Sedimentary my dear flotsam: the challenge of sediment on coral reefs
When you imagine tropical coral reefs, beautiful and colourful pictures of tranquil yet thriving communities come into mind. Yet there are struggles that corals deal with every day you may not have considered.
Let’s first cover a bit of basic biology about corals. Corals are animals, made up from many tiny creatures called polyps. These polyps can grab food with their tentacles, and contain photosynthetic cells which can provide energy from the sunlight. These coral reefs are found mostly in warm tropical waters, in fairly sheltered areas where the stress of the waves won’t damage these fragile creatures. But animals living away from the destructive power of the tide don’t always have it easy; they face their own set of complications.
One of the major problems that puts coral reefs at risk is sedimentation, where bits of mineral or organic matter floating in the water slowly sink down and settle onto the corals. Because corals are found in very sheltered environments with slow-moving currents not much of the sediment is pulled away, but instead just piles up onto the corals. This may not sound very troubling, but if the sediment builds up it can cause all sorts of dilemmas.
Most corals need clear, clean water to survive, and large amounts of sediment can create difficulties. Build-ups of sediment can cause extreme problems, smothering the coral by making it harder for it to get oxygen from the surrounding water, restricting the light that it needs for photosynthesis by reducing water clarity, and preventing it from filter feeding by covering the polyps.
What causes sedimentation?
Some small amounts of sediment are always carried into coral reef systems by the rivers into the sea. The action of water flowing along a riverbank erodes away sediment, which is then suspended in the water, and carried towards the coral reefs, where it is deposited. This naturally happens, and corals have ways to cope with these small amounts.
In some cases, high levels of sediment only become a problem after a storm occurs, stirring up lots of material from the surrounding area and suspending it in the water, where it will drift back down onto the corals. In these cases the sediment is often deposited in very large volumes all at once, and so poses a real threat of smothering. Storms can adversely affect corals, but since they don’t happen often, the coral reefs can recover from these events.
However, levels of sedimentation onto coral reefs has been rising in some areas due to increased levels of erosion. Land use near the coast and rivers for agriculture or urban development means that the land is cleared of its natural vegetation which helped hold the ground together, and as a result the river can erode away more sediment to carry into the sea, and onto the coral reefs. For example, greater amounts of sediment have been transported onto the Great Barrier Reef in Australia since the settlement of Europeans in the country beginning in the late 1700s. This is a growing problem putting extreme pressure on corals to survive unnaturally high levels of sedimentation that they’re not used to.
So how do corals deal with this problem?
For starters, corals in very calm environments with lots of sediment often grow in particular shapes to prevent the sediment from settling on the coral’s surface. Corals come in many different shapes, and branched corals are often found in areas with high sedimentation. In contrast, corals in areas with low sedimentation often have flatter shapes.
Corals also come in funnel shapes, such as Acropora clathrata, a solid table coral. The sediment sinks, falls down the sloping sides, and is trapped in the funnel centre. The tissues where the sediment is concentrated are smothered and die, but the surrounding tissues remain healthy. This “self-sacrificial” method allows funnel-shaped corals to survive in calm, high-sedimentation conditions.
You may think of corals as creatures that don’t move a whole lot, but these extraordinary organisms are capable of some rather interesting tricks. Corals living in calm conditions have been observed to use different methods remove the smothering sediment such as pulsed inflation, movement of the coral through contraction and expansion of the polyps, which shakes off the sediment. Other methods are used, such as using wave-like contractions along the coral surface to remove sediment. The undulations which push sediment off the coral take a lot of energy, but it is essential to the coral’s survival to remove it.
Not a problem for all corals
High levels of sediment suspended in the water near to the shore from river run-off can block out the light, so most corals are found in clearer, deeper water. However, some corals have been found to tolerate more turbid waters (i.e. containing more sediment), as it can protect them from extreme light and heat that would damage the coral. UV rays can be very damaging to corals and can cause bleaching, so in environments with high UV irradiance (such as near to the water’s surface) some sediment in the water allows them to survive the more extreme conditions.
Can corals cope?
Corals have many adaptions to cope with natural sediment inputs, but human impacts are causing extreme stress that many corals cannot survive. Corals worldwide are in decline, and human activities around the coastline contributing to increased sedimentation are one of the causes for this. If the escalating devegetation of the coastlines, and the growing urbanisation/agriculturalization of land near to rivers continues, we may have to say farewell to one the most beautiful ecosystems on the planet.