Aquatic Habitats

Rivers

Rivers and streams differ from other aquatic habitats in their physical characteristics (i.e., shape, substrate) and hydrology, which are dominated by flowing water and often seasonal variation. As in lakes or wetlands, habitats and biological communities in rivers vary with depth or distance from shore, and in response to seasonal changes in the environment. Significant shifts in habitats and biological communities also occur over the length of rivers, due to the changing influence of riparian vegetation on shading and organic matter inputs as the river width increases (Wetzel 2001).

The distribution of fish and other aquatic organisms in rivers and streams depends on the environmental conditions they prefer or require. Oxygen levels in streams are usually sufficient for fish, and temperatures are generally similar at the surface and the bottom. Other habitat features, however – stream substrate, current strength, water depth, aquatic vegetation, and the presence of undercut banks, pools, or woody debris – can vary over relatively small distances within a watercourse, providing a range of habitats for different species (Nelson and Paetz 1992).

The Limpopo River begins at the confluence of the Marico and Crocodile Rivers in the Limpopo Province of South Africa, north to form the border with Botswana where it arcs east, where it is joined by the Shashe River to form the border with Zimbabwe. From here it flows down the Great Escarpment and east into Mozambique at Pafuri, across the coastal plateau to the Indian Ocean at Xai Xai.

The Olifants River originates near Witbank, to the east of Johannesburg, running east, across the southern portion of the basin, to join the Limpopo River in Mozambique just after Massingir Dam.

For a description of the hydrology of the Limpopo River basin, please refer to the Hydrology chapter.

Freshwater Ecoregions

In addition to developing terrestrial ecoregions for the planet, WWF have, in conjunction with The Nature Conservancy and a broad group of global partners, developed an assessment of Freshwater Ecoregions of the World. This assessment provides a biogeographic regionalisation of freshwater biodiversity on Earth (WWF/TNC 2008). The freshwater ecoregions for the Limpopo River basin are shown in the map below.

As can be seen from this map, the basin is dominated by the Southern Temperate Highveld ecoregion, with the southwestern portion of the basin in South Africa and a small fragment of Botswana covered by the Zambezian Lowveld.

Wetlands

Wetlands are areas where the water table is at or near the surface, or where the land is covered by shallow water for long enough to result in water tolerant vegetation and altered soils (Environment Canada 2009). Wetlands are neither truly terrestrial nor truly aquatic, and are often transition zones linking land and water environments. The water table that creates a wetland can arise from a regular unconfined aquifer close to the surface, or from a perched aquifer – a region of saturated rock created by a localised body of impermeable rock.

Wetland characteristics are determined by climate, topography and landscape, soils and geology, hydrology, vegetation, and human interventions.

Although there are some small, localised wetlands in the western portion of the basin, the greatest concentration is in the lower river basin, in Mozambique, in the Lower Limpopo and Changane sub-basins.

For more information about the distribution and role of wetlands ecosystems, please refer to the Wetlands section of this chapter.

Lakes

Lakes are defined as permanent waterbodies greater than 0,25 ha in surface area and more than 2 m deep. Worldwide, lakes are the largest reserve of surface fresh water (Kalff 2002). Lakes vary in morphological features, such as depth, extent of shoreline, basin shape, and basin geology. They also vary in their surrounding vegetation, climate, and river inflows and outflows. These characteristics influence the physical and chemical environment of a lake, which in turn affects its biological characteristics. Habitats and the distribution of aquatic organisms can vary significantly even within a single lake, depending on water depth, dissolved oxygen levels and light penetration, distance from shore, and lake bottom substrate.

With the exception of Lake Pave on the Lumane River, there are no natural lakes in the Limpopo River basin. However, the numerous dams provide aquatic habitats that to a large extent mimic those found in lakes – a large body of deeper water, with slow flow, temperature gradients and a different sediment and nutrient profile compared to rivers.

Estuaries

Estuaries form the interface between a river and the ocean and an estuary is always tidal: the water level fluctuates in response to the ocean tides. The estuarine zone of the Limpopo River is located at the river mouth, close to Xai Xai, in Mozambique. The estuary reaches through the dune-field at the coast and stretches several kilometres upstream. The estuarine environment of saline seawater and freshwater in this interface between terrestrial and marine environments stretches several kilometres upstream from the river mouth and is characterised by mangrove communities on both banks.

Due to the mixture of saline and freshwater, estuaries are home to highly specialised organisms that can exist in the dynamic environment that changes on a constant basis. Mangrove communities in particular often support significant biodiversity in plant, animal and insect life.

The image below shows the mangrove communities on the banks of the Limpopo River in the estuary.