SettlementPlates

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Newly Installed Settlement Plates [1]

Settlement Plates

What are settlement plates?

Settlements plates are man-made devices that simulate the natural strata that corals settle on. Corals settle on the plates during a given time period, and the plates are removed and analyzed in a lab. The amount of coral larvae (coral spat) on each settlement plate allows scientists to learn about the recruitment and, therefore, the health of the reef. Also, scientists often use settlement plates to determine the timing of coral spawning events.


Obviously, settlement plates are found in coral reefs or places where corals are able to live. The orientation and position of settlement plates on the reef are very important. Flow and coral abundance varies greatly throughout the reef. It would not be accurate to compare a vertically oriented settlement plate at 10 meters deep to a horizontal settlement plate at 5 meters deep.


Different types of settlement plates include ceramic tiles, petri dishes, glass, dead branching coral, and slices of stony coral. Ceramic tiles are the best option because they are inexpensive, easy to replicate, easy to analyze in a lab, and are similar to the calcium carbonate strata that corals naturally settle on. [2]

What do settlement plates measure?

Brain Coral budding [3]

Settlement plates measure coral recruitment, which is the process of coral planulae settling on a substrate and commencing to grow their calcium carbonate shell. The health of a coral reef is closely linked to the amount of coral recruitment; the more recruitment, the healthier the reef, and vice-versa. [4] Scientists still do not know very much about the reproductive processes of corals and how a coral planulae ends up where it does. Settlement plates are essential to the study of coral recruitment and the health and growth of a reef.

How are settlement plates used?

Settlement plates are an important scientific tool for assessing the recruitment rates of coral planulae to the reef. They function as a substrate for coral planulae to attach, mimicing ideal conditions. When attaching settlement plates for an experiment it is extremely important that depth and location on the reef are taken into consideration. In an ideal experiment the plates are set either at a consistent depth throughout the reef or in one of the 3 main physical structures of the reef: upper fore reef, lower fore reef, back reef (lagoon).[2]


In order to avoid settlement plates causing damage to healthy reefs they should be firmly attached onto existing dead reef using wire mesh or steel bolts or on heavy, fixed, objects like concrete blocks. To attach steel bolts a hole must be drilled through the settlement plate and the substrate, typically an overhang or dead coral head, and secured with either two wing nuts or the bolt should be screwed in or cemented in. The steel bolts provide a strong, rust-resistant, binding that can survive weathering and human or animal interaction.[2]


Settlement plate experiments rely heavily on timing. Reefs are sensitive ecosystems and little is known about coral reproduction and timing. Plates are installed in a manner that minimizes tampering with healthy reef parts and time spent monitoring the plates. By infrequently checking plates it provides a general recruitment rate and limits financial costs in travel for the experiment, as most reefs are not easily accessible. Plates should be placed either directly before known spawning events, if they're well documented by species/region, or placed and left for a year to get a more general idea of the recruitment rate and survivorship. After allowing recruitment to take place the plates are carefully removed from the substrate and bleached. The bleaching kills the organisms on the plate and turns the microscopic corals white and freezes them. After drying the plate can be put underneath a dissection microscope and the recruitment can be quantified from there based on the type of study.


The best characteristic about settlement plates is that they can be easily removed from coral reefs without causing damage to healthy corals or intruding on the environment. Small unfinished ceramic tiles provide a great mimic to the natural substrate that coral planulae encounter while drifting through in the current. These ceramic tiles have been found to produce equivalent yields to, traditionally more damaging, dead coral plates. [4]

Problems with Settlement Plates [2]

Using settlements plates in coral research an extremely popular and efficient means of investigation. There are however a few problems associated with this research technique that, with careful planning, researchers must remain aware of. While settlement plates, especially unfinished ceramic tiles, provide an excellent mimic of the natural substate found in coral reefs they are perfect replicas. The settlement plates are clean surfaces that do not substitute perfectly for a reef surface. In a natural setting even a coral planulae that reaches the reef may not find the type surface that it needs and die. Just like a juvenile animal needs certain characteristics in a nursery, so do coral planulae. This means that some juvenile coral polyps might not adhere well, possibly causing bias in the data. The results gained from the recruitment might only be representing the more hardy, pioneering species. Often plates only 5 meters apart can show drastic recruitment differences caused by light exposure, plate orientation, or subtle depth change.

While settlement plates are generally considered a relatively inexpensive experimental technique, the type of settlement plate along with bolts, and installation equipment can get expensive. While it might seem practical to use a cheaper plate type (i.e. petri dish, glass plate, cinder block) these materials will each produce varied results and are only really comparable to other studies using that specific plate type.


    • Some have horizontal and vertical dimensions
    • Difficult to manipulate under a microscope
  • Destructive
    • Using live coral as a recruitment measure

Examples of settlement plates being implemented

  • St. John case study [5]
  • Phi Phi Islands, Thailand [6]
  • Australia, Great Barrier Reef near Cairns [7]



Notes

  1. "Eco Koh Tao | Eco Diving Courses, Gap Year Opportunities and Eco Internships - Koh Tao, Thailand." Eco Koh Tao | Eco Diving Courses, Gap Year Opportunities and Eco Internships - Koh Tao, Thailand. N.p., n.d. Web. 26 Feb. 2013.
  2. 2.0 2.1 2.2 2.3 Harriott, Vicki J., and David A. Fisk. "A comparison of settlement plate types for experiments on the recruitment of scleractinian corals." Marine Ecology Progress Series. 37. (1987): 201-208. Print.
  3. "Coral Reproduction." NOAA's Coral Reef Conservation Program:. N.p., n.d. Web. 28 Feb. 2013.
  4. 4.0 4.1 Salinas-de-Leon, Pelayo, et al. "Scleractinian Settlement Patterns to Natural Cleared Reef Substrata and Artificial Settlement Panels on an Indonesian Coral Reef." Estuarine, Coastal and Shelf Science 93.1 (2011): 80-5. GeoRef. Web. 25 Feb. 2013.
  5. Green, Daniel H., Edmunds, Peter J.. "Spatio-temporal variability of coral recruitment on shallow reefs in St. John, US Virgin Islands." Journal of Experimental Marine Biology and Ecology, Volume 397, Issue 2, 15 February 2011, Pages 220-229
  6. Phongsuwan, N., C. Richter, and Y. Sawall. "Coral recruitment and recovery after the 2004 Tsunami around the Phi Phi Islands (Krabi Province) and Phuket, Andaman Sea, Thailand." Helgoland Marine Research 64.4 (2010): 357+. Academic OneFile. Web. 25 Feb. 2013.
  7. Harriott, V. J., and D. A. Fisk. "Recruitment Patterns of Scleractinian Corals: A Study of Three Reefs." AUST.J.MAR.FRESHWAT.RES. 39.4 (1988): 409-16. Environmental Sciences and Pollution Management. Web. 25 Feb. 2013.
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