12.2 Coral Reef Ecosystems

Corals from Australia’s Great Barrier Reef. (Wikipedia)

Coral reefs are characterized by the structures that provide habitat for the fish and invertebrate species that make up the ecosystem. Hard corals create the reef itself. They typically consist of a layer of colonial polyps that live on the surface of a calcium carbonate skeleton that is secreted by the coral polyps. Corals rely on a symbiotic relationship with the zooxanthellae algae which can be found in the gastrodermis, the "stomach," of coral polyps. Zooxanthellae photosynthesize while residing inside their host and provide the necessary nutrients and energy for the polyp, transferring 95% of the produced sugars (Muscatine, 1990). In return, corals supply zooxanthellae with nutrients essential for photosynthesis such as ammonia and phosphate from their waste metabolism. These nutrients seem to be essential for the survival of the zooxanthellae as the water column in the tropics is usually devoid of essential inorganic compounds (Trench, 1979).

Polyps of the coral Eusmilia fastigiata (Wikipedia) Zooxanthellae

https://commons.wikimedia.org/wiki/F...pistillata.jpg

Optimal Range for Coral Growth

Shallow coral reefs are found in clear, tropical waters with temperatures around 70–85° F or 21–29° C. Temperature, salinity, nutrients, aragonite saturation state, and light are among the most important factors in controlling the geographic distribution of shallow-water coral reefs (Couce et al., 2012) (Kleypas et al., 1999). The global, annually-averaged tolerance limits for coral reefs are 21.7—29.6 °C for temperature, 28.7—40.4 psu for salinity, 4.51 μmol L-1 for nitrate, 0.63 μmol L-1 for phosphate, and 2.82 for aragonite saturation state. The averaged minimum light intensity in coral reefs is 450 μmol photons m-2 s-1 (Guan et al., 2015).

Shallow coral reef habitat flourishes in ample sunlight (Wikimedia)

Effects of Higher Temperatures on Corals

Coral reefs are one of the most vulnerable ecosystems to climate variation and change. Corals, the building blocks of carbonate reefs, have a restricted thermal tolerance. This results in ‘bleaching’ events (loss of symbiotic algae) when sea surface temperatures rise above a given threshold (Graham et al., 2008). Sea temperatures in tropical regions have increased by 1°C over the past century. These increases in temperature can exceed the thermal tolerance of corals and their photosynthetic symbionts, zooxanthellae, and cause more frequent and widespread bleaching. (Guldberg, 1999). Please see section X.X for additional information on coral bleaching.

Shown above is a reef where a significant number of corals have been bleached. (Wikimedia)

This graphic summarizes the processes behind coral bleaching (Wikimedia)

Corals and Macroalgae Phase Shifts

Other substantial influences for coral reef degradation also exist. Specific factors such as eutrophication, increased sedimentation, tourism, and increased fishing pressures may interact with climate change to produce negative synergistic effects (Wilkinson and Buddemeier, 1994). Degrading reefs undergo a phase shift in which the abundance of corals decline and switches to an increase in abundance of larger fleshy macroalgae (Done, 1992). The main drivers that have been cited to explain such a shift is primarily from eutrophication (Lapointe, 1997) and reduction in herbivory (Hughes, 1994). Eutrophication is mainly caused by high nitrogen and phosphorus runoff from agricultural lands that seep into the ocean. The increased nutrient load creates an optimal environment for the production of macroalgae, which is a direct competitor with coral reefs since they decrease the total amount of available light for zooxanthellae to photosynthesize. Intense feeding by herbivorous fish and sea urchins favor coral ecosystems by excluding the presence of macroalgae. Increased fishing pressure has reduced the number of herbivorous fish by orders of magnitude, estimated at around a 60 % decrease (Jackson, 1997) (Bellwood et al., 2004). Due to this reason it is essential to conserve species specializing in herbivory if our goal is to preserve coral ecosystems (Adam et al., 2015).

A healthy coral reef where no algae is present can sustain high amounts of biodiversity. (Wikipedia)

Helpful Tools:

1. https://www.youtube.com/watch?v=2aAfIlRjgk8: Parrotfish Keeping Macroalgae in Check Through Herbivory

2. https://www.youtube.com/watch?v=1aWoTGgVUkc: Jennifer Smith from Scripps Explaining The Basics of Coral Reefs

3. https://www.youtube.com/watch?v=60jof35WuAo: Coral Bleaching Explained

4. https://www.youtube.com/watch?v=rHHuq_COHZs: Heron Island Marine Research Station University of Queensland Coral Reef Climate Change Experiment

References

1. Adam, Thomas C., et al. "Herbivory and the resilience of Caribbean coral reefs: knowledge gaps and implications for management." Mar Ecol Prog Ser 520 (2015): 1-20.

2. Bellwood, David R., et al. "Confronting the coral reef crisis." Nature429.6994 (2004): 827-833.

3. Couce E, Ridgwell A, Hendy EJ (2012) Environmental controls on the global distribution of shallow water coral reefs. J Biogeogr 39: 1508–1523.

4. Done TJ (1992) Phase shifts in coral reef communities and their ecological significance. Hydrobiologia 247:121}132

5. Gattuso, J.-P., Frankignoulle, M., Bourge, I., Romaine, S., and Buddemeier, R. W. (1998). Effect of calcium carbonate saturation of seawater on coral calcification. Global Planetary Change 18, 3747.

6. Graham, Nicholas AJ, et al. "Climate warming, marine protected areas and the ocean-scale integrity of coral reef ecosystems." PLoS One 3.8 (2008): e3039.

7. Guan, Yi, Sönke Hohn, and Agostino Merico. "Suitable Environmental Ranges for Potential Coral Reef Habitats in the Tropical Ocean." PloS one 10.6 (2015): e0128831.

8. Hoegh-Guldberg, Ove. "Climate change, coral bleaching and the future of the world's coral reefs." Marine and freshwater research 50.8 (1999): 839-866.

9. Hughes TP (1994) Catastrophes, phase shifts and large-scale degradation of a Caribbean coral reef. Science 265:1547-1551

10. Jackson JBC (1997) Reefs since Columbus. Coral Reefs16:S23−S32

11. Kleypas JA, McManus JW, Meñez LAB (1999) Environmental Limits to Coral Reef Development: Where Do We Draw the Line. Am Zool 39: 146–159.

12. Lapointe BE (1997) Nutrient thresholds for bottom-up control of macroalgal blooms on coral reefs in Jamaica and southeast Florida. Limnol Oceanogr 42:1119}1131

13. Muscatine, L. "The role of symbiotic algae in carbon and energy flux in reef corals." Ecosystems of the world 25 (1990): 75-87.

14. Trench, R. K. (1979). The cell biology of plant animal symbiosis. Annual Reviews of Plant Physiology 30, 485-531.

15. Wilkinson, C. R., and Buddemeier, R. W. (1994). Global climate change and coral reefs: implications for people and reefs. Report of the UNEP- IOC-ASPEI-IUCN Global Task Team on the Implications of Climate Change on Coral Reefs. IUCN, Gland, Switzerland, 124 pp.