Artificial reefs

In the last 15-20 years Artificial Reefs (hereafter ARs) have attracted a lot of interest in research as well as in recreational fishing and diving. Unfortunately in this case, science and sports meet when the negative effects of human activities (recreational or commercial) affect benthic communities.

Defining univocally an AR can be pretty hard since different countries have different definitions, depending also on the different use they make of them. The London Convention and Protocol/UNEP for the “Guidelines for the Placement of Artificial Reefs“, in 2009, defined an AR as:

…a submerged structure deliberately constructed or placed on the seabed to emulate some functions of a natural reef such as protecting, regenerating, concentrating, and/or enhancing populations of living marine resources.
Objectives of an artificial reef may also include protection, restoration and regeneration of aquatic habitats, and the promotion of research, recreational opportunities, and educational use of the area.
The term does not include submerged structures deliberately placed to perform functions not related to those of a natural reef – such as breakwaters, mooring, cables, pipelines, marine research devices or platforms – even if they incidentally imitate some functions of a natural reef”.

This clear definition is also inherent to the multiple uses of ARs for scientific and recreational purposes.

Several structures, specifically created or rejections, have been used as ARs:

– oil platforms;
– submarines;
– planes;
– shipwrecks;
– and also concrete blocks, tires, etc.

AR colonizationNow… since ARs are structures susceptible to fouling followed by real assemblages (the gif on the right shows a colonization process of an AR), probably the main term coming to mind at this stage is probably “rehabilitation”: it can be seen as a condition for which a specific habitat can be replaced by a better one.
A valid alternative term can be “enhancement”… of biomass, biodiversity, physical parameters, etc. It looks like the majority of ARs has been placed for this specific purpose but the problem is that it does not consider any previous disturbance due to its placement (voluntary or not). In addition, the “enhancement” condition has mainly been used to increase fish assemblages for commercial and recreational fishing, so it is supposed to be adopted mostly in fisheries biology.
Both terms can anyway give a good idea of the ecological relevance of ARs.

Artificial reefs (2)At this stage, people reading this post could wonder why I am using the term assemblage instead than community. This bring up another important point of view we should see ARs from. In most cases, they tend to promote a different biota than natural substrata with the result of a co-existence of benthic organisms. But community and assemblages are not synonymous: while a community is an extremely high level of ecological organization originated by species interactions, an assemblage is a simple co-existence of species living together only for a common benefit.

This common benefit is the main reason for which ARs are ecologically important. They provide and enhance:
– availability of food and feeding efficiency;
– shelter from predation;
– settlement of larvae;
– buffering of physical stress;
– substratum availability, complexity and heterogeneity;
– for scientific community, they also reppresent excellent models for colonization and successional studies.

The substratum availability is one of the most important advantage of placing an AR. In hard-bottom environments, ARs can simply increase the space available for settlement but in soft-bottoms they can create a completely new habitat providing an hard substrate. And that is why sometimes their role can be emphasized to “habitat creation”.

This is the case of Zenobia shipwreck.

Recreational diving and benthic communities

Recreational diving (1)When talking about human activities and their impact on the environment, we usually think immediately on commercial activities; unfortunately, recreational sports can also have a large and underestimated impact.

In the area of diving, it is well known the damaged caused by recreational diving, especially in those very delicate ecosystems and micro-habitats, like coral reefs, caves, shipwrecks, etc, where the ecological equilibrium is much more delicate. This ecosystems, ARs included, attract an incredible amount of tourists because of their beauty.
In recent years, easy access, improvement of technology and safety in diving have increased the number of divers worldwide with a consequent negative impact that cannot be ignored.

X

Recreational diving (5)Bethic communities are the most vulnerable to physical contacts and damages due to:

– hands;
– body: e.g., photographers, wrong bouyancy control;
– equipment;
– fins: wrong bouyancy control, wrong weight.

X

According to Fernandez-Marquez (et al., 2010), a single diver or a single dive can have a pretty limited impact on benthos; the real problem is represented by cumulative effects, source of significant localized damaged.

Recreational diving (3)There is a general  idea that all this is mainly due to unexperience and wrong behaviour withing diving communities. While dive instructors usually take care of benthos, explaining what to do and not to do during their courses, divers and tourists use to ignore the recommendations due to common sense.
In addition, in diving communities is becoming very popular the idea that certificates and logged dives cannot reflect the experience of a diver. It can be pretty clear if we consider than a diver can become an instructor with less than 100 dives. And it can be much clearer if we compare a diver with 200 logged dives in coral reefs (clear water, warm temperature, hired equipment, guided dive) and a diver with 100 dives in different conditions (cold and warm temperature, drift, responsible of his own equipment, not guided dive). The diving experience would be definitely different and cannot be based on logged dives.

Zenobia shipwreck

MSc CyprusZenobia shipwreck was a swedish ferry used to carry small and large tonnage vehicles. Built in 1979, during its maiden voyage from Sweden to Syria it reported some problems in the water pumping system before reaching Larnaca (Cyprus). After a couple of days of unsuccessful work, engineers were dismissed and it was left sinking in Larnaca Bay, 800 m far from the harbour. On 7 June 1980, after 2 days, it sank completely with all its whole load, at -42 m. The real history, the sequence of the events and the origin of the problem are still very unclear.

Zenobia 1

Nowadays, Zenobia shipwreck (12,000 tons, lengh: 178 m, width: 28 m) is one of the Top 10 sites for shipwreck dives in the world (“The Times“, March 2003) and one of the Top 25 best dive sites worldwide.

This is due to:

– its short distance from the harbour;
– its easy access for recreational diving;
– its suitability to any kind of dive, from open water to technical dives;
– all the ecological beauties common to every wreck dive.

It is lying on its left side in a sandy-bottom at the depth of -42 m, the right side is instead between -16 and -19 m.

Unexperienced divers can enjoy an easy dive around -18 m on the right side of the shipwreck; experienced divers can go down till -30/-40 m reaching the bottom with all the 104 trucks and the accomodation block; technical and advanced divers can deal with the extremely adventurous dive inside the shipwreck till the dangerous engine room.

The shipwreck

Marine life

Underwater scientific photography and softwares

In order to assess the impact of recreational diving on the benthic community of Zenobia shipwreck, I used a now very common methodology well knows as photoquadrats techinique.
This technique has almost completely replaced visual methods since it improved a lot the amount of data collected and the efficienty of data collection in subtidal communities with a limited margin of informations loss, provided to be conducted in the appropriate way. Indeed, digital photography offers an excellent compromise between data accuracy and safe diving.

Advantages:

– rapid data collection;
– permanent photographic record;
– massive amounts of data collected;
– less tedious data collection: accuracy increased.

Disadvantages:

– digital equipment susceptible to breakage/flooding;
– equipment failure can result in data loss;
– hiding and tiny organisms can be omitted;
– a minimum experience is required.

Generally, we use the transects for sampling photoquadrats and we can use three different sampling methods:

– continuous sampling;
– random sampling;
– systematic sampling.

In my MSc thesis I also tested the difference of statistic significance among the three methods applied.

CPCe (1)IMAGE ANALYSIS WITH CPCe If digital techniques have replaced visual methods for data collection, softwares have replaced visual methods for data process.
Coral Point Count with excel extensions, from Nova Southeastern University, is an excellent software specificly designed for that: a matrix of point is randomly distributed over a digital image and the species or substrate-type under each point is visually identified.

CPCe (2)

Materials and methods

MSc Materials and methods (2)PRELIMINARY OPERATIONS. Before sampling to quantify the impact of recreational diving, some preliminary operations were setup:

– definition of the minimum area to optimize data collection;
– collection, physical and digital, of macroalgae sample for first identification;
– definition of the optimized parameters for sampling in order to maximize the effort;
– construction of the framework.

SAMPLING. I choose three areas to be sampled according with the standard diving routes and “use” of the shipwreck: a control (R), a middle (B) and a impact zone (A).
Three transects were placed in each area and three different methodologies were tested, in order to assess the statistical significance of these and, above all, detecting any significant difference between the methods.

MSc Materials and methods (1)

ANALYSIS OF PICTURES. In the laboratory, pictures were downloaded to a PC, renamed with codes able to allow a correct identification, cropped and edited with Adobe Photoshop before the analysis with CPCe.

MSc Materials and methods (3)

Results

The transect A was, as expected, the most impacted with an average coverage around 23% while the transect B has a slightly higher coverage, a bit less than 30%. Both of them were pretty far from the reference one, which went up to almost 90%.
Some species of seaweeds (Sargassum sp., Peyssonnelia sp., Padina pavonica) used as bioindicator, confirmed my data with their relative coverage.

TRANSECT R
LOW IMPACT


TRANSECT B
MEDIUM IMPACT


TRANSECT A
HIGH IMPACT


My results suggest that the diving is having a significant impact on the macroalgae coverage of the shipwreck, especially in areas subject to high levels of use, and that the coverage and biological composition of the substrate can be successfully used as an important and significant indicator of health status of the assemblages.