Coral Reefs
Introduction
Types of coral reefs
There are eight main categories of corals: branching, corals with meandering ridges and valleys, massive or thick colonies, thin plates and crusts, solitary/isolated/free-living corals, coral with large, daytime expanded polyps, column corals and blue/fir/organ pipe/lace corals. Some examples of coral are found below:
- Coral Reefs support a large number of plants and animals.
- Coral Reefs are built by millions of tiny animals called polyps.
- Some coral polyp species receive more than 60% of their food from algae.
- Each coral colony begins life as a single polyp, which then reproduces itself as a single polyp, which then reproduces itself by budding or by dividing.
Types of coral reefs
There are eight main categories of corals: branching, corals with meandering ridges and valleys, massive or thick colonies, thin plates and crusts, solitary/isolated/free-living corals, coral with large, daytime expanded polyps, column corals and blue/fir/organ pipe/lace corals. Some examples of coral are found below:
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Spatial patterns
Spatial patterns - Where are coral reefs found?
Special conditions are needed for reef-building corals. Coral reefs will only grow in waters warmer than 18oC, and no deeper than 50metres. Therefore coral reefs are limited to clear, shallow tropical seas found either side of the equator. Corals may not develop properly in waters that receive freshwater runoff or sediments from rivers.
Location
Reefs tend to be most extensively developed along the eastern edges of continents which are bathed in warm equatorial currents.
Turbidity and salinity restrain coral growth. Muddy waters along coast lines at the point of the river mouths dampen coral reef growth.
A common place for coral reefs to grow is around volcanic islands in tropical waters. When the volcano first forms corals begin to accumulate on the upper part of the volcano in the shallow water. This process often forms fringing reefs attached to the side of the volcano (keep in mind that fringing reefs can also form in other locations). The coral continues to grow upwards and over time the volcano sinks. When the volcano has sunk the coral will form a coral ring with a lagoon in the centre. When the volcano has completely sunk below the water's surface, the no land can be seen it now becomes an atoll.
Altitude
Coral reefs are found below sea level, in shallow, warm water (temperature and light intensity are important factors in their growth).
Water temperature decreases with water depth an as such coral reefs tend to be limited to the sunlit waters of the upper 30m of water. The presence of photosynthetic zooxanthellae in the coral's flesh effectively limits the coral growth to the upper zone of water.
Latitude
Coral reefs are located in tropical and subtropical areas of the world with decreasing species diversity with increasing latitude (between the Toprical of Cancer and the Tropic of Capricorn).
Size
Coral reefs cover less than 1% of the oceans floor.
Shape
The shape of a coral reef is determined by the type of coral reef. Types of coral reefs and their shapes are as follow:
Barrier (Ribbon) Reefs: run parallel to the coastline in a ribbon shape. They create a barrier between the shoreline and the open sea.
Patch reefs: oval in shape and grow outwards
Fringing reefs: formations linked to the coastline
Atolls: ring-shaped coral islands
Coral Cays: various shapes - based on sand banks - can be changed/swept away by cyclones or by currents.
Continuity
Many types of coral reefs require a solid land mass for the ecosystem to develop. As such there is some coninuity around large landmasses, however in many island areas (e.g Indonesia, Pacific Islands), coral reef ecosystems can be separated by many miles from the next reef.
Barrier Reefs are rarely continuous - they have open channels, some of which can be extremely large.
Case study: Great Barrier Reef
- stretches 2000 kilometres along the east coast of Queensland from just above Fraser Island at latitude 24 degrees S to near the mouth of the Fly River in the Gulf of Papua New Guinea, at around 8 degrees S.
- covers 348,000 kilometres
- It is not a single structure, but a collection of over 2000 reefs, including barrier reefs, fringing reefs, islands and cays.
Case study: Coral Triangle
- located in the western Pacific Ocean
- includes the waters of Indonesia, Malaysia, Papua New Guinea, the Philippines, the Solomon Islands, and Timor Leste.
Dynamics of ecosystem functioning
Important interactions that allow coral reefs to function:
- shallow water (an interaction between the lithosphere and hydrosphere)
- warm water (an interaction between the hydrosphere and atmosphere)
- clear water (an interaction between the hydrosphere, lithosphere and human impacts)
- salinity (an interaction between the hydrosphere and lithosphere)
- high oxygen levels (an interaction between the hydrosphere and atmosphere)
Coral reef basics - symbiosis
Symbiotic relationships
Symbiosis is a long term relationship between two organisms. There are three types of symbiotic relationship: mutualism (where both organisms benefit), commensalism (where one species benefits, but there is no benefit or harm to the other species), and parasitism (where one organism benefits tot he detriment of the other). There are many examples of symbiosis on coral reefs.
Corals and zoozanthallae
The relationship between the corals and the zoozanthallae is beneficial to both. Corals provide the zoozanthallae with an environment suitable for survival. It is moist and the coral's waste gives energy to the zoozanthallae. Through the process of photosynthesis the zoozanthallae produce compounds that the coral use for food.
Clownfish and Sea Anemones
The Sea Anemones have tentacles with stinging cells. These stinging cells kill many organisms and it is in this way the anemones get their food. Clownfish hide in amongst the tentacles of the Sea Anemone, but are not harmed by them. In this way the clownfish are protected from other predators. Occasionally the Clownfish will catch food for the Sea Anemone.
Sharks and Remoras
Sharks sometimes get parasites which live on the external surface of the shark. Remoras are cleaner fish, and they attach themselves to the shark and kill the parasites. When the shark feeds the Remora are able to eat the scraps from the feed.
Watch the videos below, and take notes about the following symbiotic relationships:
- Christmas tree worms and porites corals
- Coral shrimps and corals
- Goby shrimp and Hawaiian Shrimp Goby
- Cleaner Wrasse and cleaning stations
Weather and climate
Temperature
- Areas have temperatures averaging up to 26 degrees Celsius. These temperatures and clear water are essential for coral growth.
- Reef building is optimal at 20 degrees celcius.
- Contain plants and animals adapted to tropical sun and extreme insolation.
- If the temperature is too low the coral can't produce limestone, while if it is too high the coral will expel the zooxanthellae.
Precipitation
- Heavy summer rains can lead to increased run-off and large amounts of sedimentation from mainland areas. These can suffocate corals and lead to algal blooms.
- Rainfall is fresh water. At times of heavy rainfall freshwater can pool on top of the heavier salt water. Any corals which are exposed at low tide will be exposed to an increase in fresh water and may result in coral deaths.
- Precipitation can increase the river flow of nearby rivers and as a result carry more sediment into the oceans.
Wind
- Wind has played a role in shaping exposed structures through wind erosion.
- During extreme weather events, wind and waves can break corals.
Tropical Cyclones
- Tropical cyclones, which are common features in these areas, can have a major impact on reef survival. They can result in heavy rainfall (increased freshwater), increased river flow (sedimentation entering the ocean), increased wind (coral breakage).
Geomorphic processes
Continental drift
The movement of the earth's plates and the associated rifting, have enabled coral to grow as oceanic plates have moved into tropical areas.
Subsidence
The rate of subsidence and sea level has determine reef growth over time. If the lithosphere subsides too far the water will be too deep for coral growth. If land subsides into the water it can create the right conditions for coral growth.
Most coral reefs consist of thick sequences of accumulated skeletal fragments of calcium carbonate (limestone). Life on the reef is confined to the surface of the massive limestone rocks. When corals and other organisms die their hard parts are added to the reef edifice. This results in upward and lateral growth of the coral reef over long periods of time.
Weathering and erosion
The calcium carbonate (or limestone) that the reef is made of is subject to chemical weathering. The limestone is broken down by carbonic acid formed partly from rain (freshwater) and carbon dioxide.
Waves and winds cause mechanical weathering of the corals. This is where parts of the corals are broken off, and then these broken pieces rub and crash against live coral causing more coral to break off.
Bioeroders, e.g. grazing fish, boring sponges and bivalaves, and algae contribute to the physical breakdown of the reef by biting bits of coral off or boring into the coral colonies, weakening them and eventually causing them to collapse or dissolve.
Hydrological processes
Wave action
The movement of water to form waves creates tiny bubbles of oxygen which are essential for the growth of corals.
Waves can break corals. During times of cyclones and storms waves can turn large areas of coral reefs into piles of rubble. It can take anywhere from 10-25 years for the reef to recover, depending on what state the reef was in.
Currents
Currents are responsible for much of the transport and deposition of sediment, plant life and seeds and marine migration of organisms.
Biogeographical processes
Invasion
Corals are prone to invade the territory of neighbouring colonies. Corals send out long stinging tentacles to search for and destroy the invader. Defensive mechanisms that corals use to ward off such attacks and the onslaughts of other organisms include armour, camoflauge, and toxic and unpalatable substances incorporated in their body tissues. These kind of mechanisms provide some defence for outbreaks of Crown of Thorns starfish and Drupella.
Most invaders on coral cays are plants.
Succession
When communities of plants and animals are replaced over time by a series od different and usually more complex communities, it is called ecological succession. Coral reefs are built by millions of tiny animals called polyps, which form enormous colonies.
Reproduction: Every year in late spring or early summer, the coral release their eggs and sperm in a spectacular synchonised event. The eggs that are fertilised by sperm begin to divide rapidly forming larvae.
On coral cays, succession is aided by wind. Butterflies are known to travel up to 60km to outlying islands, spores of ferns, mites, aphids and ticks can travel similar distances by wind. Beetles, lizards, etc and their eggs can be carried by violent storms. Bird droppings provide the nutrients for plants to colonise.
Resilience
For coral reefs in general, inertia and resilience are both low, so any minor changes in environmental conditions such as water temperature, turbidity levels, sedimentation and any forms of pollutant can place coral in a state where survival is unlikely.
The resilience of coral reefs comes predominantly from the wide range of genetic and species diversity within the ecosystem, and the time over which they have developed. Most reefs have developed over tens of thousands of years (most are between 5000-10 000 years old) and have been able to adapt to changing conditions. The resilience of ecosystems is threatened when reefs are separated from other reefs (by destruction of joining reefs). This restricts the ability of species to move to other areas. Sudden changes to the ecosystem (i.e. many human-induced changes) also restrict the ability of the ecosystem to adapt over time.
Many symbiotic relationships exist between species which add to the resilience of reefs.
Corals and peanut worm: keeps the coral upright and stops it from being smothered by sediment.
Shrimp can bore into the feet of Crown of Thorns starfish saving coral.
Sponges act as "vacuum cleaners" filtering out bacteria, detritus and coral mucus.
On the other hand, the symbiotic relationship within the reef can contribute to their vulnerability, as some species are highly specialised. A decline or local extinction of one species can have a detrimental impact on it's partner species.
Adjustments to natural stress
Sea level changes over time
Coral reefs have responded to changes in sea level. Keep-up reefs are those most suited to deal with changes to sea level at a particular time. Some reefs have not survived, while others are trying to survive under changed conditions.
Temperature change over time
In response to changes in temperature, coral reefs may be able to adapt to life in warmer sea. Some scientists believe that new species of coral will evolve to cope with the change (this is obviously dependent on the speed of the climate change). Fossil records show that corals have been subjected to warming trends many times and have survived. This does not take into account the added carbon dioxide in the atmosphere which is the result of this particular type of climate change.
Cyclones and extreme weather
The effects of cyclones and storms can devastate whole areas of coral reefs. The natural processes involved in breaking down dead organisms ensures that a basis is provided from the growth of new polyp colonies.
Wind and extreme weather events are a form of natural stress. Leeward sides of islands generally have more species diversity than windward sides due to wave energy and reef formations and structures.
Crown of Thorns Starfish
Crown of Thorns Starfish (COTS) are a naturally occurring species which at times increase in numbers and become a problem. They suffocate the corals and cause coral bleaching. In small numbers the reefs can recover from COTS, but in large numbers COTS can decimate whole reefs.
Fears crown of thorns starfish plague will wreak havoc on Great Barrier Reef
Case study: Great Barrier Reef
The northern part of the GBR is affected by the Trade Wind Drift, while the southern part of the reef is influenced by the East Australian Current. Movement of water brings in warm salty water from equatorial regions, contributing to biodiversity (movement of species is also helped by movement of water). The movement of water has also shaped the underwater landforms of the GBR.
In the ancient past the GBR was affected by tectonic movement of the continental plate. The types of corals present in the area would have first colonised the area as the plates were moving through warm tropical areas, and then moved southwards. GBR is now located in a tectonically stable environment.
Temperature
- Areas have temperatures averaging up to 26 degrees Celsius. These temperatures and clear water are essential for coral growth.
- Reef building is optimal at 20 degrees celcius.
- Contain plants and animals adapted to tropical sun and extreme insolation.
- If the temperature is too low the coral can't produce limestone, while if it is too high the coral will expel the zooxanthellae.
Precipitation
- Heavy summer rains can lead to increased run-off and large amounts of sedimentation from mainland areas. These can suffocate corals and lead to algal blooms.
- Rainfall is fresh water. At times of heavy rainfall freshwater can pool on top of the heavier salt water. Any corals which are exposed at low tide will be exposed to an increase in fresh water and may result in coral deaths.
- Precipitation can increase the river flow of nearby rivers and as a result carry more sediment into the oceans.
Wind
- Wind has played a role in shaping exposed structures through wind erosion.
- During extreme weather events, wind and waves can break corals.
Tropical Cyclones
- Tropical cyclones, which are common features in these areas, can have a major impact on reef survival. They can result in heavy rainfall (increased freshwater), increased river flow (sedimentation entering the ocean), increased wind (coral breakage).
Geomorphic processes
Continental drift
The movement of the earth's plates and the associated rifting, have enabled coral to grow as oceanic plates have moved into tropical areas.
Subsidence
The rate of subsidence and sea level has determine reef growth over time. If the lithosphere subsides too far the water will be too deep for coral growth. If land subsides into the water it can create the right conditions for coral growth.
Most coral reefs consist of thick sequences of accumulated skeletal fragments of calcium carbonate (limestone). Life on the reef is confined to the surface of the massive limestone rocks. When corals and other organisms die their hard parts are added to the reef edifice. This results in upward and lateral growth of the coral reef over long periods of time.
Weathering and erosion
The calcium carbonate (or limestone) that the reef is made of is subject to chemical weathering. The limestone is broken down by carbonic acid formed partly from rain (freshwater) and carbon dioxide.
Waves and winds cause mechanical weathering of the corals. This is where parts of the corals are broken off, and then these broken pieces rub and crash against live coral causing more coral to break off.
Bioeroders, e.g. grazing fish, boring sponges and bivalaves, and algae contribute to the physical breakdown of the reef by biting bits of coral off or boring into the coral colonies, weakening them and eventually causing them to collapse or dissolve.
Hydrological processes
Wave action
The movement of water to form waves creates tiny bubbles of oxygen which are essential for the growth of corals.
Waves can break corals. During times of cyclones and storms waves can turn large areas of coral reefs into piles of rubble. It can take anywhere from 10-25 years for the reef to recover, depending on what state the reef was in.
Currents
Currents are responsible for much of the transport and deposition of sediment, plant life and seeds and marine migration of organisms.
Biogeographical processes
Invasion
Corals are prone to invade the territory of neighbouring colonies. Corals send out long stinging tentacles to search for and destroy the invader. Defensive mechanisms that corals use to ward off such attacks and the onslaughts of other organisms include armour, camoflauge, and toxic and unpalatable substances incorporated in their body tissues. These kind of mechanisms provide some defence for outbreaks of Crown of Thorns starfish and Drupella.
Most invaders on coral cays are plants.
Succession
When communities of plants and animals are replaced over time by a series od different and usually more complex communities, it is called ecological succession. Coral reefs are built by millions of tiny animals called polyps, which form enormous colonies.
Reproduction: Every year in late spring or early summer, the coral release their eggs and sperm in a spectacular synchonised event. The eggs that are fertilised by sperm begin to divide rapidly forming larvae.
On coral cays, succession is aided by wind. Butterflies are known to travel up to 60km to outlying islands, spores of ferns, mites, aphids and ticks can travel similar distances by wind. Beetles, lizards, etc and their eggs can be carried by violent storms. Bird droppings provide the nutrients for plants to colonise.
Resilience
For coral reefs in general, inertia and resilience are both low, so any minor changes in environmental conditions such as water temperature, turbidity levels, sedimentation and any forms of pollutant can place coral in a state where survival is unlikely.
The resilience of coral reefs comes predominantly from the wide range of genetic and species diversity within the ecosystem, and the time over which they have developed. Most reefs have developed over tens of thousands of years (most are between 5000-10 000 years old) and have been able to adapt to changing conditions. The resilience of ecosystems is threatened when reefs are separated from other reefs (by destruction of joining reefs). This restricts the ability of species to move to other areas. Sudden changes to the ecosystem (i.e. many human-induced changes) also restrict the ability of the ecosystem to adapt over time.
Many symbiotic relationships exist between species which add to the resilience of reefs.
Corals and peanut worm: keeps the coral upright and stops it from being smothered by sediment.
Shrimp can bore into the feet of Crown of Thorns starfish saving coral.
Sponges act as "vacuum cleaners" filtering out bacteria, detritus and coral mucus.
On the other hand, the symbiotic relationship within the reef can contribute to their vulnerability, as some species are highly specialised. A decline or local extinction of one species can have a detrimental impact on it's partner species.
Adjustments to natural stress
Sea level changes over time
Coral reefs have responded to changes in sea level. Keep-up reefs are those most suited to deal with changes to sea level at a particular time. Some reefs have not survived, while others are trying to survive under changed conditions.
Temperature change over time
In response to changes in temperature, coral reefs may be able to adapt to life in warmer sea. Some scientists believe that new species of coral will evolve to cope with the change (this is obviously dependent on the speed of the climate change). Fossil records show that corals have been subjected to warming trends many times and have survived. This does not take into account the added carbon dioxide in the atmosphere which is the result of this particular type of climate change.
Cyclones and extreme weather
The effects of cyclones and storms can devastate whole areas of coral reefs. The natural processes involved in breaking down dead organisms ensures that a basis is provided from the growth of new polyp colonies.
Wind and extreme weather events are a form of natural stress. Leeward sides of islands generally have more species diversity than windward sides due to wave energy and reef formations and structures.
Crown of Thorns Starfish
Crown of Thorns Starfish (COTS) are a naturally occurring species which at times increase in numbers and become a problem. They suffocate the corals and cause coral bleaching. In small numbers the reefs can recover from COTS, but in large numbers COTS can decimate whole reefs.
Fears crown of thorns starfish plague will wreak havoc on Great Barrier Reef
Case study: Great Barrier Reef
The northern part of the GBR is affected by the Trade Wind Drift, while the southern part of the reef is influenced by the East Australian Current. Movement of water brings in warm salty water from equatorial regions, contributing to biodiversity (movement of species is also helped by movement of water). The movement of water has also shaped the underwater landforms of the GBR.
In the ancient past the GBR was affected by tectonic movement of the continental plate. The types of corals present in the area would have first colonised the area as the plates were moving through warm tropical areas, and then moved southwards. GBR is now located in a tectonically stable environment.
Human-induced modifications
Coral Bleaching
Vulnerability and resilience of ecosystems is determined by factors such as location, extent, linkages and diversity. Climate change and associated ocean acidification and increased ocean temperatures are resulting in widespread coral mortalities, The ability of corals to return to close to their natural state is determined by how resilient the reefs remain. A resilient reef will either be able to resist a bleaching event (i.e. not bleach to any great extent) or recover from it.
The changes in temperature and associated bleaching are resulting in a different mix of species on the reef. This will impact reefs in the long term.
Loss of species
Fish, whales, dolphins, sharks, rays and the many other organisms found in reefs rely on the complexity of the ecosystem for survival. Some fish rely on the colour of the corals for camoflauge and the structure of the coral for hiding. Many organisms are unable to carry out normal functions and processes as a result of the increased ocean acidification associated with climate change. Shellfish are less able to create their shells due to increased pH. Slow growing corals will take 100-200 years to recover, meaning that the reef will not exist in the form that we have known it in the past.
Dispersal of spawn
Ocean warming impacts on the dispersal or coral spawn (eggs). Increased ocean temperatures result in a decline in the dispersal distance of coral spawn from the origin (parent coral) to the destination site. This change in dispersal patterns can impact on species' distribution, abundance or corals in particular areas and genetic diversity across reefs. Changes to dispersal patterns can also impact on the connectivity (interconnections) between different areas of the reef by limiting the areas of reef that particular coral species are located.
Poleward shift of species
Ocean warming can also result in a poleward shift of species from tropical zones to more temperate zones. Warmer waters are found further from the tropics and species are able to take advantage by increasing their range.
In Western Australia, a species of wrasse - cheorodon rebuscens has started to shift its range with displacement of recruits south of its usual habitat. There is evidence of high recruitment at the temperate edge and no recruitment at the tropical edge. The range shift provides limited expansion opportunities, reducing resilience of the species.
Irukandji are migrating further south on the Great Barrier Reef as a result of warmer waters and are also having longer seasons in other areas. There have been anecdotal reports of increases in reports of stinging and hospitalisations on islands within the Great Barrier Reef (e.g Fitzroy Island) and snorkellers are being strongly advised to wear stinger suits outside of usual peak Irukandji seasons. Irukandji and associated stingings have also been reported on western side of the southern tip of Frazer Island where they haven't previously been found.
Symbiotic relationships
Following bleaching events or even natural disasters, corals can become overgrown with algae, making it difficult for coral recruits to settle and grow. The mix species on a reef can impact on how resilient that particular reefs is. For example in Moorea in French Polynesia experienced high coral mortality in the 1980s. Recovery of the reef was enabled in part as a result of grazing fish such as parrot fish removing some of the algae in the process of eating corals. In this way the biodiversity of the reef contributed to high levels of resilience on the reef in comparison to some other reefs globally. The scale of the bleaching on GBR make it unlikely that these types of natural processes will have much of an impact on recovery.
Other human impacts
Turbidity
Turbidity and salinity restrain coral growth. Muddy waters along coast lines at the point of river mouths dampen coral reef growth in 2 ways: sediment particles interfere with the ability of zooxanthellae to photosynthesise by decreasing the amount of light in the water. Secondly, sediment may be deposited on top of the coral itself burying them, or preventing them from feeding.
Sewage
Sewage outflows can impact negatively on reefs by introducing high levels of phosphorous and encouraging algal growth. Eutrophication can result in extensive algal blooms and reduced levels of invertebrates. Rates of coral morbidity are increased due to smothering. It can also result in a shift from an area with hard coral species to softer corals and fleshy algaes. The severity of the impact of sewage can be determined by ocean currents, tides and flushing rates.
Fishing
Pollution
Marine debris (human generated litter) in our marine environment is one of the major pollutants of oceans. Plastics such as shopping bags, containers, bottle, discarded nets etc result in the death of many animals.
Chemical runoff.
Oil spills
Dumping
Discharge from different buildings and industries.
Agriculture
Cattle grazing, urban development and land clearing - increased erosion and sediments in the water block out sunlight for coral and fish.
Feed and faeces from tuns pens can cause marine pollution as well as extensive damage to the sea floor.
Vulnerability and resilience of ecosystems is determined by factors such as location, extent, linkages and diversity. Climate change and associated ocean acidification and increased ocean temperatures are resulting in widespread coral mortalities, The ability of corals to return to close to their natural state is determined by how resilient the reefs remain. A resilient reef will either be able to resist a bleaching event (i.e. not bleach to any great extent) or recover from it.
The changes in temperature and associated bleaching are resulting in a different mix of species on the reef. This will impact reefs in the long term.
Loss of species
Fish, whales, dolphins, sharks, rays and the many other organisms found in reefs rely on the complexity of the ecosystem for survival. Some fish rely on the colour of the corals for camoflauge and the structure of the coral for hiding. Many organisms are unable to carry out normal functions and processes as a result of the increased ocean acidification associated with climate change. Shellfish are less able to create their shells due to increased pH. Slow growing corals will take 100-200 years to recover, meaning that the reef will not exist in the form that we have known it in the past.
Dispersal of spawn
Ocean warming impacts on the dispersal or coral spawn (eggs). Increased ocean temperatures result in a decline in the dispersal distance of coral spawn from the origin (parent coral) to the destination site. This change in dispersal patterns can impact on species' distribution, abundance or corals in particular areas and genetic diversity across reefs. Changes to dispersal patterns can also impact on the connectivity (interconnections) between different areas of the reef by limiting the areas of reef that particular coral species are located.
Poleward shift of species
Ocean warming can also result in a poleward shift of species from tropical zones to more temperate zones. Warmer waters are found further from the tropics and species are able to take advantage by increasing their range.
In Western Australia, a species of wrasse - cheorodon rebuscens has started to shift its range with displacement of recruits south of its usual habitat. There is evidence of high recruitment at the temperate edge and no recruitment at the tropical edge. The range shift provides limited expansion opportunities, reducing resilience of the species.
Irukandji are migrating further south on the Great Barrier Reef as a result of warmer waters and are also having longer seasons in other areas. There have been anecdotal reports of increases in reports of stinging and hospitalisations on islands within the Great Barrier Reef (e.g Fitzroy Island) and snorkellers are being strongly advised to wear stinger suits outside of usual peak Irukandji seasons. Irukandji and associated stingings have also been reported on western side of the southern tip of Frazer Island where they haven't previously been found.
Symbiotic relationships
Following bleaching events or even natural disasters, corals can become overgrown with algae, making it difficult for coral recruits to settle and grow. The mix species on a reef can impact on how resilient that particular reefs is. For example in Moorea in French Polynesia experienced high coral mortality in the 1980s. Recovery of the reef was enabled in part as a result of grazing fish such as parrot fish removing some of the algae in the process of eating corals. In this way the biodiversity of the reef contributed to high levels of resilience on the reef in comparison to some other reefs globally. The scale of the bleaching on GBR make it unlikely that these types of natural processes will have much of an impact on recovery.
Other human impacts
Turbidity
Turbidity and salinity restrain coral growth. Muddy waters along coast lines at the point of river mouths dampen coral reef growth in 2 ways: sediment particles interfere with the ability of zooxanthellae to photosynthesise by decreasing the amount of light in the water. Secondly, sediment may be deposited on top of the coral itself burying them, or preventing them from feeding.
Sewage
Sewage outflows can impact negatively on reefs by introducing high levels of phosphorous and encouraging algal growth. Eutrophication can result in extensive algal blooms and reduced levels of invertebrates. Rates of coral morbidity are increased due to smothering. It can also result in a shift from an area with hard coral species to softer corals and fleshy algaes. The severity of the impact of sewage can be determined by ocean currents, tides and flushing rates.
Fishing
- Sea floor trawling ruins marine habitats, destructive trawling practices.
- Overfishing
- Modern fishing equipment increases the number of fish caught.
- Fishing nets become "dolphin and turtle graveyards", because animals become trapped in the nets and die. This has implications for the whole ecosystem.
- Any physical contact with anchors, feet, fishing nets or diving gear can result in sections of coral being broken and destroyed.
Pollution
Marine debris (human generated litter) in our marine environment is one of the major pollutants of oceans. Plastics such as shopping bags, containers, bottle, discarded nets etc result in the death of many animals.
Chemical runoff.
Oil spills
Dumping
Discharge from different buildings and industries.
Agriculture
Cattle grazing, urban development and land clearing - increased erosion and sediments in the water block out sunlight for coral and fish.
Feed and faeces from tuns pens can cause marine pollution as well as extensive damage to the sea floor.
