Effects of Climate Change on Volta Lake Resources (VOLTRES)

Project summary

Africa is very likely to experience higher temperature increases than the rest of the earth during the 21st century. Extreme rainfall events are predicted to increase for West Africa. Climate change (CC) will influence the ecosystem in lakes by changes in the physical (water temperature, stratification, transparency), chemical (nutrient loading, oxygen) and biological (structure and functioning of the ecosystem) components. The Volta Lake is the main source of inland fish in Ghana, a source of livelihood for over 1,200 riparian communities and the main medium for cage fish farming in Ghana. The fishery of the Lake is heavily exploited and CC is expected to worsen the situation. VOLTRES will assess the impacts of CC on hydrology, water quality, primary production and fish production of the Lake and improve the understanding of the ecosystem functioning through a combination of field data sampling with empirical biophysical and mechanistic ecological modeling of the catchment-lake system. The river basin and lake ecosystem models will be validated using existing data as well as data generated through monitoring. Results, lessons, skills and recommendations from the project will be disseminated through stakeholder workshops, seminars, policy briefs, and journal publications. The output will contribute significantly towards addressing the major needs identified in the Ghana National CC Adaptation Strategy in the design and implementation of programmes on fishery management.

Outputs

Project completion report:

Average air temperature in the Volta Basin (VB) has increased by about 1.6°C in the historic period (1961 – 2005), with an average increment of about 0.35°C/decade. This is expected to further increase by about 1.4°C - 2.5°C and 2.2°C - 3.2°C, for the IPCC medium-low (RCP4.5) and high (RCP8.5) emission scenarios, respectively. Contrary to air temperatures, annual rainfall amount in the VB has reduced significantly (average of 5 mm/year) over the same historic period. However, decadal analysis suggests the reducing trend in the annual rainfall came to a halt in 1983 and some recovery started from 1984 to date, though the recovery so far is not significant. The trend and magnitude of future rainfall in the basin is uncertain but an ensemble mean of +2% (not significant) is projected for the future. Based on the projected air temperatures and rainfall for the basin, river discharge into the Volta Lake in the future is projected to reduce, on average, between 20% and 27% of the historic discharge. Future annual sediment load into the lake is projected to decrease, on average, between 4% and 15% of the historic average load. Based on the climatic condition in the VB, and insights from the project’s biophysical model of the lake, Lake Volta stratifies and mixes twice in a year (January-February and July-August). In each mixing event, phytoplankton (algae) biomass temporarily increases as a result of increased nutrients in the surface waters. However, the production of algal biomass in the Lake is limited by the availability of light, due to suspended solids in the water column, than nutrient loading. Considering a projected reduction of suspended sediment into the lake, the challenge of light limitation is likely to reduce. The increasing temperature in the VB will intensify stratification of Lake Volta which could reduce the amount of oxygen available to the fish community and other organisms. The important fish species in order of abundance were: Silver catfish (Chrysichthys), Tilapias, Silversides (Alestes) and Bayad (Bagrus) and Squeakers (Synodontis) Water level appeared to be a major environmental variable influencing the structure of fish community. A reduction in river discharge, and consequently lake water level, as projected by this study and which is expected to occur mostly in the peak period of August-October, coincides with the spawning of some important and commercial species of fish (e.g. Aletes, Hydrocynus). This could lead to low productivity and profound impacts on the Lake fisheries. The projected increase in ambient temperature in the basin appears to have no significant effect on fish productivity as it translates to lake water temperature that is still within the optimum ranges of water temperatures (22 °C–35 °C) for survival of major fish species.

Brief popularized abstract:

The study determined how climate change will impact the hydrology, nutrient processes and fish productivity of Lake Volta. Our findings show that the basin is warming and expected to be warmer in the future. The rainfall pattern has become more uncertain and erratic. The combined effect is a reduction of river discharge to Lake Volta during the peak inflow period, which coincides with spawning of fishes. The reduced inflow is likely to result in shrinkage of the flood plains for fish reproduction, affecting fish productivity of the lake. Also, warmer temperatures will strengthen the partitioning of the water column and increase the depth of de-oxygenated layer, reducing the area suitable as fish habitat. This could cause massive fish kills. Major challenges include lack of long term data (e.g. fish catch, water temperature, dissolved oxygen, nutrients); data gaps; non-availability of some data (e.g. fish species composition). VOLTRES influenced decision makers and managers to incorporate climate change considerations in policies and plans for managing fishery resources. The project has improved the understanding of the occurrences of intermittent fish kills in Lake Volta. The lake is dynamic and there is the need for a comprehensive characterization for effective use and management