Original article (in Croatian) was published on 05/10/2023; Author: Melita Vrsaljko
Posidonia meadows produce oxygen, store CO2 and preserve the biodiversity of the Croatian undersea. They are threatened by human activity.
A few years ago, the process of mapping and monitoring marine habitats intensified in Croatia, the protection of which the state is committed to by implementing the Directive on Birds and the Directive on Habitats.
Among at least 322 species and habitat types for which conservation monitoring programs will be established as part of the project of the Ministry of Economy and Sustainable Development is Posidonia oceanica, a species of marine flowering plant endemic to the Mediterranean Sea (two similar species have been recorded only in Australia), where its biggest threat is man.
It is a plant that is also called the “lungs of the Mediterranean”. It is a kind of “factory” of oxygen; it is estimated that 20 percent of known Mediterranean species live in its meadows, and as many as 30 types of algae live on its leaves. Posidonia meadows are habitats, hatcheries, breeding grounds and feeding grounds for more than 100 species of fish (HAOP).
Destruction of Posidonia in Croatia occurs daily, and is most intense in the summer period when the Adriatic Sea is crowded with nautical vessels. The reduction of meadows not only of this species, but also of other marine flowering plants that are important for marine biodiversity, is invisible to most, and this is mostly witnessed by researchers and divers who participate in projects whose purpose is to protect them.
Even though the key elements of the system for monitoring the state of preservation of marine flowering plants will be developed only now, several associations in Croatia have been working on their mapping and spreading awareness of the importance of their preservation for years. We talked about these topics with biologists Hrvoj Cizmek from the Zadar Society of 20,000 Mile Sea Explorers and Matea Spika from the Split-based Association for Nature, Environment and Sustainable Development Sunce, which conducted the first pilot of Posidonia transplantation on the eastern side of the Adriatic.
More systematic monitoring
These days, Cizmek plans to present the results of the research related to the mapping of species of flowering plants Zostera marina, Zostra noltei, Cymodocea nodosa in the Karin Sea, which was carried out by the “20,000 Miles” Society of Sea Explorers in cooperation with several other associations from the Zadar hinterland as part of the project “Sea meadows: It’s not too late”. In parallel, he is working on a proposal for monitoring marine habitats, which should also be presented soon.
We asked him if Croatia is late in implementing such a project at the national level, which would provide insight into marine habitats of special interest. It is true, says Cizmek, that in some ways the period is just beginning when marine habitats, including Posidonia, will be more systematically and properly monitored.
“However, Croatia is not the only one lagging behind. Yes, we are only now starting to work, but the problem is that Italy has only recently done so. Greece did it two years ago, and Bulgaria is doing it in parallel with us. We are no better or worse. This process is going very slowly and we can say that all countries are equally behind in it”, says Cizmek.
Biologists and other members of the Society of Sea Explorers “20,000 Miles” have so far been mapping flowering plants at numerous locations along the Croatian coast.
“So far, we have worked as we were paid, and as much as we could finance ourselves from a project, and last year and this year, as part of the creation of a national monitoring system, we are truly going all over the Adriatic”, he says.
Matea Spika, whose association also works on similar projects, says that so far marine flowering plants, specifically Posidonia, have been more systematically monitored only at the level of marine national parks and nature parks, whether these institutions finance the research with their own or project funds.
“The most successful Croatian park in this regard is the Telascica Nature Park. Since 2011, when the program for monitoring Posidonia oceanica meadows in Croatian marine protected areas was established as part of the MedPAN project, we have conducted with the Public Institution Telascica Nature Park a program for monitoring the condition within the boundaries of the Park every two years, and so far it has been conducted a total of 7 times, says Spika and adds that the focus of monitoring the condition of Croatian parks is mainly on the impact of anchoring.
Endangered sea meadows
It is this human activity, along with wastewater pollution, construction and dumping in the sea, fish and shellfish farms, marinas and harbours, and some fishing gear, that is the biggest threat to Posidonia, which normally grows in areas where there is a lot of pressure on the coast.
“Until the anchor is “caught”, it drags along the seabed and ploughs across the Posidonia meadow. Even after the anchor is hooked to the bottom, the anchor chain drags along the bottom and destroys the Posidonia again. When the anchor is pulled out, new ploughing takes place and thus the Posidonia meadow is destroyed”, says Spika.
Anthropogenic influence, Cizmek confirms, is the main cause of the reduction of sea meadow habitat under this climate.
“About 40 percent of the world’s fleet of charter ships is located in Croatia. This is essentially a lot of small boats and private speedboats that drop anchor into the sea daily. If we take into account that there are about 100,000 charter boats in Croatia, and if we determine that, for example, 50 percent of them drop anchor and pull a half-meter or four, five-meter long line, we can say that in our country the destruction of these plants occurs on a daily basis”, says Cizmek.
We asked him why, when it comes to the protection of marine flowering plants, Posidonia is the most talked about, even though several other species live in the Adriatic Sea; sea silk (Zostera marina), dwarf silk (Zostera nolti) and knotted sea fringe (Cymodocea nodosa).
“It is a slow-growing and slow-living species. In the areas where it was destroyed a few years ago, even a few decades ago, the closing of the holes and those scars is very slow. It can be seen that it is expanding, but it expands a maximum of one centimetre per year. If it is a hole of half a meter, it will only close in 25 years and in ideal conditions with a lot of light, and that is relatively shallow, up to 10 meters. Even deeper than that, the constant growth of Posidonia has slowed down even more”, says Cizmek.
He points out several more reasons why slow-growing Posidonia is especially important. Namely, in the Adriatic Sea, there is also an invasive species of Caulerpa algae that spreads rapidly. This species appears in places where there are scars in Posidonia meadows. However, it stays only on the scar because the Posidonia settlement is too soft for the algae to settle on the leaves, and it does not go between the stems because there is not enough light. Healthy Posidonia is important because it is a natural barrier for this invasive species, points out Cizmek.
In addition, Posidonia meadows store significant amounts of carbon in their sediments, which is another reason why it should be protected since its destruction can lead to the release of additional amounts of carbon dioxide into the atmosphere.
The UN states that seagrass makes up ten percent of the ocean’s capacity to store carbon (so-called blue carbon). It occupies only 0.2 percent of the sea floor, however, it can capture carbon from the atmosphere up to 35 times faster than a tropical rainforest. Since about 159 countries have seagrass on their coasts, the potential of sea flowering plants to contribute to the fight against climate change is significant.
“It takes the CO2 that has dissolved in the sea, takes it into itself and after some time deposits it under itself. Posidonia settlements have much more deposited carbon than some pure sediment”, says Cizmek.
Additionally, we asked Cizmek what criteria he uses when he and his colleagues observe the state of sea meadows.
“The density of shoots must be measured and how many dead remains of Posidonia are present in the settlement itself. Based on these two parameters, we can say whether Posidonia is in good condition, and whether it is threatened or endangered. Monitoring, the assessment of the situation, which must go on for several years, is very important. If the density is seen to decrease, it means that something is happening. Most often, this is a sign of some organic pollution, which is why Posidonia shoots die within the settlement itself. If an increased number of areas with dead Posidonia is noticed, it means that the cause is usually mechanical. When the destruction of Posidonia is fast, it can be attributed to mechanical causes, and when it is slow, that is, when the density of the settlement decreases, then it is most often an organic cause, such as, for example, an increased amount of organic substances that damage the plant”, says Cizmek.
How to save sea meadows?
This is a question that scientists have been trying to answer since the middle of the last century when the first studies related to the protection of sea meadow habitats were published (1, 2).
Numerous examples of self-renewal of sea meadows after certain human interventions have been recorded, such as what happened in Tunisia where, due to the increase in pollution in the period 1920-1980, the collapse of the flowering plant Z. nolte was recorded. After the authorities implemented an ambitious wastewater treatment plan, the recovery of mixed meadows of Z. noltei, R. maritima and C. nodos was recorded.
In addition to such types of “rehabilitation”, scientists are also working on restorations that include planting and transplantation, interventions aimed at returning degraded habitats to their original state. Projects are also being implemented to establish completely new habitats in areas suitable for the establishment of sea flower settlements.
Some authors emphasize that the problem with transplant projects is that there is often a lack of monitoring of results, that some transplant operations were carried out in places where the species was never present, and that transplant operations were conducted in locations where the cause of regression had not yet disappeared.
“Transplanting to an area where seagrass is still in decline is clearly absurd. This happened in Marseille in 2004. The municipal environment department commissioned a study to find out if Posidonia planting is possible and where. The conclusion was that in the western part of Prado Bay, the deterioration has stopped and therefore transplanting is possible. However, on the other hand, in the eastern part of that bay, the decline continued. The public relations department of the same municipality decided to carry out a highly publicized transplant operation in front of the marina, the worst possible place for such an undertaking. Last but not least, the worst possible technique was used – the use of cement frames. All the cuttings died quickly, and the cement frames will remain at the bottom for decades, maybe even a century”, state the authors of the paper published in January 2021 in the journal Water.
Experiences from Croatia
Around the world, projects to restore and create habitats for sea flowers are taking place. In this, Australia is leading the way, where work is underway to reestablish an 18-hectare seagrass meadow, and the United Kingdom, where the WWF is collaborating with the state on a project to plant 18 hectares of seagrass, to restore 15 percent of that habitat by 2030 (1, 2).
Croatia is just at the beginning with such projects. As part of the “Safe Anchoring and Seagrass Protection in the Adriatic Sea” (SASPAS) project, the Association Sunce participated in the pilot transplant of Posidonia within the Kornati National Park in Kraljacica Bay.
Posidonia was transplanted from healthy meadows to those where it had disappeared as a result of years of anchoring. The transplants are attached to supports made of degradable material (corn starch, wood) which have the role of supporting the rhizome while the plant takes root and is anchored in the seabed.
We asked Spika if the transplant project was successful.
“After a couple of years, we see that some shoots have taken root and continued to grow, but some of the shoots did not survive, and some of the transplants were destroyed by anchoring. In order to obtain a realistic estimate of the survival rate, it is important to control the anchoring in these research areas well in the future. Considering that Posidonia grows very slowly, only one centimetre a year, it will take a lot of time to be able to draw final conclusions”, says Spika, emphasizing that these are research pilot activities and she is convinced that the methodology will continue to develop in the Mediterranean countries, which will affect a higher survival rate in the future.
For this reason, many scientists who deal with the topic of marine habitat restoration have highlighted general advice that should be followed when planning a marine flowering plant restoration project.
First of all, before planting in a certain place, the question should be asked whether the species has ever existed there. Replanting must be integrated into the overall meadow management strategy at the bay or regional level, with the strategy taking into account the total area of existing meadows, the area lost each year due to decline and the causes of that decline, the area restored each year by natural regeneration (if it happens) and the area that can be reclaimed by replanting within a time frame of 10, 20 or 50 years.
In addition, the cost of transplanting should be taken into account, as well as the cost of some other methods, such as wastewater treatment, setting up reefs against trawlers, and providing environmentally friendly moorings for pleasure boats…
Seagrass transplanting is the world’s most expensive restoration method. Spika also tells us that such projects, in addition to significant financial resources, also require great diving effort.
“The cost of around 20 m2 of transplants goes up to 20,000 euros, while the price of one ecological buoy for mooring ships is around 4,000 euros”, says Spika.
Education, prevention
Cizmek, like the interlocutor, agrees that education should be primarily focused on.
Posidonia is a strictly protected species in Croatia under the Nature Protection Act, but many skippers do not know this. For this reason, as an association, they plan to devote more attention to campaigns aimed at boaters in the coming years.
“The problem is that you can’t educate all the skippers, among whom there are a lot of foreigners who drop anchor wherever they want. Not only foreigners do it, but also domestic skippers. This is not good not only for the Posidonia but also for the safety of navigation since it is not good to anchor above it because, in case of any strong winds, the Posidonia does not hold the anchor well. It would certainly be good to go with some kind of documentation that the skippers receive with the ship, in which there is information that anchoring at Posidonia is prohibited”, he says.
Matea Spika states that in the last two decades, as much as 20 percent of this extremely important habitat has been lost, so it is necessary to act immediately.
“We don’t have time to wait for the perfecting of the transplantation method itself. We believe that it would be much more effective for protection to establish a ban on anchoring in some extremely valuable locations and the establishment of ecologically acceptable anchorages. It is important to emphasize that during the installation itself, and later the exploitation of the anchorage, there is no contact with the plant cover, which is extremely important for the preservation of marine flowering plants and which is why this kind of anchor system is more environmentally friendly compared to the installation of concrete blocks”, he says.
In addition to educating boaters, there is also a need to educate the general public, part of which is not yet familiar with why sea meadows are important.
This is confirmed by the case of Sakarun Beach on Dugi otok, whose visitors for the past few years complained about the accumulation of Posidonia deposits on the sand, considering that it makes it untidy. However, as the experts explained even then, even dry Posidonia thrown out by the sea is also important for the ecosystem because it protects the sand layer from erosion and thus participates in the biogeomorphological function of the area’s survival (1, 2).
In other words, it gives Sakarun Beach exactly the characteristics that make tourists visit it.