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Coral

We need reefs. Can we make some?

Article: Achilleos, K., Patsalidou, M., Jimenez, C., Kamidis, N., Georgiou, A., Petrou, A., & Kallianiotis, A. (2018). Epibenthic Communities on Artificial Reefs in Greece, Mediterranean Sea. Water, 10(4), 347. doi:10.3390/w10040347

Ever wonder what happens to a ship or boat if it sinks in shallow depths between 30-150ft where sunlight is abundant and nutrients are low? The answer is exciting! Over time, numerous organisms begin to colonize the ship until it forms an ecosystem similar to a coral reef. This newly formed artificial reef (AR) is equipped with stony and soft corals, sponges, anemones, polychaete worms, coralline algae, sea urchins, and a wide variety of fish and other invertebrates. They even act as refuge for the bigger animals such as turtles, sharks, stingrays, skates, and dolphins.    

For divers, this would be a perfect weekend dive and for fisherman, a lucky day! But what if we create these reefs on purpose? We know it would boost tourism and commercial fishing, but could ARs also be used to help restore threatened natural reefs, their ecosystem functions (biodiversity, nutrient cycling, productivity, and gas exchange), and important ecosystem services they provide (food, storm protection, sand formation, and erosion control)?

Fig 1. Shipwrecks are examples of artificial reefs. Resource: Photo by Courtney Platt

Background

Coral reefs worldwide have suffered massive coral declines over the past decades due to cumulative human impacts and natural disturbances. Reefs in the Mediterranean are no exception. Their alarming loss has forced coastal cities’ governments to make reef management, conservation, and restoration top priority. Past solutions to fish declines and loss of esthetically pleasing reefs included the deployment of artificial reefs (any man-made structure placed underwater). Today, interest in ARs has shifted to an integrated ecosystem approach with the need to understand what organisms make up the community. Is that community stable over time or does species composition (all the different organisms that make up that community) change in response to biotic (living things) and abiotic factors (non-living = temperature, light, chlorophyll)?  

Thus, scientists from Greece set out to do three things: 1) describe species composition and diversity; 2) learn more about local marine fauna; and 3) note any community change with seasonal variation in 3 ARs deployed in 2005. With these results, they could effectively direct reef management efforts using ARs.  

Fig 2. Examples of modules deployed in Europe as artificial reefs. Resource: Fabi et all, 2011

The Study

Three ARs made up of concrete blocks and pyramids organized in modules were deployed in 2005 in different regions of the Mediterranean Sea (Ierissos Gulf, Kalymnos, and Preveza). Samples of macrofauna (animals that can be seen with the naked eye) were collected and identified up to species during each season (Winter, Spring, Summer, and Fall) in 2013 and 2014. Sea surface temperature and chlorophyll concentrations were measured remotely by use of satellite. Chlorophyll is a pigment that traps light energy from the sun. Organisms that have these pigments make their own food by photosynthesis. Measuring chlorophyll gives an estimate of how much phytoplankton is in the water column. Temperature, salinity, and density were also measured in each location at five different depths.

Results

Fig 3.  Richness of individuals (%) per season, year, and sampling site. Spring: SPR; Summer: SUM; Autumn: AUT; Winter: WIN; n: Total number of samples.

Overall, scientists counted 2,235 animals, making up 117 different species between the three ARs (Fig 3). Each one had a unique and distinctive diversity. Ierissos and Preveza were dominated by polychaete worms while Kalymnos was dominated by gastropods (snails and slugs). The most diverse AR was Kalymnos with 80 species, followed by Ierissos with 69, and Preveza with 43 (Figure). All three ARs had the same dominant groups, including gastropods (43%), Polychaetes (37%), Bivalves (clams, oysters) (8.9%), and crustaceans (crabs, shrimps, lobsters) (6.8%).

Fig 4. The most common bivalve was Coralliophaga lithophagella (top), resource: Photo by Alessandro Falleni.

Fig 5. The most common gastropods were the needle welk or Cerithium vulgatum (middle), resource: Photo by Schnecken & Muscheln.

Fig 6. The most common polychaete were the Lysidice ninetta (bottom), resource: Aphotomarine.

These differences in community structure are dependent on their seasonal variation exposure to surface temperatures, chlorophyll concentrations, salinity, depth, temperature, and density. The AR in Ierissos is exposed to colder water, while Preveza is exposed to warmer temperatures throughout all seasons. Chlorophyll concentrations in Ierissos are higher than this at Preveza and Kalymnos during the entire seasonal cycle, but chlorophyll concentrations are more consistent in Preveza and Kalymnos. Both Ierissos and Preveza receive freshwater that flows from the continent, meaning they have lower salinities. In turn, Kalymnos is characterized by being exposed to consistent high salinities with very rare freshwater intakes. Depth, temperature, and density were consistent throughout seasons in all sites. Preveza and Kalymnos had similar conditions but differed greatly in Ierissos (Fig 7).

Fig 7. Satellite-derived sea surface temperature seasonal mean (SST) (2003–2014) and location of the artificial reefs. (a) Winter; (b) Spring; (c) Summer; (d) Autumn; 1: Preveza; 2: Ierissos; 3: Kalymnos.

Conclusion

The seasonal cycle did not have an effect on species community structure. Communities were stable and well adapted to local conditions. However, differences in salinity values between all three ARs explain their difference in species composition.

What does this mean?

Studies such as this one are highly needed to compare ecological processes between ARs and natural reefs in order to determine viability of using ARs to restore and rehabilitate damaged reefs ecosystems. If community structure remains similar to that of natural reefs, they may yield similar ecosystem services such as provide storm protection and erosion control. With the increasing need for reef resource management, studies should focus on investigating the dynamics affecting developing communities on ARs by short and long term monitoring and comparing exposure to different environmental conditions due to location. In the past, ARs have increased commercial fishing and tourism value. If ARs could also increase ecological value and restore ecosystem services, we may have an effective new management strategy of restoring reefs.

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