Swimming amongst seagrasses
Date of publication: Apr 4, 2023

It’s mid-October in Sardinia, and the once packed beaches in the Summer, are bare and wild. It’s windy and not necessarily warm, but I can’t resist jumping in the water for a little snorkelling session in the crystalline turquoise waters Sardinia is known for.

Me walking along the Li Itriceddi beach in Sardinia. Seagrass patches are visible amongst the rocks.

The moment I put my head underwater I see a combination of sand and rock, and a variety of small fishes and invertebrates, nibbling on the algae growing on the substrates. I swim a little bit further from the coast and amongst the sand and rocks, there are now seagrasses, reflecting the sunlight as they gently rock from side to side. It’s Posidonica oceanica, an endemic seagrass species to the Mediterranean that can form large underwater meadows. While swimming along the seagrasses I can’t help but notice an increase in bigger fish hiding amongst the green blades. These habitats are extremely important in providing food and shelter to many organisms. They also serve as nursery to many important commercial fishes.

Posidonia oceanica off the coast of the East coast of Sardinia. Video by Maria Pinto

Unfortunately, like many seagrass meadows around the world, those in the Mediterranean are in decline, due to the combined effect of trawling, touristic activities, and climate change. This is a serious problem, not only because it won’t be possible to admire these mesmerizing underwater meadows anymore, but also because they play a crucial role in protection against coastal erosion and in maintaining the oceans healthy and rich. They are also probably one of our best allies against climate change. Seagrass meadows are 35 times faster at removing carbon dioxide from the atmosphere than rainforests, by efficiently storing it in the oxygen-depleted seabed.

Posidonica oceanica is only one amongst the 72 known seagrasses species worldwide. In my home country Portugal, there are three different species living along the coast (Zostera noltii, Zostera marina and Cymodocea nodosa), also facing the same fate as their Mediterranean cousins. However, over the past years, the establishment of marine protected areas and restoration efforts, which some of our partners have participated in, have brought back some of the previously lost meadows.

Left: Zostera marina. Centre: Zostera noltii; Right:Cymodocea nodosa.

I had the pleasure of diving amongst some recovered meadow patches off the coast of Sesimbra, where I was lucky enough to spot a couple of seahorses, chilling amongst the blades as they usually do. With the decline of seagrasses, this is a species that is in danger of disappearing from Portuguese waters. We are working for that not to happen.

Seahorse (Hippocampus guttulatus). Photo: Mare Per Sempre

Even after years of restoration efforts, seagrass meadows off the coast of Portugal are no where near their previous size, and in some areas they are still declining. It is difficult to re-build an ecosystem that took centuries to develop by itself, especially when some of the threats that led to its disappearance remain. Seagrasses are also naturally slow-growing plants and become even slower with increasing ocean temperatures. Therefore, to ensure that the seagrass meadows in the turquoise Mediterranean waters of Sardinia and those in the powerful Atlantic Ocean remain the underwater guardians of our costs, it is crucial to combine restoration efforts with conservation in order to restore what has been lost and preserve the ecosystems that remain.

Written by Maria Pinto (NHMW)