Wetlands and Lakes II - Lake Victoria

In this post, I will be discussing ecosystem services delivery at a specific African location. I have chosen to focus on the lake-wetland system at Lake Victoria basin, East Africa. 

Location

Lake Victoria is located in East Africa and its catchment area spans across 6 countries. It is the largest lake in Africa and the second largest freshwater body in the world. Lake Victoria covers an area of 69,000 km2 with its total catchment area spanning over 180,000 km2. Located in the upper reaches of the Nile River, the only outflow of Lake Victoria is the Victoria Nile river within the complex White Nile river basin. Formed 400,000 years ago, the lake is formed by structural uplift around the basin and is one of the Great Lakes of Africa. 

Source

Ecosystem services

The Lake Victoria basin is characterized by a complex array of rivers, wetlands and satellite lakes which supports the livelihoods of over 25 million people and maintains hydrological balance of the Nile River system downstream. This interdependent relationship is a prime example of the balance between the different types of ecosystem services.

Population within the basin depends on rain-fed and flood recession agriculture from riparian wetlands and satellite lakes as well as hydroelectric power and commercial industry in and around the lake (Balirwa et.al 2003). Products and provisioning services of wetlands are largely understood and extensively exploited by local communities. These services include flood recession cultivation, wetland fisheries, grazing land and potable water resources. This results in Lake Victoria being Africa's largest and most important inland fishery source. However, the regulating services within the basin are often poorly understood/managed and taken for granted (Kansiime et.al 2007). A prime example of this is the water quality and buffer services provided by wetlands and satellite lakes. The presence of riparian wetlands around the lake determines transport characteristics of nutrients and sediments into the lake. Highly productive macrophytic vegetation in wetlands direct absorbs nutrients and filtrates any undesired releases into the lake. Wetlands also serves as a buffer for external impacts, increasing the ecological and hydrological resilience of the lake (Kassenga 1997). 

Threats

Despite the high reliance on provisioning services and importance of regulatory services, management in the Lake Victoria basin can be characterized as being highly extractive for the past 50 years. Overfishing, demographic change, expansion of urban settlements and expansion of intensive agriculture since the 1970s are the major threats to the capabilities of freshwater ecosystems to deliver ecosystem services (Odada et.al. 2009). The fisheries industry at Lake Victoria had been subjected to the introduction of more productive fish species in the early 20th century. Overfishing, increased nutrient runoff and a 'fishing down' process driven by population growth and food demands led to the depletion of native cichlid populations and the domination of the introduced Nile perch in the 1980s (Bilirwa et.al. 2003). The introduced Nile perch dominated over the previously cichlid-dominated ecosystem, leading to the deterioration in services provided by cichlid-phytoplankton feedback loops which previously regulated oxygen depletion. This had led to both a collapse in the fishery industry and a reduction in regulatory services when deep changes in food web structures were revealed and no more economically viable fish stocks were available.

The high variety of ecosystem services delivered by the lake-wetland system also caused dramatic demographic change with population growth within the basin being significantly higher than the rest of Africa (Fig.1). This has led to increased conversion of riparian wetlands into urban settlements and agricultural land, resulting in the loss of wetland vegetation and functioning. Loss of regulatory services from wetlands led to the deterioration of water quality and increased eutrophic status of the main lake due to industrial nutrient runoff and urban discharge (Odada et.al. 2009). Knock-on impacts to human well-being were experienced due to to fact that water abstracted from the lake for direct consumption and industrial use often undergo little to no additional treatment. 

Fig 1 Changes in population density between 1970 and 1990
- large scale conversion of riparian wetlands to urban settlements

Trade-offs

The economic valuation of ecosystem services are integral to the ecosystem services concept which also reveals the nature and inevitability of trade-offs. Trade-offs occur when management choices changes the relative magnitude and type of ecosystem services provided. Ecosystem services delivery are often in conflict with the need to meet increasing demands and the need to maintain ecosystem biodiversity and functioning. Large loss of forests and gains in agricultural land in upland regions and the increase in vegetation production and loss of intact wetlands in the lowlands led to increased food production and food provisioning services but was succeeded by a simultaneous decline in other provisioning services and the majority of regulating services (Swallow et.al. 2009). Furthermore, the introduction of invasive Nile perch dramatically boosted economic activity of the area initially but led to the overfishing and the eventual demise of native biodiversity and the fishery industry (McCauley 2006). 

Using a bio-economic model of the fishery industry including wetland-water quality interactions, Simonit and Perrings (2011) concluded that if wetlands in sub-basins were reduced by 60%, nutrient enrichment will lead to an expected loss of $1.98 mil USD/year and $216 USD per ha per year in fishery production. This reflects the inevitability of management decisions resulting in trade-offs among ES which may have varying economic implications. There should therefore be better monitored agricultural development along with informed and integrated water and land management practices in order to resolve conflict between development and ecosystem integrity.