Module 3: Reefs Under Siege

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Background:

Reefs built by organisms (bioherms) have been a feature on this planet in one form or another for more than 600 million years (Fig. 1). The first modern reefs built by corals and algae appeared during the age of reptiles. Examination of fossil and living reefs shows they are truly repositories of high biodiversity. Today, coral reefs cover much less than a percent of the ocean, yet they are home to an overwhelming number of vertebrate, invertebrate, and plant species.  Bioherms in the past have been subject to disturbances of many kinds including hurricanes, tusunamis (Hagan et al. 2007), sea level rise and fall, glacial periods, shifting of tectonic plates, and even mass extinctions. Today’s modern coral reefs are no different, except there is a new source of disturbance: you and me. Coral reefs are in serious danger from a variety of stressors, that all seem to link back to the activity of huge numbers of humans now living on Earth. Degradation of water quality from coastal eutrophication and development, overfishing, emergence of new diseases and underwater pandemics affecting keystone species like sea urchins, phase shifts of the coral community to one dominated by algae and sponges, and a host of other insults has led to a serious loss of coral cover planet wide (Jackson et al. 2001, Bruno et al. 2007). There are now no pristine reefs left.

Fig. 1. Reefs through time.
The left axis is time before the present in millions of years. Rudists are a kind of bivalve. Stromatoporoids were a type of cnidarian. Stromatolites are sedimentary structure made by microbial mats (often dominated by cyanobacteria). Archaoecyathids are organisms that superficially resemble sponges; there is also isotopic evidence from their fossils that they contained photosynthetic symbionts. Stromatoporoids were a subphylum of Porifera (sponges) allied with, or perhaps identical to, modern sclerosponges.

The most serious threat is that posed by global warming. As sea surface temperature rises, the symbiosis between the photosymbiotic dinoflagellates, the zooxanthellae (Genus Symbiodinium) (Fig. 2) and the coral host, breaks down. The reefs turn snow white; the coral skeleton shows through the now transparent tissue (Fig. 3). If the warm water lasts too long, the coral can be killed. Bleached corals can recover; the symbiotic algae return and repopulate the coral host. But this can take months, and during that recovery period the corals essentially stop growing, in part because their calcification rate is greatly reduced without their symbiotic algae present. Boring organisms like sponges, bivalves, and polychaetes that naturally break apart reefs can get the upper hand. Although other stressors like ultraviolet light and sedimentation can induce coral bleaching, warm water, only a degree or two above the average temperature on a reef, poses the greatest long-term threat to reefs as we know them.

 

Fig. 2. Micrograph of zooxanthellae, the symbiotic dinoflagellates living inside the coral’s endoderm. These algae photosynthesize and pass excess carbon products, usually glycerol, to the coral host. Their presence also facilitates calcification by the coral. Some shallow-water corals can meet almost all their energy requirements from the zooxanthellae. During bleaching episodes, causes by stressors like warm water, these cells are expelled from the coral host. These algal exist in a variety of clades, each with its own thermal stress tolerance. Sometimes several clades of algae can inhabit a single coral colony. During this classroom module, we will investigate the photosynthetic performance of these algal when exposed to a flow stress.

The rise of seawater temperature planet-wide is now well-documented. Global warming of the world ocean has also caused a measurable change in the pH of the coastal ocean, which will make it harder for organisms that calcify to make their skeletons (Buddemeir et al. 2004). Species that survive will be ones that can tolerate the increasing frequency of bleaching events. There is now some evidence that the degree to which corals can store lipids and feed on zooplankton will influence the new order we will see on reefs as the planet warms (Rodrigues and Grottoli 2007, Grottoli et al. 2006), and its an open question as to whether dermersal zooplankton, investigated in the last classroom module, are changing globally as the sea warms.

 

Fig. 3. Heads of Montastrea annularis, a major reef species in Florida and the Caribbean
Sea, turning white during a bleaching event. During a bleaching event, the corals slow or stop their growth. Bioerosion can get the upper hand. In extreme events, the coral themselves can be killed by the heat stress, especially if combined with other stressors or diseases.

The amount of carbon dioxide now in the Earth’s atmosphere is now the highest it has been in the last 650,000 years and is expected to double again before leveling off. Some unknown consequences of the increased warming are how corals will fare as hurricanes, a natural disturbance that they have evolved with, increase in severity, and how increased stratification of the coastal ocean near reefs may affect the nutrient balance near reefs. And as sea level rises, some reefs may not be able to grow fast enough into the optimal zone for growth, effectively drowning.

 

Fig. 4. Aquanaut Jo Gascoigne using a Walz PAM  fluorometer to measure the photosynthetic efficiency within a colony of Montastrea annularis, subject to a unidirectional heated flow in a chamber powered by Aquarius. (The coral is temporarily outside the chamber in this picture.)

Using Aquarius, and its predecessor habitat, Hydrolab, Dr. Patterson and his colleagues showed that moving water can have a direct effect on coral metabolism (photosynthesis and respiration - Patterson et al. 1991) and on the process of bleaching (Carpenter and Patterson 2007). (Charles Darwin was one of the first to note that vigorous water motion is necessary for coral reef growth.) Patterson’s experiments were done on the seafloor next to Aquarius using recirculating flow chambers (see Multimedia movie). By changing the flow past the corals, the thickness of the boundary over the colony can be changed. The boundary layer, a concept explored more thoroughly in Module 5, is a slower moving layer of fluid that forms over all surfaces in nature. It arise because fluids have internal friction (dynamic viscosity), and this layer of retarded fluid acts a barrier through which compounds must move into or out of organisms living on the seafloor. If the flow speed goes up, the boundary layer becomes thinner. Dr. Patterson’s work showed that boundary layer thickness can act as a direct control on metabolism in organisms like corals. In some situations it seems flow can exacerbate the stress experienced by a coral during a bleaching episode.

One instrument used to investigate the effects of flow on bleaching in individual coral colonies is a neat underwater instrument made by a company in Germany (Walz), called a Diving PAM (Fig. 4). PAM is short for Pulse Amplitude Modulation, and is a kind of fluorometer, a device the probes the photosynthetic machinery of the zooxanthellae using different pulses of light. Photosystem II’s potential to harvest light and turn it into energy (in the form of ATP), later used to synthesize new carbon compounds from CO2, is what is measured by the PAM, described more fully below. (Fig. 5)

Fig. 5. Photosystems II and I, the engine of metabolism in most plants. The capture
of the energy of photons by the reaction center of PS II, boosts electrons to a
higher orbital; the energy is subsequently captured through a series of redox reactions
into an energy-containing compound, ATP, the currency of the cell. The PAM
fluorometer measures both how efficient the capture is (quantum yield), and also
can approximate the overall rate a function of light intensity (relative light curve).

What students will see during the show:

This classroom will take a closer look at the corals next to the Aquarius habitat, and make some measurements of their photobiology using the PAM fluorometer. The aquanauts will have set up two experiments the previous day. One experiment covered up some corals to shield them from sunlight. The other experiment is blowing water past some coral heads. The aquanauts will take measurements of the quantum yield, a measure of photosynthetic efficiency on the same species exposed to light and kept in the dark. It will be interesting to see if the corals’s efficiency depends on the exposure of the photosynthetic machinery to photons previously (it should, as most plants adapt to changing light conditions). We will also follow up on a discovery made by Dr. Patterson and his student, now Dr. Carpenter, during some previous Aquarius work: the upstream side of coral colonies shows reduced photosynthetic efficiency during heat stress. This time, we won’t heat the corals but will instead blow water past them steadily using submersible pumps powered by Aquarius. We will visualize the flow past the colonies using dye, so students can see for themselves the difference in flow speed from the upstream to the downstream side, where there is an eddying wake. Will flow alone cause a difference in photosynthetic efficiency? We will find out together!

We will also perform a couple of quick video and still photo transects over the reef, to estimate how much coral cover (% of the bottom occupied) is present near the habitat. These data will be put up on the LOF website. On a healthy reef, the bottom should be around half covered by coral. What is is like near the world’s only underwater research habitat? Estimation of percent cover can occur using a variety of methods. In addition to the random dot method we will demonstrate during the module, see http://www.isa-arbor.com/publications/tree-ord/ordprt3c.aspx for an overview of other techniques. During this percent cover survey we will be on the lookout for bleaching, and some of the diseases that can affect corals, some of which seem to be on the rise.

 

Learning outcomes:

At the end of the module, students will have learned the following:

1. The operating principle of the PAM and how it can be used to assess a coral’s photosynthetic health through the measurement of quantum yield.
2. The importance of flow to the metabolism of a coral colony, including the nature of wakes and eddies and how flow might affect the reef during a bleaching event.
3. Some of the diseases that can strike coral colonies and other threats to reefs including over-fishing, and coastal eutrophication.
4. How to calculate the percent cover for a given taxon during a reef transect.

 

Quantitative exercises:

1. PAM fluorometry and percentages: During this module, the aquanauts will read numbers of the quantum yield as determined by the PAM. Quantum yield is dimensionless. It is the percentage of photons that pass through the plant that are actually used in photosynthesis. Percentages are non-normally distributed, so sometimes scientists perform a transformation first on the data, if they are going to perform a statistical test for differences between means (a Student’s test or ANOVA). The proper transformation for these data, since they are percentages, would be to compute the arc sine of each value, before calculating the mean, standard deviation, etc. This function is found on most calculators and in any spreadsheet.

A. Graph the quantum yield as function of shaded/unshaded and upstream/downstream for the two experiments. (If you didn’t write the numbers down during the classroom, they will be posted later in the day at the LOF website.) First, use the actual values, they repeat with the arc sine values. Finally compute the means and standard deviations, using transformed and untransformed data. Did the treatments show any differences? Did transformation matter to your result?  If you are familiar with computing a two-tailed t-test, perform a statistical analysis of your data.

B. Sometimes a picture make a graph more understandable. Exercise your artistic or computer art talents on this exercise. Replot your data from A, but use a diagram or drawing or photo from the Internet to make the experimental results immediately understandable. In other words, if you combine your graphy with some telling artwork, the reader wouldn’t have to read the figure caption, to know what the aquanauts did with the PAM.

2. Percent cover of coral: Analyze for percent cover, the data taken by the aquanauts for the 6 bottom types they used: live coral, dead coral, sponge, sand, other invertebrate, and macroalgae. Which type dominates at Conch Reef, Aquarius base? Now jump on the Internet and find two other places on Planet Ocean where percent cover was analyzed for a reef environment and make a comparison. You may have to pool some data from your web findings to make the other studies fit into the bottom type model used by the aquanauts.

 

Reading:

Bruno, J.F., and E.R. Selig. 2007. Regional Decline of Coral Cover in the Indo-Pacific: Timing,
Extent, and Subregional Comparisons. PLoS ONE 2(8): e711 doi:10.1371/journal.pone.000071 - available online at
http://www.plosone.org/article/info:doi/10.1371/journal.pone.0000711 - A sobering analysis of several decades worth of field transects over a huge region, showing the rate of loss of reefs in the Indo-Pacific is higher than previously thought.

Buddemeir, R.W., J.A. Kleypas, and R.B. Aronson. 2004. Corals Reefs and Global Climate
Change. Pew Center on Global Climate Change. 42 pp. Click Here to read the brief.

Carpenter, L.W., and M.R. Patterson. 2007. Water flow influences the distribution of photosynthetic efficiency within colonies of the scleractinian Montastrea annularis (Ellis and Solander 1786): implications for coral bleaching. Journal of Experimental Marine Biology and Ecology 351: 10-26. - In situ work at Aquarius that showed the importance of flow to photobiology during bleaching events.  Click Here to read the study.

Grottoli, A.G., L.J. Rodrigues, and J.E. Palardy. 2006. Heterotrophic plasticity and resilience in
bleached corals. Nature 440: 1186-1189. - Nice concise description of experiments showing how heterotrophy can help corals cope with bleaching.

Hagan, A.B., R. Foster, N. Perera, C. A. Gunawan, I. Siliban, Y, Yaha, Y. Manuputty, I. Hazam,
and G. Hodgson. 2007. Tsunami impacts in Aceh Province and Northern Sumatra, Indonesia.
Atoll Research Bullentin 544: 37-54. - Rapid response assessment shortly after the 2004 Pacific tsunami by an expedition led by Dr. Annelise Hagan, Living Oceans Foundation Chief Project Scientist and Aquarius aquanaut.  Click Here to read the bulletin.

Jackson, J.B.C., M.X. Kirby, W.H. Berger, K.A. Bjorndal, L.W. Botsford, B.J. Bourque, C.B.
Lange, H.S. Lenihan, J.S. Pandolfi, C.H. Peterson, R.S. Steneck, M.J. Tegner, and R.R. Warner. 2001. Historical overfishing and the recent collapse of coastal ecosystems. Science 293: 629-638. - A consensus view that humans have been altering marine ecosystems planet-wide for centuries. Includes an analysis of coral reefs.

Patterson, M.R., K.P. Sebens, and R.R. Olson. 1991. In situ measurements of flow effects on
primary production and dark respiration in reef corals. Limnology and Oceanography 36(5):
936-948. - In situ work at habitats Hydrolab and Aquarius that documented flow-modulation of photosynthesis and respiration in reef corals. Also proposed using chemical engineering theory to understand the scaling of flow-modulated metabolism.

Rodrigues, L.J., and A.G. Grottoli. 2007. Energy reserves and metabolism as indicators of coral recovery from bleaching. Limnology and Oceanography 52(5): 1874-1882. - A detailed examination of the energy budgets two coral species showing big differences in recovery from experimental bleaching.  Click Here to read the study.

Multimedia:

Movie of recirculating heated flow chambers used by Dr. Patterson and his team at Aquarius to investigate coral bleaching. Second part of movie shows corals being removed from the chambers to delicately sample the tissue of the coral, to test it for the expression of heat shock protein. Heat shock proteins are made by all living things when under temperature stress:

Web Resources:

NOAA’s Coral Reef Watch (http://coralreefwatch.noaa.gov/) - A truly excellent website run by NOAA devoted to using remote sensing and in situ tools to monitor and manage coral reefs. The satellite products, showing ‘hotspots’ where bleaching may be imminent, are used by scientists world-wide. These data are also available as .kmz files for use in Google Earth (http://earth.google.com/).

Living Oceans Foundation overview of current projects involving Coral Reefs:
(http://www.livingoceansfoundation.org/index.php?option=com_content&view=article&id=30&Itemid=162) - The Foundation supports several projects planet-wide concerned with inventorying reefs and conducting long-term coral research. Its operating principle, Science without Borders^TM, has allowed rapid progress in these multi-institutional, multi-disciplinary projects.

In Hot Water: Coral Bleaching video from Tobago
(http://video.google.com/videoplay?docid=-7519185976400549466&q=coral+bleaching&total) - A sobering look at what happens to a reef when it bleaches and how it can affect local people.

Coral bleaching: a video from the Coral Reef Multimedia project:
http://video.google.com/videoplay?docid=-1026110807523174319 - Nice overview of the multiple factors that affect the etiology of coral bleaching, including thermal stress, ultraviolet light, diseases (including the possible roles of bacteria and viruses), and the importance of different clades of zooxanthellae to bleaching.

Shifting baseslines website: http://www.shiftingbaselines.org/ - A provocative web site that makes the case that we are forgetting what the oceans, including coral reefs, used to look like. Be sure to check out the award-winning videos and Public Service Announcements.

Analysis by Dr. Steven Miller (former Aquarius Director) of Florida Reefs: “Why Do Reefs
Look the Way they Do in Florida?”
(http://www.uncw.edu/aquarius/education/reef_information.html) - Not all change in the ocean is due to humans (although our fingerprints are everywhere).