophytes and an increase in cyanobacteria has led to reduced zooplankton production and a consequent decline in fish stocks, all of which can be associated with the elevated water temperatures. This indicates that even moderate climate warming may destabilise phytoplankton dynamics in tropical African lakes, thereby reducing water quality and food resources for planktivorous fish, with consequent negative impacts on human livelihoods

Ecological Changes in the Zambezi River Basin

This research article was published in Council for the Development of Social Science Research in Africa, 2021.Africa faces a plethora of challenges and chief among these is a change in the climate (Zakaria and Maharjan 2014) which is one of the key factors affecting the ecology and hydrology of its river basins (Kusangaya et al. 2014). Beilfuss (2012) proposed that Africa’s arid regions are highly vulnerable to climate change with the Zambezi River Basin (ZRB) being particularly at risk (Kling et al. 2014). After the Nile and Niger rivers, the ZRB is the next most trans-boundary river basin in Africa as it serves eight African countries. Consequently, water resource development planning is crucial, since any changes in climate will impact the hydrological cycle and the amount of water retained in hydrological systems (Beilfuss 2012) of which only up to 3 per cent is readily available as usable and shared freshwater. Like some Sub-Saharan countries, which have experienced up to 0.5 C increases in temperature (Hendrix and Glaser 2007), the Zambezi River Basin is also facing changes in climate (Ndhlovu 2013). A recent study by Kling et al.(2014) reported rises in temperature and more variable precipitation in the basin since the 1980s. Such historical climatic changes, and those projected towards the mid-century (2050), are of concern with serious social and economic implications to local communities (Mubaya et al. 2012). The Intergovernmental Panel on Climate Change (IPCC) projected a global decadal temperature rise of 0.2 C (IPCC 2007). However, regional climat

City to city learning and knowledge exchange for climate resilience in southern Africa

This paper argues that learning is essential for cities to become resilient to challenges. The Future Resilience for African CiTies And Lands (FRACTAL) project is profiled. Following FRACTAL’s city-to-city learning approach of sharing good practices, knowledge and experiences framed around transdisciplinary research, the study cities of Harare, Lusaka, Windhoek and Durban conducted city learning exchange visits between 2017 and 2018. FRACTAL contributes towards climate resilient development by providing relevant climate information for decision-making at the city regional scale in southern Africa. Transferable lessons and practices included effective water conservation and waste management and the use of public-private partnerships.UK’s Department for International Development (DFID)Natural Environment Research Council (NERC) (2015-2019)START International’s Global Environmental Change (GEC) grants (2016-2017

A review of phytoplankton dynamics in tropical African lakes

This paper provides a synthesis of current knowledge on phytoplankton production, seasonality, and stratification in tropical African lakes and considers the effects of nutrient enrichment and the potential impacts of climate warming on phytoplankton production and composition. Tropical African lakes are especially sensitive to climate warming as they experience wide fluctuations in the thermocline over a narrow range of high water temperatures. Recent climate warming has reduced phytoplankton biomass and production in the lakes. A decline in the production of palatable chlorophytes and an increase in cyanobacteria has led to reduced zooplankton production and a consequent decline in fish stocks, all of which can be associated with the elevated water temperatures. This indicates that even moderate climate warming may destabilise phytoplankton dynamics in tropical African lakes, thereby reducing water quality and food resources for planktivorous fish, with consequent negative impacts on human livelihoods

Decline of Kapenta fish stocks in Lake Kariba : a case of climate changing?

This paper analyzes a 44-year data set of climate and lake water levels and a 29-year record of Kapenta fish stocks in Lake Kariba, Zimbabwe, in order to examine the relationships between environmental factors and determine whether they exert any influence on the Kapenta fish stocks. Changes in lake thermal properties have negatively affected plankton production and nutrient levels. The study brings a wealth of new knowledge to the understanding of the ecosystem from an ecological perspective, which can be used as part of an integrated water resources management framework

Sixty years since the creation of Lake Kariba: Thermal and oxygen dynamics in the riverine and lacustrine sub-basins

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 func-tioning 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 scientifi-cally 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 spa-tiotemporal 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.ISSN:1932-620