Artificial Reefs (Final)
This topic submitted by Chad Peters (
firstname.lastname@example.org) at 11:55 PM on 6/8/06.
A view of Grahams Harbor from the air. See other
from the Bahamas.
My family has never been known for great family vacations. While other kids would go to cool places like Myrtle Beach or Hawaii, my family would go for the bargain basement vacations. That is until1999. After years of vacations to exotic Indianapolis and Gatlinburg, my parents decided to cash in the money they had saved to finance the quote, ultimate vacation, a Disney Vacation with a Caribbean cruise included. There are a lot of things that I remember about that trip, but perhaps my fondest memory was snorkeling off the shore of Castaway Cay, where Disney was proudly starting construction on an artificial coral reef. I recall being very unimpressed with their handy work; their so-called reef looked to me like a bunch of masonry domes sitting in some sea grass. However, as I look back on the experience, I canŐt help but think about that reef. What does it look like now? Was it a success or a failure? Can projects like that ever work? To answer these questions and more, I chose to do my discussion paper on artificial coral reefs.
The dictionary definition for a coral reef is an erosion-resistant marine ridge or mound consisting of compacted coral, algae and the biochemicals Magnesium and Calcium carbonate. An artificial reef on the other hand, is any material sunk off shore for the purpose of attracting fish, corals and other organisms associated with the coral reef environment.
As will be discussed at some length later, artificial reefs (or AR) are not a replacement for natural reefs (NR). It too the natural reefs hundreds of years to reach the point they are today, artificial reefs can help to augment them, even expand them but they cannot replace them. A major difference has been that even the most successful artificial reefs contain less species richness than similar natural reefs, and the population density is much lower. It should also be noted that NR are normally built on a limestone substrate, where as AR can be based on any number of substrates, from stone to metal.
Artificial reefs do not of course start out as the highly diverse and complex structures that we think of when we hear the word reef. Virtually all artificial reefs are made up of multiple similar unites also known as reef modules. These modules can be made from a variety of material, in a variety of shapes and sizes. The actual construction usually involves the lowering of these modules onto set locations off the shore using a crane. In doing this, crane operators must take care not to lower the modules too quickly. Depending on their construction, this may damage the unit and it would certainly stir up the sediments.
The materials used can actually be quite diverse and inexpensive. Materials have been known to include limestone, concrete, gravel aggregate mixed with pieces of old tires, metal mesh and even old cars and boats. The only real requirement for module material is that is heavy enough to not be moved by strong ocean circulation (like the currents associated with a hurricane) and that it will not degrade over time and become a pollutant. The possible materials are nearly endless, making it a nice way to recycle certain waste materials.
The shapes of modules can be just as diverse as the materials from which they are made. Modules can be made in shapes ranging from boulders, to domes, to cylinders, to pyramids. The shapes of the modules are often tailor made to create ideal colonization opportunities for target coral species. Substrate preferences can vary; some species preferring horizontal substrates with others preferring more graded ones. In general however, vertical substrates tend to have more success in recruiting corals because of decreased sediment deposition and increased water circulation. Another factor is texture and arrangement. Substrates that are more structurally complex are generally more desirable, because complexity leads to a greater number of niches, which can lead to a greater richness of species.
ItŐs no secret that coral reef ecosystems are in decline all over the world due to a variety of factors. In most cases, decline can be linked to human activities. As the once small cities neighboring the reefs develop into major population and agricultural centers. Reefs located close to cities are often victims of nutrient pollution, causing algal blooms, which shade the corals effectively suffocating them. These same reefs can also be literally buried by increased sediment loading due to deforestation and agriculture on the land. Perhaps the most solvable threat however, would be the practice of excessive and often destructive fishing practices by the locals. It is in this area that artificial reefs can be beneficial.
Artificial reefs can serve a variety of different purposes in terms of conservation. Although there will be other uses discussed, the most basic function of an artificial reef is to provide a substrate for corals and other organisms to colonize. In human terms however they have three main uses, to speed up reef recovery, to expand an existing reef and to reduce pressure on an existing reef.
As mention before, it is unlikely that an artificial reef can replace a natural reef however, what it can do is provide extensive substrate for the establishment of corals. This ability comes in handy in areas where much of the natural substrates have been destroy by practices like blast fishing or the extraction of coral as building material. If left to their own devices, reefs will normally recover on their own. However, if a reef goes through a disturbance and as a result few substrates are available, providing some certainly canŐt hurt. Say for example, the local corals prefer to colonize vertical substrates; however most of the standing structures have been eliminated due to blast fishing. In this case, introducing vertical structures in the form of artificial reef modules would be quite beneficial in the reefŐs speedy recovery.
The second major use for artificial reefs is to expand existing ones. This is often deployed in a reef that has shrunk in size due to human activities or a reef that has been fragmented. In a case such as this, artificial reefs have been used to create corridors between reef fragments much as how corridors have been used to connect forest fragments in terrestrial systems. However, artificial reefs have also been used to expand existing reefs, even if the reef in question is doing comparatively well. There are many different reasons why this could be done, tourism, increased fishing or simply the idea that the more reef there is the better.
Perhaps the most commonly discussed use however (at least in my research) is adding an artificial reef to reduce pressures on the natural one. Many coral reefs are located within human communities that rely on fishing as their primary source of income (not to mention food). This can place a great deal of pressure on the reef, in fact many of these fishermen have been forced to abandon the reef for deeper waters because nearly all of the desired fish species have been removed. Needless to say, this is a serious problem. There is more than one way to deal with this and one method that is growing in popularity is building an artificial reef. It has been shown that artificial reefs, among other things; increase the population and species richness of an area relatively quickly. In fact one study term s in Florida saw a 30% increase in richness over two years). The belief is if the fishermen do most of their fishing over the artificial reef it will give the natural reef time to recover. Similarly, it is becoming more popular to create artificial reefs specifically for divers thus sparing the natural reef from the chronic disturbance of tourism.
Artificial reefs can be made anywhere that corals reefs occur in nature. The basic range is between 30 degrees North latitude and 30 degrees South latitude. Within this area, coral tend to occur less than 30 meters down at temperature no lower than 22 degrees Celsius. Corals also require nutrient poor water, with low turbidity and high clarity, so that their symbiants can photosynthesize.
As useful as they are however, artificial reefs are not a cure all; certain ecosystems benefit more from the introduction of artificial reef structures than others. Since artificial reefs are really just substrates for corals and hiding places for fish, they should come in most handy where such structures are limiting. Also, an artificial reef cannot thrive in an environment that is not conducive to the growth of a natural reef. Therefore, one should attempt to reduce factors such as nutrient and sediment pollution before establishing the artificial reef.
Overall, artificial reefs seem to be growing in use. Their reputation for success as species diversity enhancers and human disturbance buffers coupled with the comparatively low cost of construction (just sink an old striped ship off an existing reef and you have an artificial reef) has helped to make them a success. However, they are far from perfected as experimenters continue to attempt to find the ideal structure and orientation of modules for given sites. In fact, one of the greatest concerns with artificial reefs today is that they will be over used, or rather that they will be used in areas where substrate availabilities are not limiting.
To be truthful, I was very skeptical coming into to this report. I did not believe that artificial coral reefs would be a viable conservation option. My experience has been that the more one interferes with natural processes in the name of conservation, the more difficult the task becomes. However, this opinion with regard to artificial reefs was born of a lack of understanding in their function. Artificial reefs are not an attempt by science to create artificial nature; rather they are an attempt to encourage natural processes; kind of like building a bird house and waiting for the birds to colonize it on their own. Who knows, maybe some day I will get to revisit Castaway Cay and find that the reef has gone from and eyesore to rich and thriving ecosystem. One can only hope.
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Walker, Brian K., Bruce Henderson, and Richard E. Spieler. "Fish Assemblages Associated with Artificial Reefs of Concrete Aggregates." Aquatic Living Resources (2002): 1-11. 5 June 2006 .
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