This project is currently a research proposal supported by 17 scientists belonging to different research institutions (Spanish Institute of Oceanography, IEO; University of Basque Country, UPV-EHU; Institute of Marine Sciences, ICM-CSIC; Dalhousie University, Halifax, Canada; University of Vigo; Marine Research Center, CIMA-Xunta de Galicia; University of Santiago de Compostela, UsC) and led by IEO (Oceanographic Centers of Murcia, Baleares, Málaga and Canarias). Funding for this proposal titled “Physiological and technical bases for the use of flat oyster, Ostrea edulis, in bioremediation actions in the Mar Menor lagoon (SE, Spain)” is pending.
The Mar Menor is a hypersaline protected lagoon located in the southeast of Spain. It is one of the largest coastal lagoons in the Mediterranean region, as well as in Europe. The lagoon has a maximum depth of 7 m and an average depth of 3.6 m. It has an extension of 135.5 km2 and the total volume of water in the lagoon is 610 hm3. A sandbar called La Manga, of about 23 km in length, with a maximum width of 900 m, acts as a barrier between the lagoon and the Mediterranean Sea. Three shallow main channels in the sandbar connect the lagoon with the open sea.
Traditionally, the Mar Menor has been an oligotrophic and hypersaline (53-70 ups) ecosystem that was characterized by the transparency of its waters, unlike other coastal lagoons, with sedimentary bottoms dominated by seagrasses. But since the 1970s, this ecosystem underwent a great oceanographic transformation, due to the enlargement of one of its channels which caused the decrease of Mar Menor salinity to 42-48 ups. During the same decade, there was a change in agricultural practices where rainfed crops were replaced by intensive irrigated crops, a fact that led to a great increase of enriched agricultural waters to the lagoon through its numerous watercourses of its watershed. In addition to the agriculture impact, uncontrolled tourism was developed, since the 60’s, around the perimeter of the lagoon. As a consequence of all these effects, a massive proliferation of phytoplankton bloomed in 2015, in a slight way, and more obviously in 2016, which transformed the transparent waters of the lagoon into a green soup. Since then, eutrophication crises have been recurrent, indicating the instability and fragility of the environmental status of the lagoon. The situation reached its worst moment after episodes of torrential rains caused by a cold air pool that abruptly decreased salinity in the lagoon in September 2019. As a consequence, an major episode of anoxia occurred causing the death of all organisms above 3 m of depth.
The progressive decrease in the salinity of Mar Menor in the last decades allowed the entry and settlement of new organisms and, in 1972, the first flat oysters (Ostrea edulis) were documented. During the 1980s, the flat oyster population developed, being commercialized by local fishermen, registering a production of around 140 t. However, the extraction of commercial-sized oysters was abandoned because they showed an unattractive appearance for the consumer (developing extremely thick shells), together with the difficulty of their extraction since they were distributed on muddy bottoms covered by dense meadows of Caulerpa prolifera, another organism that initially dominated the bottom of the lagoon after the salinity drop. The studies of those years estimated an oyster population of 135 million with an average density of 2 oysters/m2 and a maximum density of 22 oysters/m2, where 25% of the population reached commercial sizes. The last evaluation of the oyster population was carried out in 2006 and a reduction of almost 10 times was verified with respect to the 1992 census. Since then, no study has been conducted on the flat oyster in Mar Menor and the current status of the oyster populations in the lagoon is unknown.
Project aims to:
- gain KNOWLEDGE about the feeding physiology of the oyster and its nutrient extraction capability throughout a phytoplankton bloom and to
- develop the necessary TOOLS for a future oyster restoration action within the framework of a comprehensive plan for the restoration of the lagoon.
The project has been articulated along four axes:
- Science. Studies on the status and characteristics (biology, physiology, pathology, genetics, microbiome) of the current oyster population and on the feeding behavior (individual and population level) in different environmental scenarios including phytoplankton blooms.
- Restoration. Given the actual situation of the lagoon, the oyster population needs to be supported with seed produced in a hatchery. The project includes the construction of a pilot hatchery at the IEO facilities in the Mar Menor in order to assess the viability of seed production from broodstock obtained from the lagoon itself.
- Education. In order to involve and introduce this project and the future bioremediation actions to the community, the construction of multifunction floating platforms are planned. The platforms will serve, at water level, as nurseries and broodstock maintenance, and, at surface level, as a laboratory for physiological measurements and outreach activities.
- Blue Growth. Since the lagoon is affected by pollution from agriculture and mining, a complete study will be carried out on the levels of contaminants accumulated in oysters, in order to establish its possible suitability for human consumption and thus generate added value to the bioremediation actions.