Wednesday, September 28, 2011

Settling Sediment


I found a nice paper on the coral reef environment in the Caribbean. But first, because I haven't introduced it yet, and so I can refer back to it in the future: Coral. What is it?

Coral is an animal that belongs to the phylum cnidaria (ni dare ee ah, that c is silent folks). This is a very diverse group that consists of jellyfish, corals, and other stingers. These animals sting because they are armed with stinging cells called nematocysts. If you've ever come face to tentacle with a jellyfish then you are aware of their painful potential. There are four major groups of cnideria: Anthozoa (true corals, anemones, sea fans, and sea pens), Cubozoa (box jellies), Hydrozoa (siphonophores, hydroids, fire corals, and medusae), and Scyphozoa (true jellyfish).

We will be concentrating on the first because it includes the corals that build the large reefs found in tropical waters, generally between 30 degrees N and 30 degrees S latitudes. Various types of coral species can be found at various depths, but reef-building corals are usually found at depths of less than 46m (150ft). This a warm depth that allows sunlight to get through to feed their zooxanthellae. Zooxanthellae (zoo zan thell ee) are a type of microscopic algae that have a symbiotic relationship with coral. This shallow water depth also tends to be close to shorelines where wave action is abundant and brings in food to these filter feeders. The temperature and salinity of these waters also allows for the precipitation of calcium which the corals use to build their hard skeletons. Within a skeleton lives an individual coral polyp. A polyp starts off life as a floating animal but when it reaches adulthood it becomes sedentary and its body changes to a tubular saclike shape with a central mouth surrounded by a ring of tentacles. These tentacles are used for defense and for moving food to the mouth, and they contain the nematocysts which are double-walled structures containing a folded, venom-filled thread with a barbed tip that can shoot out and penetrate prey. The wonderful colors of the coral reef come from the natural pigments of the coral tissue or the zooxanthellae. Coral reefs are formed as old coral dies and new coral builds on top of it, a process taking hundreds or thousands of years, and a reef is made up of millions of individual polyps. The corals within this ecosystem remove and recycle carbon dioxide, shelter and provide food for other reef animals, and can even offer some protection to the land.

Humans contribute to the deterioration of coral reefs in a number of ways. Physical damage caused by boats, recreational contact (touching coral kills it, so hands and fins off!), and over-fishing cause a lot of ecological stress to reefs. The runoff of sediments, contaminants and nutrients from agriculture, industry, sewage, and land clearing are also stressful. Additionally, coral bleaching is occurring at a rapid rate. This is when the environmental conditions no longer support the symbiotic relationship of corals with zooxanthellae. No colored zooxanthellae, no colored coral. No symbionts equals coral death.

An older, but still relevant, paper published in the Bulletin of Marine Science reports on a project that monitored the effects of land development on a near-shore reef in St. Thomas, USVI. Land development in the Virgin Islands has rapidly increased, particularly up from the shoreline. This development means the removal of natural vegetation and construction of roads, increasing erosion rates. That means lots more silt and clay that gets into the water. As I mentioned above, sedimentation is very stressful and deleterious to coral. It is just dirt, you may think, so how is that bad? Well, sediments decrease light penetration (which reduces photosynthesis in zooxanthellae and coral growth), settling sediment contributes to tissue loss or mortality (it reduces larval settlement and it literally smothers the coral polyps), it increases coral energy costs (increasing respiration while decreasing photosynthesis), and chronic sedimentation may reduce the abundance and diversity of corals and other reef organisms (increasing susceptibility to diseases, bleaching, and predation and decreasing recovery rates after disturbances). Because of these detrimental effects the Virgin Islands Coastal Zone Management (CZM) has implemented a strict code for developers proposing building projects in sensitive shoreline areas, and academic and government organizations conduct reef assessments to quantify the impact of development on the coral reef environment.

This study's objectives were to measure the rates of sedimentation, monitor water quality, quantify changes in the abundance and diversity of sessile reef organisms, document the acute and chronic effects of sedimentation on coral condition, and develop management guidelines for evaluating the effectiveness of sediment control measures. The conducted their study a fringing coral reef in Caret Bay, on the northwest side of St. Thomas, particularly looking at the Caret Bay Villas construction site in relation to this reef. The conducted a sediment analysis using sediment traps at 5 and 12 meters, calculating sediment load and composition. They also did seawater analysis, collecting water along transects to look for suspended solids and turbidity. They also measured the percent cover of scleractinian (stony) corals, sponges, encrusting gorgonians (sea whips and sea fans), and macro algae. Using quadrants they identified and counted gorgonians and focus monitored individual coral heads of Porites astreoides and Montastrea annularis.

They found that the distribution of fine sediments along the reef were more concentrated in shallow depths, and that coarse sediments were only deposited in traps during months when large ground swells occurred.  The patterns of sedimentation relative to rainfall was related to the progress of the Villas' construction. The percent cover of living coral was found to have declined 14% over 18 months. This large decline may have resulted from dislodgement by large waves in the winter, smothering from sedimentation, or death due to pigment loss. They did document dislodgement but it is unlikely that that this was the sole mortality factor.  The authors also rule out much mortality due to direct smothering since they never actually observed it. They concludef that it is most likely that the majority of the mortality was related to the indirect effects of sedimentation and bleaching. They found strong evidence that the bleaching of corals was strongly correlated to sedimentation rates. So more suspended sediments equals an increased likelihood that corals will bleach. And this is in line with other studies.

This study is nice because it provides direct evidence that sedimentation (in this case from land development) cause stress to corals and may lead to their decline due to bleaching. It also provides good evidence for management practices along sensitive shorelines.

Here's the paper:
Nemeth, Richard S. and Joshua Sladek Nowlis (2001) Monitoring the effects of land development on the near-shore reef environment of St. Thomas, USVI. Bulletin of Marine Science: 69(2), 759-775. (LINK)

If you like this then I also recommend this more recent marine ecology survey:
Pittman, Simon, Ron Kneib, Charles Simenstad, and Ivan Nagelkerken (2011) Seascape ecology: application of landscape ecology to the marine environment. Marine Ecology Progress Series: 427, 187-190. (DOI: 10.3354/meps09139)

And here's some more info on corals:
U.S. Environmental Protection Agency's Page on Coral Reefs
Ocean World's Coral Page
Berkeley's Introduction to Cnideria
SeaWorld's Corals and Coral Reefs Page
Kansas Geological Survey's Biogeoinformatics of Hexacorals

(image from treehugger.com)
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