African Wildlife & Environment Issue 76 FINAL

CONSERVATION

floats on top of saline water, and will not easily mix. This is a fundamental driver of the ocean currents of the planet, where fresh water flows into the sea, and creates areas of less dense water floating above masses of denser saline water. In technical terms, these two types of water are separated by a ‘halocline’. This can be seen if a scuba diver goes through a 'thermocline' (temperature-derived where warmer water is less dense than colder water, so the two separate across a clearly defined boundary) that also coincides with a halocline. Often denser saline water is also colder than less dense fresh water.

Image 3 : 24 Februar y 2016

But because microbiota can flourish, the soil is also more friable, so more of the rain infiltrates the ground to become soil water. It is soil water, present in the root zone of plants, that sustains ecological processes more than anything else. Once that water goes deeper than the roots can penetrate, it becomes groundwater, eventuallyflowing intoaquifers. Adesiccated landscape thus delivers a four-fold whammy in that groundwater infiltration is reduced, soil temperature is increased, surface soil erosion is common, and fertility declines as microbiota die in hot desiccated sand. But what happens when this surface water flows beyond the point where it originally fell? It is in this

If one looks along the halocline, light is distorted much the same as a mirage gives the impression of pools of water in a desert. It is inthisregardthatestuariesareimportant,because it is here that freshwater meets saltwater ecosystems. It is also in estuaries that biodiversity is spawned. Estuaries provide highly varied habitat in a relatively small area, so these are ideal spawning grounds for fish that later migrate out to sea. Typically, estuaries have a mud bank created by the tides, sometimes associated with mangroves. This mud is rich in organic nutrients and the tidal pulse determines behaviour of the living things that call this home.

regard that the storybecomes interesting because of physics and chemistry. Fresh water differs fundamentally from saltwater in many ways. The obvious differentiator is that fresh water is not saline, but saltwater is. Herein lies a myriad important aspects we need to understand. Salinity is the relationship between a water molecule and a salt molecule. Salt is a characteristic of an arid environment. Think of a salt pan in the Kalahari or Karoo. But salt is hydrophilic, so it attracts water. Now think of a given volume of salt in a defined volume of water – say a teaspoon of table salt in a one litre jug of water. All the salt will dissolve, so if poured out nothing will be left behind in the container. However,

Image 4 : 13 Januar y 2019

if the container is left in the desiccating sun, only the water will evaporate, but no salt will leave the solution. Salinity is thus a ratio of fresh water to salt within a given system. And it is right there that things get interesting, because of the physics of water. Saline water is denser thanfreshwater, so it sinks. Stateddifferently, freshwater

However, the fresh and saltwater remain separate in estuaries, because of the physics and chemistry noted above. Consequently, we see a wedge of saline water penetrating upstream along the benthic zone of the river – the bed where land meets water – with an opposite wedge of fresh water floating above. This wedge pulses with each tide, moving back and forth over the riverbed,

9 | African Wildlife & Environment | Issue 76 (2020)