12
2 Overview of the Aquatic Ecosystem
oxygen to survive. The constant flow also prevents the settling of pathogenic larvae
of organisms such as mosquitoes on the surface of the water. Several aquatic and
terrestrial life-forms depend on the water cycle in the river. For example, some lifeforms such as the tadpole stage of frogs and the dragonflies begin life in the river
until they reach adulthood and transform to live on land or fly. Even in adulthood,
some of these organisms still depend on the water as a source of food and refuge.
Rivers are linked to streams, pools, water channels and floodplains. The life-forms
and structure of the abiotic and biotic components around a river vary at different
parts of the river. Figure 2.2 is an example of a stream flowing through the town
of Stellenbosch, South Africa. The water from this stream also serves as irrigation
water for the farms in the area.
2.2.4 Lakes
A large standing body of water surrounded by land is referred to as a lake. Over a
thousand lakes exist in the world. A lake is formed by the deposition of water from
rainfall, ice formation, river water and other factors which lead to water flow and
formation ending up in the lake. Since the lake is formed from these variable factors,
the level of water and conditions in a lake therefore varies at different periods as
water evaporates and is deposited. Lakes form a habitat for diverse life-forms such
as water plants, snails and fish. Waterbirds such as swans, ducks and geese are also
a regular feature in lakes, and these in turn serve as food for land animals as well as
aesthetics and recreation for humans. Example of lakes are the Lake Chad which is
surrounded by the seven Chad basin countries: Nigeria, Cameroon, Central African
Republic, Niger, Chad, Algeria and Sudan (FAO 1997), Lake Victoria in Tanzania
and numerous other lakes across the world.
The life in the lake is held in a sensitive balance between the photosynthesizing
life-forms and the decomposition of the organic matter. A lake is comprised of the
oxygen-dissolved zone, the light-penetrating zone and the dark oxygen-depleted
layer. These lake zones are classified as littoral, limnetic and profundal zones,
respectively (Table 2.3).
Some of the lakes in the world are drying up due to climate change, pollution and
some due to the natural cycle of a lake. Lake Victoria, the second largest freshwater
lake in the world, is possibly an example of such, which is located in Tanzania and
covering an area of over 69,000 km
2 as of 2018. It was reported to have shrunk
by 203 km
2 from 1984 to 2018 with 0.3% shrinkage in area (Awange et al. 2019).
Similarly, the Lake Chad has been reported to have shrunk from a surface area of
25,000 to 2000 km
2 (Mahmood and Jia 2019). This shrinkage of the lakes is attributed
to a combination of climate change and human activities.
2 Overview of the Aquatic Ecosystem
oxygen to survive. The constant flow also prevents the settling of pathogenic larvae
of organisms such as mosquitoes on the surface of the water. Several aquatic and
terrestrial life-forms depend on the water cycle in the river. For example, some lifeforms such as the tadpole stage of frogs and the dragonflies begin life in the river
until they reach adulthood and transform to live on land or fly. Even in adulthood,
some of these organisms still depend on the water as a source of food and refuge.
Rivers are linked to streams, pools, water channels and floodplains. The life-forms
and structure of the abiotic and biotic components around a river vary at different
parts of the river. Figure 2.2 is an example of a stream flowing through the town
of Stellenbosch, South Africa. The water from this stream also serves as irrigation
water for the farms in the area.
2.2.4 Lakes
A large standing body of water surrounded by land is referred to as a lake. Over a
thousand lakes exist in the world. A lake is formed by the deposition of water from
rainfall, ice formation, river water and other factors which lead to water flow and
formation ending up in the lake. Since the lake is formed from these variable factors,
the level of water and conditions in a lake therefore varies at different periods as
water evaporates and is deposited. Lakes form a habitat for diverse life-forms such
as water plants, snails and fish. Waterbirds such as swans, ducks and geese are also
a regular feature in lakes, and these in turn serve as food for land animals as well as
aesthetics and recreation for humans. Example of lakes are the Lake Chad which is
surrounded by the seven Chad basin countries: Nigeria, Cameroon, Central African
Republic, Niger, Chad, Algeria and Sudan (FAO 1997), Lake Victoria in Tanzania
and numerous other lakes across the world.
The life in the lake is held in a sensitive balance between the photosynthesizing
life-forms and the decomposition of the organic matter. A lake is comprised of the
oxygen-dissolved zone, the light-penetrating zone and the dark oxygen-depleted
layer. These lake zones are classified as littoral, limnetic and profundal zones,
respectively (Table 2.3).
Some of the lakes in the world are drying up due to climate change, pollution and
some due to the natural cycle of a lake. Lake Victoria, the second largest freshwater
lake in the world, is possibly an example of such, which is located in Tanzania and
covering an area of over 69,000 km
2 as of 2018. It was reported to have shrunk
by 203 km
2 from 1984 to 2018 with 0.3% shrinkage in area (Awange et al. 2019).
Similarly, the Lake Chad has been reported to have shrunk from a surface area of
25,000 to 2000 km
2 (Mahmood and Jia 2019). This shrinkage of the lakes is attributed
to a combination of climate change and human activities.