Lake Victoria

Front 1

Lake Victoria

Back 1

• Lake Victoria is one of the three Great Lakes of the African Rift Valley

• Lake Victoria is a huge and important ecosystem

• It also holds a world-famous example of adaptive radiation and at least 2 world-famous examples of disastrous invasive species

Front 2

Facts

Back 2

• Worlds largest tropical lake

• Worlds second largest freshwater body

• Divided politically one the ratio 6:45:49 between Kenya, Uganda & Tanzania

Front 3

Numbers

Back 3

• Surface altitude: 1135m

• Surface area: approx. 59,947km2

• Shore length: 3,300 km

• Mean depth approx. 40m and max depth approx. 80m

• Source of the river Nile (only outflow)

• ~33 million people live off this lake

Front 4

Formation of Lake Victoria

Back 4

• The lake is probably the youngest of the Eastern Rift Valley. Formed ~400,000 years ago

• Dried our and filled up at least 3 times since then

• Tectonic movements are thought to have changed direction of flow of the western rivers. This may have allowed Congo cichlids to spread into the lake in the past

• Very different to other African Great Lakes: on average only 14% as deep as Lake Malawi and only 7% as deep as Lake Tanganyika

Front 5

Water level

Back 5

• Changes in water balance have a great effect on water quality, biotic and abiotic factors

• Some changes are due to climate but mostly to human activity

• Frequent floods

• Precipitation accounts for 85% of the inflow

• Nutrient loaded rain has been linked directly to increase eutrophication a problem increased by the invasive Water Hyacinth

• Particularly vulnerable time abiotic and biotic changes because of lakes shallow depth

• In 2006, satellites recorded the lowest water level ever recorded

Front 6

The climate of Lake Victoria

Back 6

• Tropical Lake

• Mild

• Monthly average air temperature: 19°C and 25°C

• Daily average: 15°C to 30°C

Front 7

Wildlife

Back 7

• Hippopotamus

• African Clawless Otter

• Williams Mud Turtle

• Nile Crocodile

• Midges

Front 8

Aquatic Invertebrate

Back 8

• 4 freshwater crab species

• 1 shrimp species

• 28 snail species

• 17 bivalve species

Several more likely to still be discovered

Front 9

Schistosomiasis

Back 9

• Schistosomiasis affects 250 million people annually worldwide

• It is classed as one of the most damaging ‘neglected tropical diseases’

Front 10

Cichlids

Back 10

• Remarkable adaptive radiations and convergent evolution of very similar physiologies in two or more of the African Great Lakes

• All Lake Tanganyika cichlids are more closely related to each other than all Lake Malawi cichlids, yet have evolved remarkable similarities to species from the other lake

• Lake Victoria used to have approx. 500-600 endemic haplochromine cichlid species

• After the latest dry period, a few species entered the lake

• They then underwent the quickest speciation to fill different ecological niches ever observed in mature

Front 11

Cichlids

Back 11

• The Lake Victoria cichlids descend from hybrids resulting from the long-distant interbreeding of an Upper Nile and Upper Congo lineage

• This provides abundant genetic diversity that has been mixed and recombined repeatedly to greatly assist the subsequent and remarkable adaptive radiation generating 100s of different cichlid species

• Recently, an estimated 50% of cichlids vanished in the 1980s and they appear to be disappearing ten times faster than they can be described

• Descend from hybrids resulting from the long-ago interbreeding of an Upper Nike with an Upper Congo lineage

• This provided abundant genetic diversity that has been mixed and recombined repeatedly to greatly assist the subsequent remarkable adaptive radiation

Front 12

Fisheries

Back 12

Today’s fishery is dominated by 3 species:

1. Nile Perch

2. Nile Tilapia

3. Dagaa

Front 13

Human Population Changes

Back 13

• The collapse of locally available fish has coincided with a great increase in human population density around the lake

Front 14

Cichlids status

Back 14

• In 2019, IUCN lists 184 species as vulnerable, 52 as endangered and 106 as critically endangered

• At least 6 species are extinct in the wild

• Nile perch and invasive plants to be blamed

• By 1980, cichlid catch has declined by 80%

Front 15

Water Hyacinth

Back 15

• A major threat to the lake

• Native to South America, invasive to Lake Victoria

• Introduced in the late 1980s/ early 1990s

• Few local herbivores that consume it

• Abundant space and resources boost its growth, hence > exponential growth

Front 16

Water Hyacinth Effects

Back 16

• Choking Lake Victoria

• Changing the balance of water transpiration

• Promoting growth of other grasses

• Eutrophication (increased nutrients released due to rotting plant parts)

• Reduces penetration of light

• Slowing large boats and almost stops endemic fishing industry

• Affecting intake for power plants

• Increases prevalence of skin rash, cough, malaria, encephalitis, gastric-intestinal disorders and bilharzia/schistosomiasis

Front 17

Water Hyacinth: Control Effects

Back 17

• Water hyacinth weevil (feeds on it)

- First introduces to US as biological control agent in 1972

- Has been introduced into approx. 35 countries in total, to control the devastating effects of water hyacinth

Front 18

Water Hyacinth: Control Effects

Back 18

Physical extraction & removal. A weed-clearing machine can only clear approx. 10 hectares per day

Chemical methods. These cost Sudan at least £1 million annually, and are likely to have very damaging ecological side-effects

Biological Control

Introduce Hippos? This failed to pass through the US Congress by just a single vote in 1910

Front 19

Water Hyacinth

Back 19

• Hard to control

• Covered up to 10% of the lake at one point

• Image from NASA (2006) shows it was back as recently as 2006