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1.4.2 Human Impact on Modern Reefs

As said earlier, there is no single one living reef which does not have a human fingerprint on it. Actually some consider that at least since Christopher Columbus discovered America, there is pressure on these reefs. It is said that Columbus could anchor his ship in the lagoon and just walk across sea turtles which were so frequent that his foot did not become wet.

(## quotation from Jeremy Jackson in Coral Reefs, to be added and checked##)

However, human impact has increased so drastically that our tropical coral reefs probably will disappear rapidly unless reef and climate protection will be pushed as fast as possible. Let's have an overview of the hazards and threats on modern reefs. We will subdivide into various aspects of water pollution, reef quarrying, tourism, overfishing and global climate change.

1.4.2.1 Water Pollution in Reef Areas

Water pollution chiefly encompasses two fields, sediment pollution (i.e. disturbance by finely dispersed inorganic particles, and pollution by other substances, both of which interact.

a) Sediment pollution

Finely dispersed particles, i.e. muds, clays and silts, being washed into reef areas remain floating for long periods. The longest floaters are terrigeneous clays which may float up to five years in the water. They cause many disturbances which we will discuss in more detail when treating reaction of corals to sedimentation. Here is a brief overview:

  1. Dispersed particles cause turbidity, decreasing light availability for the symbionts of the corals. Corals will grow slower.
  2. Terrigeneous clays frequently carry ionic nutrient complexes and are accompanied by particulate organic matter such as plant debris. Both may cause overnutrification of reef waters (see below for malign effects)
  3. Finely dispersed matter lowers oxygen dissolution in waters. Corals need high amounts of oxygen.
  4. Fine material settling down on corals force corals to use a lot of energy to clean themselves. This energy is lacking then for the growth process. Corals have very different abilities to clean themselves (see ##ecology section)
  5. If fines are continuously, or at frequent intervals, deposited in the reef, hard substrates and hence attachment areas for coral larvae will disappear and corals and the entire reef may eventually be buried.

Here are a couple of sediment pollution causes. You might imagine others as well:

  1. Construction activities in cities and directly at coasts. This causes a lot of fines to be washed into the coastal seas.
  2. Coastal road works. Fines might be continuously washed in the sea unless roads will be covered by tar or other pavement
  3. Deforestation of tropical rain forests: Even if these chopped down forests are far away. It does rain a lot in a tropical rain forest (or what remained after getting all its timber) and soil erosion will be intensified after deforestation. This material eventually reaches tropical coastal areas and pollutes reef settings.

The occasional influx of coarser sediments normally does not cause such severe problems. Corals may as well grow on a siliciclastic cobble as on a carbonate substrate. Siliciclastic material shed by wadi floods into reef areas of the Red Sea may easily be used as new substrate by the corals. However, these siliclastic streams are mostly channeled through the reef (see ###reef and siliciclastic section). However, exclusive influx of coarser sands and cobbles is normally only possible in arid regions. Sands being introduced in more humid areas are normally accompanied by fines owing to intense chemical weathering.

will be continued and figs will be added....

for images on reef dangers and hazards see our preliminary slide show versions (pdf 780 kb)

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References for chap. 1

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last changes 25/11/02 by R. Leinfelder