Extra for Examwizard Topic 1 Quesion Biology
Content
In a naturally occurring ecosystem, vegetation occurs in a patchwork of different community types and age classes (or seral stages). This diversity is important for the health and sustainability of the landscape. These different age classes and their arrangement on the landscape also provide varying uses for humans and for plant and animal communities. The patches occur in an array of shapes, sizes, and arrangements. This complex matrix (or mosaic) is in a constant state of dynamic equilibrium. Natural disturbances such as drought, fire and disease continually modify the landscape by resetting the vegetation cycles. When developing management strategies it is important to produce or enhance this complex vegetation mosaic
Seral stages
seral stages (Science: botany, ecology) The series of relatively transitory plant communities that develop during ecological succession from bare ground to the climax stage.
A seral community (or sere) is an intermediate stage found in ecological succession in an ecosystem advancing towards its climax community. In many cases more than one seral stage evolves until climax [1] conditions are attained. A prisere is a collection of seres making up the development of an area from non-vegetated surfaces to a climax community. A seral community is the name given to each group of plants within the succession. A primary succession describes those plant communities that occupy a site that has not previously been vegetated. These can also be described as the pioneer community. Computer modeling is sometimes used to [2] evaluate likely succession stages in a seral community. Depending on the substratum and climate, a seral community can be one of the following: Hydrosere Community in water Lithosere Community on rock Psammosere Community on sand Xerosere Community in dry area Halosere Community in saline body (e.g. a marsh) Dune Plant Communities - Determining the Distribution, Abundance and Dispersion of Organisms Based on Cover Field ecologists are usually unable to measure the entire population of organisms or variables being studied. For example, if we wanted to know the average length of Batillaria attramentaria in the Elkhorn Slough we would NOT measure all of the snails in the population. Instead, ecologists usually depend upon sampling techniques to reduce the number of measurements that must be taken. The basic idea of all sampling techniques is to measure some of the individuals in a population but to be able to make inferences about all the individuals in the population of interest. Sampling techniques vary with the nature of the question being asked and the population sampled, but desirable characteristics of all sampling techniques are that they are:
1) Unbiased; the sample should be representative of the population as a whole. For instance, in sampling our snails we would not want to use a technique that did not sample small snails. 2) Repeatable. 3) Logistically feasible in the field. In this lab we will use two different sampling techniques to: (1) estimate the percent cover of plants and substrates in two different coastal dune communities, and (2) determine if sampling methodology affects estimates of populations. Analysis of Vegetation There are several methods used to obtain quantitative information about the composition and structure of plant communities. Some methods are more appropriate for certain vegetation types than others. In this lab we will sample the dune communities at Elkhorn Slough and Franklin Pt (north of Ano Nuevo). Two techniques that can be used with this type of vegetation are the quadrat sampling method and the line intercept of line intercept method. Each group of students will use both methods along the same transect and compare the results between the two methods. Transect lines (50m+) will be set up perpendicular to shore. In consultation with the instructors groups will decide their placement along the shore. Each transect line will start on the beach in front of the dune and will run across the dune crest into the back dune. With these data the class will be able to compare the vegetation found from the foredune - through the mid - to the back dune from multiple transects. Procedure Transect placement - first the groups must decide two things; 1) where to initiate the transects and, 2) how long the transects need to be to reach from the beachfront to the back dune. A. Quadrat Method This method of sampling with quadrats or plots of a standard size is a widely applicable technique. It may be used in all major types of small vegetation. The size, shape, number and arrangement of quadrats will vary depending on the type of community and the type of information desired. Each group of students will set up a transect line through the dune starting at 0 m at the beach front. Square 0.25 m2 quadrats will be used for this analysis. After deciding how long the transect must be to reach across the dune, divide that value by 20 to get the number of sections we will sample (e.g., if the distance is 100 m then 100/20= 5, every 5 m we will place a quadrat). Quadrats are positioned along the transect line at intervals in the middle of the section. (Therefore in our example, the quadrat between 0-5 m will be placed at 2.5 m). Each quadrat is divided into 16 squares. Identify the individual plants within the quadrat. Then count the number of small squares in which the species occurs (this value will range from 0-16). The species needs only to OCCUR in the square to be counted in that square. If, for example species A occurs in all 16 squares and species B also occurs in all 16 squares then both A and B would have abundance values of 16 for that quad. Do not include a plant whose rooted base lies outside the quadrat. B. Line-Intercept Method In this method we will estimate coverage of species over the entire transect by writing down all of the plant species and substrates along the transect, starting at 0. You will write the Distance from - to (in meters to the nearest cm) and the species/substrate. Eg. 2.0-2.50 - sand, 2.50 m-2.58 m - beach rocket (Cakile), 2.58-2.70- American dune grass (Elymus). We will provide or create codes for these species, rather than writing the entire scientific or common name. Determining % cover
A. Quadrat Method 1) 2) 3) For each quadrat determine the total number of species occurrences (e.g., 16 for species A, 16 for species B, 10 for species C and 8 for Sand = 50 occurrences) Determine the relative percent cover of the species by dividing the species occurrence by the total occurrence and multiplying by 100 (e.g., for species A: (16/50)*100= 32%). Calculate average percent cover by species by averaging across all four transects for each quadrat location (e.g., for 2.5m). You should also calculate a standard error.
B. Line -Intercept Sampling Method 1) 2) Prior to sampling divide your transect into 20 equal parts. Determine percent cover for each of these sections (which will be comparable to the 20 quadrat locations). For calculating the % cover imagine that each section is 5 m in length. A 5 m transect can be thought of as a line with the area divided into 500 one centimeter segments. Therefore, to estimate the percentage cover along the transect using the line-intercept sampling method, sum together the intercept lengths (in centimeters) of each species from the entire section and divide by the number of centimeters in the section (e.g., 500). This will give the fraction of the section occupied by each species, and multiplying by 100 will convert this number into a percentage. Calculate average percent cover by species by averaging across all four transects for each sections location (eg. For 0-5 m). You should also calculate a standard error.
3)
Analysis Graphs - you will generate graphs for each method to show the species' distribution through the dune system (fore dunes to back dunes). The graphs can have all species combined or separate graphs for separate species. A. Quadrat Method 1) Plot the average percent cover (Y axis) as a function of the distance along the transect (X axis). Each value will have the average from all four transects +SE by distance (e.g., 2.5, 7.5... or whatever division the groups have decided on). Do this for all species separately and perhaps combined to examine patterns of zonation. Calculate the correlations among species to evaluate possible interactions among species. As an example, is the relationship between the cover of species A correlated (+ or - ) with that of species B? We can help with this analysis.
2)
B. Line-Intercept Method 1) Plot the average percent cover for each distance subdivision along the transect. Each value will have the average from all four transects and the associated standard errors. .
C. Comparison of Methods To compare the two methods to determine if they sampled species differently, plot quadrat averages (X value) vs. transect averages (Y value) for each species. As an example if you had the following data set for species A. In the graph below the 45 degree line is the line on which all points would lie if the methods sampled identically.
1
Quad distance 2.5
Transect distance 0-5
Cover of A from Quads 10
Cover of A from transects 15
2 3 4 5 6 7
7.5 12.5 17.5 22.5 27.5 32.5
5-10 10-15 15-20 20-25 25-30 30-35
15 65 30 20 45 90
12 50 20 15 60 75
Percentage cover is usually used with quadrats when investigating populations. It is the estimate of the area a plant species covers in a quadrat, so if you have 100 little squares in 1 quadrat, then you count the squares in which the plant species is present = you count a square only when it is half or more covered by the plant. So if the plant is in about 25 squares within the quadrat you can say the plant covers 25% of the area.
Phenology is the study of periodic plant and animal life cycle events and how these are influenced by seasonal and interannual variations in climate, as well as habitat factors (such as elevation). The word is [1] derived from the Greek φαίνω (phainō), "to show, to bring to light, make to appear" + λόγος (logos), [2] amongst others "study, discourse, reasoning" and indicates that phenology has been principally concerned with the dates of first occurrence of biological events in their annual cycle. Examples include the date of emergence of leaves and flowers, the first flight of butterflies and the first appearance ofmigratory birds, the date of leaf colouring and fall in deciduous trees, the dates of egg-laying of birds and amphibia, or the timing of the developmental cycles of temperate-zone honey bee colonies. In the scientific literature on ecology, the term is used more generally to indicate the time frame for any seasonal biological phenomena, including the dates of last appearance (e.g., the seasonal phenology of a species may be from April through September). Because many such phenomena are very sensitive to small variations in climate, especially to temperature, phenological records can be a useful proxy for temperature in historical climatology,
especially in the study of climate change and global warming. For example, viticultural records of grape harvests in Europe have been used to reconstruct a record of summer growing season temperatures [3][4] going back more than 500 years. In addition to providing a longer historical baseline than instrumental measurements, phenological observations provide high temporal resolution of ongoing changes related [5][6] to global warming. The concept of Growing-degree day contributes to our understanding of phenology.
Coral bleaching
From Wikipedia, the free encyclopedia
Bleached corals
Healthy corals
Coral bleaching is the loss of intracellular endosymbionts (Symbiodinium, also known as zooxanthellae) through either expulsion or loss of algal pigmentation.[1]The corals that form the structure of the great reef ecosystems of tropical seas depend upon a symbiotic relationship with unicellular flagellate protozoa that arephotosynthetic and live within their tissues. Zooxanthellae give coral its coloration, with the specific color depending on the particular clade. Under stress, corals may expel their zooxanthellae, which leads to a lighter or completely white appearance, hence the term "bleached".[2]
Mass bleaching events[edit]
Bleached branching coral (foreground) and normal branching coral (background), Keppel Islands, Great Barrier Reef
Most evidence indicates that elevated temperature is the cause of mass bleaching events. Sixty major [21] episodes of coral bleaching have occurred between 1979 and 1990, with the associated coral mortality affecting reefs in every part of the world. Correlative field studies have pointed to warmer-than normal conditions as being responsible for triggering mass bleaching events. These studies show a tight association between warmer-than-normal conditions (at least 1°C higher than the summer maximum) and the incidence of coral bleaching. Factors that influence the outcome of a bleaching event include stress-resistance which reduces bleaching, tolerance to the absence of zooxanthellae, and how quickly new coral grows to replace the dead. Due to the patchy nature of bleaching, local climatic conditions such as shade or a stream of cooler water can reduce bleaching incidence. Coral and zooxanthellae health and genetics also influence [22] bleaching. Large coral colonies such as Porites are able to withstand extreme temperature shocks, while fragile branching corals such Acropora are far more susceptible to stress following a temperature [23] [24] change. Corals consistently exposed to low stress levels may be more resistant to bleaching.
Monitoring reef sea surface temperature[edit]
The US National Oceanic and Atmospheric Administration (NOAA) monitors for bleaching "hot spots", areas where sea surface temperature rises 1°C or more above the long-term monthly average. This [25][26] system detected the worldwide 1998 bleaching event, that corresponded to an El Niño. NOAA also uses a satellite with 50k resolution at night, which covers a large area and does not detect the maximum [citation needed] sea surface temperatures occurring usually around noon.
How does climate change affect coral reefs?
The warmer air and ocean surface temperatures brought on by climate change impact corals and alter coral reef communities by prompting coral bleaching events and altering ocean chemistry. These impacts affect corals and the many organisms that use coral reefs as habitat.
Climate change leads to coral bleaching
When warm waters persist, corals bleach and become less able to combat disease. As climate change continues, bleaching is predicted to become more common.
Warmer water temperatures brought on by climate change stress corals because they are very sensitive to changes in temperature. If water temperatures stay higher than usual for many weeks, the zooxanthellae they depend on for some of their food leave their tissue. Without zooxanthellae, corals turn white because zooxanthellae give corals their color. White, unhealthy corals are called bleached. Bleached corals are weak and less able to combat disease. Bleaching events on coral reefs around the globe were observed in 1998 (West and Salm 2003). In some Pacific islands , a little bit of bleaching is common in the summer; however, there have been times when bleaching is particularly bad in this region (Craig 2009). For example, larger than normal bleaching events in the National Park of American Samoaoccurred in 1994, 2002, and 2003 (Craig 2009). As climate change continues, bleaching will become more common, and the overall health of coral reefs will decline.
Climate change alters ocean chemistry leading to ocean acidification
Much of the carbon dioxide that enters the atmosphere dissolves into the ocean. In fact, the oceans have absorbed about 1/3 of the carbon dioxide produced from human activities since 1800 and about 1/2 of the carbon dioxide produced by burning fossil fuels (Sabine et al. 2004). As carbon dioxide in the ocean increases, ocean pH decreases or becomes more acidic. This is called ocean acidification. With ocean acidification, corals cannot absorb the calcium carbonate they need to maintain their skeletons and the stony skeletons that support corals and reefs will dissolve. Already, ocean acidification has lowered the pH of the ocean by about 0.11 units (SCOR 2009). Moving the ocean's pH from 8.179 to a current pH of 8.069, which means the ocean is about 30% more acidic now than it was in 1751 (SCOR 2009). If nothing is done to reduce carbon dioxide emissions into the atmosphere, ocean acidification will increase and more and more corals will be damaged or destroyed.
Healthy coral polyps take calcium carbonate from the ocean water to build their skeletons.
More carbon dioxide in the water makes the ocean more acidic. This coral's skeleton has been damaged by ocean acidification.
Ocean acidification affects more than just corals. Snails, clams, and urchins also make calcium carbonate shells and ocean acidification negatively impacts these organisms as well. Just like corals, ocean acidification makes it harder for these organisms to absorb the calcium carbonate they need to build their shells.
Ocean warming is driving mass coral bleaching
As coral reefs operate very near to their upper limit of heat tolerance (Glynn & D'Croz 1990), bleaching en masse happens when the surface waters get too warm above their normal summer temperature, and are sustained at this warmer level for too long. The intensity of bleaching corresponds with how high, and how long temperatures are elevated and, as one might expect, the intensity of bleaching affects the rate of survival. Small rises of 1 -2 degree C, for weeks at a time, usually induce bleaching.
This episodic ocean warming has been most pronounced worldwide during El-Nino events, when the Pacific Ocean exchanges heat to the atmosphere and surface waters. In recent years though, severe mass bleaching is happening outside of El-Nino because of the "background" ocean warming. The huge mass bleaching in the Caribbean in 2005, a non El-Nino year, and again this year is a prime example of this (Eakin 2010) . Evidence connecting warm surface waters and mass coral bleaching has strengthened to the extent that the National Oceanic and Atmospheric Administration (NOAA) has a coral bleaching alert system in place. This alert system accurately forecasts mass coral bleaching based on satellite data of sea surface temperatures.
Hot water + Coral = Dead coral
So how does hot water kill coral? It requires both high water temperatures and sunlight. Oxygen is released as waste during photosynthesis and like all chemical processes this is affected by temperature, speeding up as more energy (warmth) is applied. When water temperatures rise too high the protective mechanisms to prevent heat damage, employed by the coral and the algae, are overwhelmed. The zooxanthellae algae produce high levels of oxygen waste which begin to poison the coral polyp. In acts of self-preservation the coral kick out the algae, and in doing so become susceptible to starvation, opportunistic diseases, competitive seaweeds and macroalgae (slime to you and me) . Coral can succumb to the effects of bleaching years later, and for those coral that survive, growth effectively ceases and full recovery can take anything up to a decade.
The importance of coral reefs - the oasis in a marine desert
So what does this all have to do with the average man or woman in the street?, well, as far as humans are concerned, there is a rather large dollar value attached to coral reefs. Goods and services derived from coral reefs are very roughly estimated to be between $172 to $375 billion dollars per year (Martinez 2007). Not only that, but reefs directly provide food and income to over half a billion people worldwide. The decline of coral reefs is going to not only impact those that directly depend on them for a living and sustenance, but eventually have dramatic effects on economies worldwide, and will likely drastically drive up world food prices as fish populations nosedive. Ecologically speaking the value of coral reefs is even greater because they are integral to the well being of the oceans as we know them. It might serve to picture them as the undersea equivalent of rainforest trees. Tropical waters are naturally low in nutrients because the warm water limits nutrients essential for life from welling up from the deep, which is why they are sometimes called a "marine desert". Through the photosynthesiscarried out by their algae, coral serve as a vital input of food into the tropical/sub-tropical marine food-chain, and assist in recycling the nutrients too. The reefs provide home and shelter to over 25% of fish in the ocean and up to two million marine species. They are also a nursery for the juvenile forms of many marine creatures . I could go on, but the similarity with the rainforest should now be clear. Eliminate the undersea "trees", which mass coral bleaching is in the process of doing, and you'll eliminate everything that depends on it for survival, a point best exemplified in the following sequence of photos. (sequence of healthy coral-bleached coral-rubble & slime)
Seral stages
seral stages (Science: botany, ecology) The series of relatively transitory plant communities that develop during ecological succession from bare ground to the climax stage.
A seral community (or sere) is an intermediate stage found in ecological succession in an ecosystem advancing towards its climax community. In many cases more than one seral stage evolves until climax [1] conditions are attained. A prisere is a collection of seres making up the development of an area from non-vegetated surfaces to a climax community. A seral community is the name given to each group of plants within the succession. A primary succession describes those plant communities that occupy a site that has not previously been vegetated. These can also be described as the pioneer community. Computer modeling is sometimes used to [2] evaluate likely succession stages in a seral community. Depending on the substratum and climate, a seral community can be one of the following: Hydrosere Community in water Lithosere Community on rock Psammosere Community on sand Xerosere Community in dry area Halosere Community in saline body (e.g. a marsh) Dune Plant Communities - Determining the Distribution, Abundance and Dispersion of Organisms Based on Cover Field ecologists are usually unable to measure the entire population of organisms or variables being studied. For example, if we wanted to know the average length of Batillaria attramentaria in the Elkhorn Slough we would NOT measure all of the snails in the population. Instead, ecologists usually depend upon sampling techniques to reduce the number of measurements that must be taken. The basic idea of all sampling techniques is to measure some of the individuals in a population but to be able to make inferences about all the individuals in the population of interest. Sampling techniques vary with the nature of the question being asked and the population sampled, but desirable characteristics of all sampling techniques are that they are:
1) Unbiased; the sample should be representative of the population as a whole. For instance, in sampling our snails we would not want to use a technique that did not sample small snails. 2) Repeatable. 3) Logistically feasible in the field. In this lab we will use two different sampling techniques to: (1) estimate the percent cover of plants and substrates in two different coastal dune communities, and (2) determine if sampling methodology affects estimates of populations. Analysis of Vegetation There are several methods used to obtain quantitative information about the composition and structure of plant communities. Some methods are more appropriate for certain vegetation types than others. In this lab we will sample the dune communities at Elkhorn Slough and Franklin Pt (north of Ano Nuevo). Two techniques that can be used with this type of vegetation are the quadrat sampling method and the line intercept of line intercept method. Each group of students will use both methods along the same transect and compare the results between the two methods. Transect lines (50m+) will be set up perpendicular to shore. In consultation with the instructors groups will decide their placement along the shore. Each transect line will start on the beach in front of the dune and will run across the dune crest into the back dune. With these data the class will be able to compare the vegetation found from the foredune - through the mid - to the back dune from multiple transects. Procedure Transect placement - first the groups must decide two things; 1) where to initiate the transects and, 2) how long the transects need to be to reach from the beachfront to the back dune. A. Quadrat Method This method of sampling with quadrats or plots of a standard size is a widely applicable technique. It may be used in all major types of small vegetation. The size, shape, number and arrangement of quadrats will vary depending on the type of community and the type of information desired. Each group of students will set up a transect line through the dune starting at 0 m at the beach front. Square 0.25 m2 quadrats will be used for this analysis. After deciding how long the transect must be to reach across the dune, divide that value by 20 to get the number of sections we will sample (e.g., if the distance is 100 m then 100/20= 5, every 5 m we will place a quadrat). Quadrats are positioned along the transect line at intervals in the middle of the section. (Therefore in our example, the quadrat between 0-5 m will be placed at 2.5 m). Each quadrat is divided into 16 squares. Identify the individual plants within the quadrat. Then count the number of small squares in which the species occurs (this value will range from 0-16). The species needs only to OCCUR in the square to be counted in that square. If, for example species A occurs in all 16 squares and species B also occurs in all 16 squares then both A and B would have abundance values of 16 for that quad. Do not include a plant whose rooted base lies outside the quadrat. B. Line-Intercept Method In this method we will estimate coverage of species over the entire transect by writing down all of the plant species and substrates along the transect, starting at 0. You will write the Distance from - to (in meters to the nearest cm) and the species/substrate. Eg. 2.0-2.50 - sand, 2.50 m-2.58 m - beach rocket (Cakile), 2.58-2.70- American dune grass (Elymus). We will provide or create codes for these species, rather than writing the entire scientific or common name. Determining % cover
A. Quadrat Method 1) 2) 3) For each quadrat determine the total number of species occurrences (e.g., 16 for species A, 16 for species B, 10 for species C and 8 for Sand = 50 occurrences) Determine the relative percent cover of the species by dividing the species occurrence by the total occurrence and multiplying by 100 (e.g., for species A: (16/50)*100= 32%). Calculate average percent cover by species by averaging across all four transects for each quadrat location (e.g., for 2.5m). You should also calculate a standard error.
B. Line -Intercept Sampling Method 1) 2) Prior to sampling divide your transect into 20 equal parts. Determine percent cover for each of these sections (which will be comparable to the 20 quadrat locations). For calculating the % cover imagine that each section is 5 m in length. A 5 m transect can be thought of as a line with the area divided into 500 one centimeter segments. Therefore, to estimate the percentage cover along the transect using the line-intercept sampling method, sum together the intercept lengths (in centimeters) of each species from the entire section and divide by the number of centimeters in the section (e.g., 500). This will give the fraction of the section occupied by each species, and multiplying by 100 will convert this number into a percentage. Calculate average percent cover by species by averaging across all four transects for each sections location (eg. For 0-5 m). You should also calculate a standard error.
3)
Analysis Graphs - you will generate graphs for each method to show the species' distribution through the dune system (fore dunes to back dunes). The graphs can have all species combined or separate graphs for separate species. A. Quadrat Method 1) Plot the average percent cover (Y axis) as a function of the distance along the transect (X axis). Each value will have the average from all four transects +SE by distance (e.g., 2.5, 7.5... or whatever division the groups have decided on). Do this for all species separately and perhaps combined to examine patterns of zonation. Calculate the correlations among species to evaluate possible interactions among species. As an example, is the relationship between the cover of species A correlated (+ or - ) with that of species B? We can help with this analysis.
2)
B. Line-Intercept Method 1) Plot the average percent cover for each distance subdivision along the transect. Each value will have the average from all four transects and the associated standard errors. .
C. Comparison of Methods To compare the two methods to determine if they sampled species differently, plot quadrat averages (X value) vs. transect averages (Y value) for each species. As an example if you had the following data set for species A. In the graph below the 45 degree line is the line on which all points would lie if the methods sampled identically.
1
Quad distance 2.5
Transect distance 0-5
Cover of A from Quads 10
Cover of A from transects 15
2 3 4 5 6 7
7.5 12.5 17.5 22.5 27.5 32.5
5-10 10-15 15-20 20-25 25-30 30-35
15 65 30 20 45 90
12 50 20 15 60 75
Percentage cover is usually used with quadrats when investigating populations. It is the estimate of the area a plant species covers in a quadrat, so if you have 100 little squares in 1 quadrat, then you count the squares in which the plant species is present = you count a square only when it is half or more covered by the plant. So if the plant is in about 25 squares within the quadrat you can say the plant covers 25% of the area.
Phenology is the study of periodic plant and animal life cycle events and how these are influenced by seasonal and interannual variations in climate, as well as habitat factors (such as elevation). The word is [1] derived from the Greek φαίνω (phainō), "to show, to bring to light, make to appear" + λόγος (logos), [2] amongst others "study, discourse, reasoning" and indicates that phenology has been principally concerned with the dates of first occurrence of biological events in their annual cycle. Examples include the date of emergence of leaves and flowers, the first flight of butterflies and the first appearance ofmigratory birds, the date of leaf colouring and fall in deciduous trees, the dates of egg-laying of birds and amphibia, or the timing of the developmental cycles of temperate-zone honey bee colonies. In the scientific literature on ecology, the term is used more generally to indicate the time frame for any seasonal biological phenomena, including the dates of last appearance (e.g., the seasonal phenology of a species may be from April through September). Because many such phenomena are very sensitive to small variations in climate, especially to temperature, phenological records can be a useful proxy for temperature in historical climatology,
especially in the study of climate change and global warming. For example, viticultural records of grape harvests in Europe have been used to reconstruct a record of summer growing season temperatures [3][4] going back more than 500 years. In addition to providing a longer historical baseline than instrumental measurements, phenological observations provide high temporal resolution of ongoing changes related [5][6] to global warming. The concept of Growing-degree day contributes to our understanding of phenology.
Coral bleaching
From Wikipedia, the free encyclopedia
Bleached corals
Healthy corals
Coral bleaching is the loss of intracellular endosymbionts (Symbiodinium, also known as zooxanthellae) through either expulsion or loss of algal pigmentation.[1]The corals that form the structure of the great reef ecosystems of tropical seas depend upon a symbiotic relationship with unicellular flagellate protozoa that arephotosynthetic and live within their tissues. Zooxanthellae give coral its coloration, with the specific color depending on the particular clade. Under stress, corals may expel their zooxanthellae, which leads to a lighter or completely white appearance, hence the term "bleached".[2]
Mass bleaching events[edit]
Bleached branching coral (foreground) and normal branching coral (background), Keppel Islands, Great Barrier Reef
Most evidence indicates that elevated temperature is the cause of mass bleaching events. Sixty major [21] episodes of coral bleaching have occurred between 1979 and 1990, with the associated coral mortality affecting reefs in every part of the world. Correlative field studies have pointed to warmer-than normal conditions as being responsible for triggering mass bleaching events. These studies show a tight association between warmer-than-normal conditions (at least 1°C higher than the summer maximum) and the incidence of coral bleaching. Factors that influence the outcome of a bleaching event include stress-resistance which reduces bleaching, tolerance to the absence of zooxanthellae, and how quickly new coral grows to replace the dead. Due to the patchy nature of bleaching, local climatic conditions such as shade or a stream of cooler water can reduce bleaching incidence. Coral and zooxanthellae health and genetics also influence [22] bleaching. Large coral colonies such as Porites are able to withstand extreme temperature shocks, while fragile branching corals such Acropora are far more susceptible to stress following a temperature [23] [24] change. Corals consistently exposed to low stress levels may be more resistant to bleaching.
Monitoring reef sea surface temperature[edit]
The US National Oceanic and Atmospheric Administration (NOAA) monitors for bleaching "hot spots", areas where sea surface temperature rises 1°C or more above the long-term monthly average. This [25][26] system detected the worldwide 1998 bleaching event, that corresponded to an El Niño. NOAA also uses a satellite with 50k resolution at night, which covers a large area and does not detect the maximum [citation needed] sea surface temperatures occurring usually around noon.
How does climate change affect coral reefs?
The warmer air and ocean surface temperatures brought on by climate change impact corals and alter coral reef communities by prompting coral bleaching events and altering ocean chemistry. These impacts affect corals and the many organisms that use coral reefs as habitat.
Climate change leads to coral bleaching
When warm waters persist, corals bleach and become less able to combat disease. As climate change continues, bleaching is predicted to become more common.
Warmer water temperatures brought on by climate change stress corals because they are very sensitive to changes in temperature. If water temperatures stay higher than usual for many weeks, the zooxanthellae they depend on for some of their food leave their tissue. Without zooxanthellae, corals turn white because zooxanthellae give corals their color. White, unhealthy corals are called bleached. Bleached corals are weak and less able to combat disease. Bleaching events on coral reefs around the globe were observed in 1998 (West and Salm 2003). In some Pacific islands , a little bit of bleaching is common in the summer; however, there have been times when bleaching is particularly bad in this region (Craig 2009). For example, larger than normal bleaching events in the National Park of American Samoaoccurred in 1994, 2002, and 2003 (Craig 2009). As climate change continues, bleaching will become more common, and the overall health of coral reefs will decline.
Climate change alters ocean chemistry leading to ocean acidification
Much of the carbon dioxide that enters the atmosphere dissolves into the ocean. In fact, the oceans have absorbed about 1/3 of the carbon dioxide produced from human activities since 1800 and about 1/2 of the carbon dioxide produced by burning fossil fuels (Sabine et al. 2004). As carbon dioxide in the ocean increases, ocean pH decreases or becomes more acidic. This is called ocean acidification. With ocean acidification, corals cannot absorb the calcium carbonate they need to maintain their skeletons and the stony skeletons that support corals and reefs will dissolve. Already, ocean acidification has lowered the pH of the ocean by about 0.11 units (SCOR 2009). Moving the ocean's pH from 8.179 to a current pH of 8.069, which means the ocean is about 30% more acidic now than it was in 1751 (SCOR 2009). If nothing is done to reduce carbon dioxide emissions into the atmosphere, ocean acidification will increase and more and more corals will be damaged or destroyed.
Healthy coral polyps take calcium carbonate from the ocean water to build their skeletons.
More carbon dioxide in the water makes the ocean more acidic. This coral's skeleton has been damaged by ocean acidification.
Ocean acidification affects more than just corals. Snails, clams, and urchins also make calcium carbonate shells and ocean acidification negatively impacts these organisms as well. Just like corals, ocean acidification makes it harder for these organisms to absorb the calcium carbonate they need to build their shells.
Ocean warming is driving mass coral bleaching
As coral reefs operate very near to their upper limit of heat tolerance (Glynn & D'Croz 1990), bleaching en masse happens when the surface waters get too warm above their normal summer temperature, and are sustained at this warmer level for too long. The intensity of bleaching corresponds with how high, and how long temperatures are elevated and, as one might expect, the intensity of bleaching affects the rate of survival. Small rises of 1 -2 degree C, for weeks at a time, usually induce bleaching.
This episodic ocean warming has been most pronounced worldwide during El-Nino events, when the Pacific Ocean exchanges heat to the atmosphere and surface waters. In recent years though, severe mass bleaching is happening outside of El-Nino because of the "background" ocean warming. The huge mass bleaching in the Caribbean in 2005, a non El-Nino year, and again this year is a prime example of this (Eakin 2010) . Evidence connecting warm surface waters and mass coral bleaching has strengthened to the extent that the National Oceanic and Atmospheric Administration (NOAA) has a coral bleaching alert system in place. This alert system accurately forecasts mass coral bleaching based on satellite data of sea surface temperatures.
Hot water + Coral = Dead coral
So how does hot water kill coral? It requires both high water temperatures and sunlight. Oxygen is released as waste during photosynthesis and like all chemical processes this is affected by temperature, speeding up as more energy (warmth) is applied. When water temperatures rise too high the protective mechanisms to prevent heat damage, employed by the coral and the algae, are overwhelmed. The zooxanthellae algae produce high levels of oxygen waste which begin to poison the coral polyp. In acts of self-preservation the coral kick out the algae, and in doing so become susceptible to starvation, opportunistic diseases, competitive seaweeds and macroalgae (slime to you and me) . Coral can succumb to the effects of bleaching years later, and for those coral that survive, growth effectively ceases and full recovery can take anything up to a decade.
The importance of coral reefs - the oasis in a marine desert
So what does this all have to do with the average man or woman in the street?, well, as far as humans are concerned, there is a rather large dollar value attached to coral reefs. Goods and services derived from coral reefs are very roughly estimated to be between $172 to $375 billion dollars per year (Martinez 2007). Not only that, but reefs directly provide food and income to over half a billion people worldwide. The decline of coral reefs is going to not only impact those that directly depend on them for a living and sustenance, but eventually have dramatic effects on economies worldwide, and will likely drastically drive up world food prices as fish populations nosedive. Ecologically speaking the value of coral reefs is even greater because they are integral to the well being of the oceans as we know them. It might serve to picture them as the undersea equivalent of rainforest trees. Tropical waters are naturally low in nutrients because the warm water limits nutrients essential for life from welling up from the deep, which is why they are sometimes called a "marine desert". Through the photosynthesiscarried out by their algae, coral serve as a vital input of food into the tropical/sub-tropical marine food-chain, and assist in recycling the nutrients too. The reefs provide home and shelter to over 25% of fish in the ocean and up to two million marine species. They are also a nursery for the juvenile forms of many marine creatures . I could go on, but the similarity with the rainforest should now be clear. Eliminate the undersea "trees", which mass coral bleaching is in the process of doing, and you'll eliminate everything that depends on it for survival, a point best exemplified in the following sequence of photos. (sequence of healthy coral-bleached coral-rubble & slime)
