What do biologists measure under the sea surface?

„The sea, once it casts its spell, holds one in its net of wonder forever.“

Jacques Yves Cousteau

On land, we can very easily spot changes in the environment that occur due to human activities. But the blue of the sea hides secrets and you need to dive below the surface to see what is happening in the marine ecosystem. And for that, biologists need the knowledge and skills of a diver.

Posidonia oceanica, an endemic species of the Mediterranean Sea, is a sea flower whose meadows make it one of the most important habitats in the sea. Its meadows are oxygen factories, which is why we call them the lungs of the sea. On its leaves, we can find various epiphytes, while in the meadow between its leaves, more than 100 species of fish and many other marine organisms find shelter, spawn, grow and reproduce.

Figure 1. Posidonia oceanica

Posidonia, the source of life, a witness to maritime history, and a precondition for the future, is under increasing negative pressure. Man is destroying its meadows by filling in shores, concreting, building ports and marinas, discharging wastewater, fish and shellfish farms, and using certain fishing tools. The greatest threat to the survival of Posidonia is anchoring.

A prerequisite for the implementation of protection and sustainable management is the establishment of monitoring or monitoring of the situation, and its continuous repetition to monitor changes in the environment.

To make a comparison, we conducted field research on the meadows of seagrass Posidonia in the Kornati National Park at two locations. Baluni Bay was chosen as a location with minimal anthropogenic impact, while Kravljačica Bay was chosen as a location that is under great negative pressure, primarily anchoring. The research was conducted continuously for three years, at the beginning of June 2019, 2020, and the last at the end of May 2021. For Posidonia meadows, one of the descriptive indicators is the density, ie the number of shoots per m2, because it provides information on the condition of meadows, which can be used to assess the health of the ecosystem. The higher the density of shoots, the healthier the meadow. This indicator also reveals changes that have occurred, for example, due to human activity, over a long period. At the previously agreed stations, we randomly selected 6 squares, ie plots of the same size (40 * 40 cm), within which we counted the shoots. Also, samples of shoots were collected, which were then sent for laboratory analysis. The parameters of shoot length and width, the average number of leaves per shoot,% of leaf necrosis, and composition of epiphytes were measured to speak about the condition of the sampled meadow.

Figure 2. Measurement of shoot density

Meadow cover and its continuity/discontinuity describe how much seabed is covered with seagrass on a 0-100% scale using the Line Intercept Transect (LIT) technique. At previously agreed stations, the diver records and describes what he sees along a given line (transect) of 10 m. The collected data are used to obtain meadow cover values. The greater the coverage of the meadow in the area, the healthier the meadow. At the same time, during the research, we sampled the sediment present at the observed station, which was then sent to the laboratory for analysis to obtain information on the composition and size of the grain.

Figure 3. Measurement of meadow cover

The results of the research showed that there is a difference between the two locations. In Baluni Bay, the meadow cover ranged between 61% and 72% and was described as a continuous meadow with cross-sections of natural sand islands. In the Kravljačica bay, the meadow cover is between 31% and 56% and is described as discontinuous, with very pronounced cross-sections resulting from anchoring. Also, the meadow in Baluni Bay shows a higher average density of shoots, and the length of the shoots themselves is somewhat longer, compared to the Posidonia meadow in Kravljačica Bay, which is a consequence of high sedimentation.

All the results indicate that activities during the summer season, such as anchoring, the presence of a large number of vessels discharging wastewater, bathing activities where the dissolution of harmful sunscreens occurs in the marine environment have a very negative impact on the marine environment and seagrass meadows.

INTERREG Italy – Croatia SASPAS is a project we want to protect and restore seagrass meadows. Project activities began with the establishment of monitoring meadows in June 2019, followed by the first pilot transplantation of seagrass in the bay Kravljačica, continued monitoring activities, set environmentally friendly anchorages, and launched the campaign “Become a Posidonia Keeper “.

If you want to be actively involved in the protection of Posidonia, the treasury of marine life, more information about the campaign, as well as useful tips, see the article: “How to become an Ambassador and Posidonia Keeper”.

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