Science for sustainable reefs

This topic is sponsored by the Australian Research Council Centre of Excellence for Coral Reef Studies

A healthy coral reef is a thing of beauty and a wonder to behold. Coral reefs make a great holiday destination but for many of the world’s people they are so much more. Approximately 500 million people depend on coral reefs for food, coastal protection, building materials and income from tourism. This includes 30 million who are almost totally dependent on coral reefs for their livelihoods or for the land they live on (atolls). But this precious resource is under growing pressure and in serious decline.

Related site: Status of coral reefs of the world: 2008 Presents the current status of the world’s coral reefs, threats to reefs, and the strategies of the International Coral Reef Initiative. (Global Coral Reef Monitoring Network)

According to the report  Status of coral reefs of the world: 2008, 19 per cent of the world’s coral reefs have effectively been lost and a further 15 per cent are seriously threatened with loss within the next 10 to 20 years. In some regions the losses are significantly higher. The coral reefs of the Caribbean, for example, have declined in cover by 80 per cent in just the last three decades.

And what is threatening our coral reefs? Basically it’s us and our activities. Overfishing, pollution, disease and habitat destruction are some of the direct threats, but overarching and interacting with these are a suite of more serious problems connected with climate change – warming oceans, ocean acidification and rising sea levels.

Human activities are threatening coral reefs. By mid century, what were once healthy reefs (left) will have become slime-covered rubble (right) if carbon dioxide emissions are not curbed. (Image: J.E.N. Veron, A Reef in Time)

However, it’s not all bad news, especially in the short term. As scientists investigate how coral reefs bounce back after major disturbances (such as storms) they’re discovering that healthy coral reefs can have enormous resilience. Possibly our best preparation in the face of climate change is to focus on keeping our reef systems healthy and help them sustain themselves, but it may be that we cannot protect them all.

How are we affecting reefs?

Overfishing
Overfishing has led to the collapse of many of the planet’s biggest fisheries. While coral reef fisheries only account for around 2 to 5 per cent of global fisheries, they are vital to some of the poorest people in the developing world. Losing reefs deprives many of these people of their livelihood. Fishing pressure on many coral reefs has increased dramatically with the emergence of export markets for restaurant and aquarium trades based on rapid air transport. In places like the Coral Sea fishing pressure has grown rapidly in the past 20 years, and catches are declining.

Even within the Great Barrier Reef, regarded by many as one of the best managed reef systems in the world, overfishing is being experienced among some key fish groups. Populations of coral reef sharks, for example, are in the midst of a catastrophic collapse. Grey reef shark numbers have already dropped to around 3 per cent of unfished levels.

And when fish stocks take a battering, the coral reef is deprived of the ecosystem services provided by the fish. For example, removal of herbivorous fish can cause an increase in seaweed (algae) as it is no longer kept in check by grazing fish. The algae can then outcompete corals and overfished reefs become less resilient to disturbances such as large storms and bleaching events.

Runoff
Human activity at sea impacts on coral reefs but so too does human activity on land, as nutrients, sediment and pollution find their way to reefs in runoff.

Sediment in runoff smothers corals, prevents larvae from settling and blocks light (Image: Yusri Yusuf, ReefBase)

Nutrients, originating from agricultural fertilisers and soil, have a range of impacts. They promote higher concentrations of microscopic algae (phytoplankton) which support filter feeding organisms (like bivalves) that in turn compete with coral for space.

Algal blooms can also limit the light available to the reef community. This reduction in available light limits photosynthesis by seagrasses and symbiotic algae in corals, which in turn affects coral growth. Nutrients can also stimulate the growth of larger algae (eg, seaweed) which may overgrow coral and prevent it from re-establishing after a disturbance.

Sediment in runoff can smother coral reefs, prevent settlement of larvae and increase turbidity which blocks light from corals and seagrasses. Runoff also carries pesticide and herbicide residues and a range of urban pollutants out to the reef. There they can affect seagrasses, symbiotic algae in corals, and settlement of coral larvae which reduces the ability of coral populations to replace themselves.

Runoff can also cause problems simply by diluting the salinity of the water. When storm and flood events lower the salinity they can stress corals. The corals begin to exude large amounts of mucous and sometimes lose their symbiotic algae (zooxanthellae), causing bleaching.

Crown-of-thorns starfish
Outbreaks of this coral-eating pest are not necessarily correlated with human activities, but the ability of reefs to recover is.

The overarching menace of climate change
It now seems certain that coral reefs will be the first marine ecosystem to suffer extreme damage and possible collapse from climate change. The major consequences of increasing greenhouse gases on coral reefs include:

1. coral bleaching from warming oceans;
   
2. rising ocean acidification from dissolved carbon dioxide (CO₂);
   
3. more severe storms; and
   
4. rising sea levels.

Coral bleaching occurs when symbiotic algae are ejected from their coral homes. Repeated ocean-scale bleaching events have hammered home the message that warming oceans pose a critical threat to coral reef systems. The extreme El Niño event experienced in 1998 resulted in the most extensive coral bleaching and mortality ever recorded, with approximately 16 per cent of the world’s coral reefs being effectively destroyed (since then approximately three quarters of these have recovered). And in 2005 many coral reefs of the wider Caribbean were devastated when a series of major ‘hot-spots’ developed during the northern summer. The year 2005 was also a record hurricane year, which resulted in considerable loss of coral reefs.

But even if bleaching doesn’t occur, warmer waters have a range of other impacts as the  stressed corals become more vulnerable to disease.

Increasing concentrations of carbon dioxide in the atmosphere are raising the acidity of our oceans. If seawater acidifies by only a few tenths of a pH unit more, many corals, diatoms and shellfish will be unable to grow their skeletons and shells, posing the risk of extinctions and threats to marine food chains.

Rising sea levels are already encroaching on low lying islands and coastal regions, threatening people’s land, shelter and water supplies. However, compared to warming oceans and acidification, rising sea levels are likely to have less of an impact on the reefs themselves.

Sustaining our reefs

Healthy reefs look after themselves
In the past 50 million years, coral reefs have shown extraordinary resilience, coping with massive swings in climate and ocean level. Why is it then that many seem to be failing in the face of change now? The answer lies partly in the multiple stresses human activities are placing on reef systems, but the main reason is the speed of climate change we are undergoing.

Because of its size, not all parts of the Great Barrier Reef have been equally affected by human activities. The places affected the most are inshore areas, especially those influenced by deterioration in water quality from sedimentation, which has not affected most outer reefs. Outer reefs may also be more resilient to damage eg, from bleaching and crown-of-thorns starfish because the water in which they grow is in good shape. Inshore corals are prone to losing out to seaweed due to higher levels of nutrients and sediment, as well as coral bleaching caused by hot summers. The capacity of corals to recover from damage will be critical to their future in the face of climate change. While healthy, biodiverse reefs can be resilient to disturbances such as warmer water, multiple threats to reefs decrease their ability to bounce back (Box 1: Strength in diversity for coral reefs). We can reduce the impact of run-off through effective management of coastal development and catchment areas, and we can manage the effects of overfishing. However, in the longer term, the future of the Great Barrier Reef and other reefs of the world will depend on international initiatives to curb carbon dioxide emissions.

MPAs for a coral future
An effective way to reduce the stress of overfishing and maintain reef biodiversity is to establish marine protected areas (MPAs) in which fishing is restricted or banned. The Great Barrier Reef represents the most ambitious and possibly the most successful example of what networks of MPAs can achieve. In 2004 a strict no-fishing policy was imposed on a third of the reef area to form (what was at the time) the world’s largest network of no-take reserves. In just a couple of years there was a spectacular recovery in coral trout numbers on unfished reefs. In the longer term, the reserve system should enhance the sustainability of reef fishing and the reef ecosystem in general (Box 2: The Great Barrier Reef MPAs: how effective are they?). This in turn will provide greater protection for the tourism industry on the reef, which is worth in excess of A$5 billion.

Room for coral (with space for humans)
So if the problem is one of human impact, is the solution globally to simply remove the threats and place our healthy reefs in reserves? The answer is yes, but unfortunately there’s nothing simple about it. Human activity is an integral part of most of our coral reef systems and it can’t be turned on and off like a switch. Indeed many studies have shown that setting up no-take marine reserves that ignore the needs of the local population usually don’t work.

Take the Coral Triangle, for example, the world’s most biodiverse marine province. Millions of people depend on its reefs for their livelihoods, yet the region as a whole is deteriorating rapidly. The Coral Triangle Initiative, the biggest reef conservation movement ever, is attempting to reverse this by getting all countries involved to agree on measures to protect reefs. One of the guiding principles of the Coral Triangle Initiative includes the support of people-centred biodiversity conservation. As Professor Terry Hughes, Director of the Centre of Excellence for Coral Reef Studies states:

Coral species richness around the globe. Over 500 species are found within the Coral Triangle – around ten times the diversity of the Caribbean (Image: J.E.N. Veron, A Reef in Time).

The success of the reserve system in the Great Barrier Reef is in part due to the support built during its development. The Great Barrier Reef Marine Park Authority sought and gained the support of the public, industry and governments at all levels for putting the management of the world’s largest coral reef system onto an ecological footing. Backing all of this was the necessary legislation and regulatory powers and also good science to constantly inform the management process. Unfortunately, the example of the Great Barrier Reef is more the exception than the rule. Marine zoning in some countries has been severely limited because of poverty, inflexible institutions, lack of public support, difficulties developing acceptable legislation, and failures to achieve desired results even after zoning is established.

The future of the world’s coral reefs is looking grim on many fronts, and climate change will certainly redraw the coral distribution maps of the world. As this century unfolds, unless we curb our emissions and promote resilience in our reefs, corals are going to deteriorate to the point where we could lose them altogether.

Box 1 | Strength in diversity for coral reefs

We rely on reefs for coastal protection, fisheries and tourism. However, the true value of coral reefs is in their biodiversity, for they are the 'rainforests of the ocean'. When it comes to managing reefs, if we protect their biodiversity, their biodiversity should in turn protect us, by helping to make reefs a sustainable resource.
 
Ecosystems with many species are more stable over time. This is because, as conditions change, the system is more likely to have species that can cope with the new conditions. Having lots of species reduces the likelihood of a major ecological collapse. Some researchers believe that the range of species the Great Barrier Reef contains makes the system more robust overall. If conditions change, common species may become rare and rare species common, but the system as a whole survives. It’s very much in our interest to manage and care for the reef system so this keeps on happening.

The more researchers investigate what’s required to keep coral reefs healthy and resilient, the more they realise it’s about keeping coral systems intact and functioning. And that means more than just protecting the coral itself; like all ecosystems, reefs have a range of species that each play a role in maintaining the reef system. Many grazing fish, for example, are needed for a well-functioning reef. Worldwide the larger herbivorous fish are now the target of fish trappers, spear fishermen and gill netting. Scientists now know that these grazers are vital in keeping reefs clean, healthy and free of seaweed. Remove them and, if nothing else replaces them, you as good as remove the reef itself. Without the fish to mow the weed, it soon takes over from the coral.

And it’s not enough just having one or two species that do all the work. Some redundancy in the different groups that perform specific functions on a coral reef is important. This means that by having several different species that perform the same role, but respond differently to conditions, if one goes other species can still do the job. For example, in Caribbean coral reef systems, overfishing removed most of the grazing fish. Their function was then carried out by sea urchins for a while; when disease caused mass death among the urchins, a lack of other species to do the job meant that many of the reef systems collapsed.

Related sites

• Why is biological diversity important? (Australian Government Department of the Environment, Water, Heritage and the Arts)

• Coral reefs (Earthbeat, Australian Broadcasting Corporation)

• Resilience (Stanford University, USA)

Box 2 | The Great Barrier Reef MPAs: how effective are they?

It’s widely thought that networks of marine protected areas (MPAs) can play a large role in protecting and sustaining coral reef systems. There is evidence to support this, but to be effective the size and spacing of MPAs need to be carefully considered, particularly if they are to provide protection against threats associated with climate change.

In 2004 a third of the Great Barrier Reef was placed off limits to fishing to form the then world’s largest network of MPAs. Australian researchers have been studying what this has meant for certain fish stocks. They found that after only 1.5 to 2 years coral trout numbers rebounded by 31-75 per cent on a majority of reefs which had been closed to fishing. The researchers were amazed at the speed at which coral trout populations recovered – and also the sheer scale and consistency of the response. However, densities of coral trout on the reefs left open to fishers showed little or no change. In time, the higher fish populations on closed reefs may lead to improvements in fish numbers on open reefs, as juveniles from closed areas settle on open ones.

Recent research by marine biologists shows that even small reef fish, like baby clownfish, can travel as far as 35 kilometres between reefs. MPA networks can help sustain resident fish populations both by local replenishment and by fish larvae coming in from other neighbouring reserves. But networks of MPAs need to be quite large with individual reserves quite close to other reserves if they are to make a difference in the face of large regional disturbances like mass bleaching events. A survey of 66 sites in the Indian Ocean following a major coral die-off in 1998 showed that existing no-take areas may not be much help in enabling reefs to recover from major coral bleaching events. Many of these no-take areas were set up in the late 1960s and early 1970s to protect fish, before climate change and its impact on corals became a major issue. Several are small, and are surrounded by areas which are heavily fished or otherwise exploited. The researchers recommended that these existing zones should not be removed, but that new areas need to be added to them.

A major disturbance like bleaching can affect a huge area of the reef. If you have extensive and close reserve systems, then the chances are much higher they will contain organisms that escape bleaching which can help to recharge the reef as a whole.

Related sites

• Australian Research Council Centre of Excellence for Coral Reef Studies
• How do green zones on the Great Barrier Reef work? (web seminar)
• Nemo come home (web seminar)
• Protected fish stage a comeback

• The marine protected area (Stanford University, USA)

Activities

1. Experiment: The impact of catchment pollutants on aquatic species

Other activities

• ReefEd (Great Barrier Reef Marine Park Authority, Australia)
  • Managing the GBRMP – students gain an understanding of the processes that affect the Great Barrier Reef and the need for sustainable management practices. A link to the printable teaching unit is located at the bottom of the page. Suited to senior secondary students

• Reefs at risk – students learn about human-induced threats to the Great Barrier Reef and study ways in which it can be protected. A link to the printable teaching unit is located at the bottom of the page. Suited to students in years 8-10.

• Reef rescue – students complete a webquest in which they develop strategies to preserve Great Barrier Reef communities from rising sea levels due to an asteroid impact. Suited to middle school students.

• ARC Centre of Excellence for Coral Reef Studies
  
  • Educational Resources – provides multiple links to coral reef websites that are useful, scientifically accurate, and easy to navigate. They are loosely arranged by age group and topic and have been chosen for the wealth of useful links and information they contain. You will find lots of information and pictures for both school projects and general interest.
  • Coral Reef Futures – lists a collection of web-seminars recorded at the Australian Academy of Sciences. The talks present (to a general audience) the latest science which supports the sustainable management of coral reefs, in Australia, our region, and globally. Key issues include pollution, climate change, ocean acidification, fisheries management, conservation planning, capacity building, ecosystem-based management and adaptive governance.
• National Oceanic and Atmospheric Administration (USA)
• Save a reef! – students learn about the value of reefs, threats to reefs and design an information campaign on ways to reduce stresses on coral reef systems. Suited to middle school students.

• Protect this! – students learn about the design of marine protected areas to protect biodiversity. They then select a combination of ‘sites’ to protect, based on species abundance and location information.

• No fishing? – students complete a worksheet on marine protected areas in various countries using internet research.

• Coral Literature, Education and Outreach (CLEO) educational modules – provides teaching modules on coral spawning, coral bleaching and effects of carbon dioxide on coral reefs. Each module includes background information and student activities.

• Reef relief (USA)
• Coral reef teacher’s guide – presents comprehensive information on coral reefs, threats to reefs and ways to protect reefs. A large number of lessons for a range of age groups are included with printable worksheets.

Activity 1. Experiment: The impact of catchment pollutants on aquatic species

Many reefs worldwide are situated near catchment areas associated with agriculture, coastal development or mining activities. The high sediment, pesticide and nutrient loads in rivers draining from these catchment areas can affect many reef species including corals, fish, sea grasses and algae.

In groups of 4, design an experiment to test the effect of one of the following on microscopic algae and plant growth:

sediment
fertiliser
nitrate
phosphate

Remember you will need a control, replicates and hypothesis for your experiment.

The following equipment will be available:

4 x 250mL beakers or containers
digital balance
marker for labelling your beakers
Pasteur pipette
100 mL measuring cylinder
haemocytometer (if available)
aquarium pump and air tubing  (if available)
microscope
1.5 L pond water
sediment suspension
sodium nitrate (choose an amount between 1-5g/beaker)
calcium monohydrogen phosphate (choose an amount between 0.2-0.5g per beaker)
liquid fertiliser
4 x small pieces of pond weed eg. Elodea

Set up your experiment and place your group’s labelled beakers in a sunny position. The experiment should run for 3-4 weeks. At least once a week observe your beakers for:

• relative algal growth by comparing the colour of water in the beakers using a colour chart (you may wish to create your own chart)
• density of algae using a haemocytometer (if available)
• pond weed appearance/leaflet number

What did you find?

Present your results using clearly labelled diagrams, tables and/or graphs.

Answer the following questions:

1. What effect did your pollutant have on algal growth? Why?
2. What effect did it have on aquatic plant growth? Why?
3. How could you improve your experiment?

Although your experiment was conducted in fresh water, these pollutants can have similar effects in coastal waters.

4. Explain how this change in algal and/or plant growth in a coral reef might affect other reef species? Give at least two examples.

Use the information in Science for sustainable reefs to answer the following questions:

5. How do you think run-off would affect reef biodiversity?
6. Would it be necessary to worry about run-off if a reef was threatened by a greater problem of increasing water temperatures? Explain your answer.

Technicians’/Teachers’ notes

• Preparation of sediment suspension
  
Mix 250g clay and 250g sand in distilled water to make a 1L suspension. Provide each group with 100 mL of the sediment suspension. Instruct students to mix the suspension prior to using it. An aquarium air pump will help to keep the sediment in suspension.

• Students should use disposable gloves when handling the algal cultures that grow in their beakers.

• Treat algal cultures prior to disposing of them by adding bleach to the beakers to make a 1% solution. Sit culture overnight then pour treated solution down the drain.
  
  

Further reading

Australian Academy of Science
August 2010
The Science of Climate change: Questions and Answers.
A document summarising the current understanding of climate change science for non-specialist readers.

Australasian Science
July 2009, pages 32-34
Tropical fish follow Nemo south (by Peter Pockley)
Reports that tropical fish are successfully migrating to temperate waters in response to global warming.

Australian Academy of Science
August 2010
The Science of Climate change: Questions and Answers.
A document produced by the Australian Academy of Science that aims to summarise and clarify the current understanding of the science of climate change for non-specialist readers.

Useful sites

The ARC Centre of Excellencefor Coral Reef Studies (Australia)

Provides information on the Centre and its science programs for sustainable use and management of coral reefs. Includes research, latest news, web seminars, over 600 research articles, and links to education resources.
http://www.coralcoe.org.au

Australian Broadcasting Corporation

• GBR fish respond to fishing ban (The Science Show, 27 February 2010)
  Transcript of an interview discussing the large improvements in fish biomass due to no-fish zones on the Great Barrier Reef.
  http://www.abc.net.au/rn/scienceshow/stories/2010/2829737.htm

• Coral protector: Reefs keep our coast nice (Environment, 10 February 2010)
  Explains the value coral reefs in terms of their value to humans as well as the ecosystem services they provide.
  http://www.abc.net.au/environment/articles/2010/02/10/2815881.htm

• Protected coral reefs more resilient to environmental stress (The Science Show, 28 February 2009)
  Explains that coral reefs that are protected from overfishing and pollution are more able to cope with environmental stresses such as increasing temperature.
  http://www.abc.net.au/rn/scienceshow/stories/2009/2503500.htm

• The plea of the Great Barrier Reef (Ockham’s Razor, 6 April 2008)
  Discusses the impact of climate change on the Great Barrier Reef.
  http://www.abc.net.au/rn/ockhamsrazor/stories/2008/2207734.htm

• Saving Nemo (The Science Show, 12 January 2008)
  Describes how climate change is affecting coral, the effects of marine protected areas and shark research.
  http://www.abc.net.au/rn/scienceshow/stories/2008/2123829.htm

• Coral reefs (Earthbeat, 26 June  2004)
  Discusses the need for management strategies that encourage resilience of reef systems.
  http://www.abc.net.au/rn/science/earth/stories/s1139715.htm

The Great Barrier Reef: Designed to survive (built to last?) (Public lecture series 2006–07, Australian Academy of Science)

Transcript of a lecture by Professor Terry Hughes. Covers the evolutionary history of corals and reef fishes, threats to reefs and what we can do to help them survive.
http://www.science.org.au/events/publiclectures/os/hughes.htm

Marine climate change impacts and adaptation report card Australia 2009 (Climate Adaptation National Research Flagship, Australia)

Provides an overview of the impacts of climate change and adaptation options for protection of Australia’s marine environment.
http://www.oceanclimatechange.org.au

microdocs (Stanford University, USA)

Provides clear information and 2-4 minute videos on elements of sustainability, sustainability on coral reefs, species on coral reefs and solutions around the world.
http://www.stanford.edu/group/microdocs/index.html

Status of coral reefs of the world: 2008 (Global Coral Reef Monitoring Network)

Presents the current status of the world’s coral reefs, threats to reefs, and the strategies of the International Coral Reef Initiative to protect coral reefs globally.
http://www.gcrmn.org/status2008.aspx

Ocean acidification (Oceana)

Provides information on ocean acidification and a link to an extensive, clearly written report on the same topic.
http://www.oceana.org/climate/impacts/acid-oceans/

National Oceanic and Atmospheric Administration (USA)

• Corals
  A comprehensive tutorial on coral reef biology, threats to reefs and their protection.
  http://www.oceanservice.noaa.gov/education/kits/corals/lessons/corals_tutorial.pdf

• Coral reefs and oil spills: A guided tour
  Provides a basic overview of coral reefs, some of the threats they face including oil spills and bleaching, and ways that damaged reefs can be restored.
  http://response.restoration.noaa.gov/audience_subtopic_entry.php?entry_id=5&subtopic_id=27&audience_id=2

• The Coral Literature, Education and Outreach Program
  Provides up-to-date data, information and education modules on coral reefs.
  http://www.coral.noaa.gov/cleo/

The Great Barrier Reef Marine Park Authority (Australian Government)

• Great Barrier Reef outlook report 2009 - Full report
  An extensive report covering the state of the Great Barrier Reef, including its value, health, the pressures on the reef, management strategies, ecosystem resilience and its likely future.
  
http://www.gbrmpa.gov.au/corp_site/about_us/great_barrier_reef_outlook_report/full_report
• Principal water quality influences on Great Barrier Reef ecosystems
  http://www.gbrmpa.gov.au/corp_site/key_issues/water_quality/principal_influences.html#nutrients

• Management
  Describes the various roles of Commonwealth, state and local government in managing the Great Barrier Reef Marine Park.
  http://www.gbrmpa.gov.au/corp_site/key_issues/water_quality/management

• About zoning
  Explains the purpose and the types of zones in the Great Barrier Reef Marine Park.
  http://www.gbrmpa.gov.au/corp_site/management/zoning/about_zoning

Glossary

atoll. Circular shaped coral islands that either partly or completely surround a central lagoon.  Atolls form around submerged volcanic islands. They are typically low lying, so their human populations are susceptible to rising sea levels due to climate change.

biodiversity. Biological diversity. A measure of the variety of life. It is usually calculated from the number of species of organisms – although genera, families, classes and phyla can all be counted too. For more information see Biodiversity and its value (Australian Government Department of the Environment, Water, Heritage and the Arts).

bivalve. A mollusc with the shell in two parts hinged together as in mussels, scallops or clams. Bivalves are bilaterally symmetrical, have a reduced head and large gills that are used for gas exchange and filter feeding.

coral bleaching. Loss of colour of corals due to an environmental stress from such causes as increased water temperature, pollution or sedimentation. Environmental stress can cause corals to expel microscopic algae from their tissues. These symbiotic algae provide up to 90 per cent of the coral’s energy needs. Loss of these algae results in the bleached appearance of corals as they provide most of the coral’s colour. Bleached corals often starve then die if the stress persists. For more information see Coral bleaching – will global warming kill the reefs? (Nova: Science in the news).

diatom. Microscopic, single-celled algae that have a silica-based skeleton. Diatoms are found in a range of aquatic environments including freshwater, marine and moist soils.

El Niño-Southern Oscillation (ENSO). A sporadic climatic phenomenon that occurs because of changes in the usual atmospheric pressure patterns and in the sea surface temperature in parts of the Pacific Ocean. The results include the substantial reduction of the normal upwelling off the Peruvian coast, failure of the anchovy fishery in the same area, excessive rain in western South America, and droughts in Australia, parts of Asia and southern Africa.
 
ocean acidification. The lowering of the pH of oceans due to increasing levels of carbon dioxide. Ocean acidification affects the growth, structure and health of certain types of microscopic algae (phytoplankton), corals and other marine organisms with calcium carbonate skeletal structures. For more information see Acid test for the seas (Nova: Science in the news).

phase-shift. A shift from one type of ecosystem to another. Each has a distinctive set of species, and each can be highly persistent. A common example is a shift from ecosystems dominated by corals to degraded systems that are characterized by large amounts of seaweeds or other weedy species. Overfishing and pollution undermine the resilience of coral-dominated ecosystems, making phase-shifts more likely to occur.

phytoplankton. Microscopic, photosynthetic algae that live in water. Plant-like plankton.
 
resilience. The ability of an ecosystem, such as a coral reef, to cope with repeated external disturbances (such as a major warming of the ocean or a large storm) without fundamentally changing. Resilience is an aspect of ecosystem dynamics that prevents phase-shifts. Ecosystems with greater biodiversity are usually more resilient to disturbances.

rising sea levels. Increasing height of sea levels due to global warming. A warmer world will have a higher sea level because as the land and atmosphere warm, heat is transferred into the oceans. When materials are heated they expand (thermal expansion). So the heat that is transferred causes sea water to expand, which then results in a rise in sea level. In addition, water from land-based ice such as glaciers and ice sheets may enter the ocean, thus adding to the rise. For more information see Getting into hot water – global warming and rising sea levels (Nova: Science in the news).
 
sustainable. An activity that is capable of being maintained at a steady level without depleting natural resources or causing excess damage to an ecosystem. A sustainable yield in fisheries is achieved when the number of fish removed does not eventually cause the fish stock to collapse.

symbiotic. Describes a relationship between two species that live together, to the benefit of at least one of them. Some symbiotic species are unable to live without each other. Algae (called zooxanthellae) live within corals. The zooxanthellae gain protection, access to sunlight for photosynthesis and nutrients from the nitrogen waste that the coral produces. The coral benefits from the removal of wastes and from the sugars produced by the zooxanthellae.

turbidity. A measure of the suspended solids or cloudiness of water. Turbidity can be increased by sediment, often fine clay or silt that is discharged into the sea by rivers. Turbidity can also be caused by floating micro-organisms such as in an algal bloom. Increased turbidity reduces the penetration of light through water, reducing the growth of aquatic plants and corals. For more information see Turbidity (Waterwatch, Australian Government).