Aquatic Habitats

Rivers

Rivers and streams differ from other aquatic habitats in their physical characteristics (i.e., shape, substrate) and Hydrology, which is dominated by flowing water and often varies seasonally. 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).

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

Wetlands

Wetlands are areas where the water table is at or near the surface, or where the land is covered by shallow water for a long enough time 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.

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

Lakes

Lakes are defined as permanent water bodies with a surface area greater than 0.25 ha 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.

Estuaries

Estuaries form the interface between a river and the ocean and an estuary is always tidal, i.e. the water level fluctuates in response to the ocean tides. The combination of seawater and freshwater develop a high level of nutrients: estuaries are among the most productive natural habitats.