Mucilage Problem in the Sea of Marmara
The Impact of Mucilage on Soft Corals and Gorgonians in the Sea of Marmara
Nur Eda Topçu Eryalçın, Cansu Saraçoğlu, Bayram ÖztürkSoft corals and gorgonians (Alcyonacea) are important components of Mediterranean habitats where they increase local biodiversity by modifying habitat conditions and structural complexity. Most of them are long-lived, slow-growing species with low recruitment success, therefore, particularly sensitive to anthropogenic pressures. The Sea of Marmara, with its peculiar two-layered oceanography and eutrophic structure, supports high biodiversity and biomass in Alcyonaceans. Species composition and distribution in the Prince Islands region have been monitored since 2013. Recently, an excessive sedimentation in the Princes Islands has caused a massive mortality of suspension feeders, resulting in rarefication of soft corals, and shrinkage of gorgonian populations in the area. Two Mediterranean endemic gorgonians (Spinimuricea klavereni and Paramuricea macrospina), generally found below 50 meters but exceptionally common as from 20 meters in Princes Islands, significantly decreased during that period. These two common gorgonians to the region should be considered emblematic species, especially to mobilize awareness and conservation actions, because their depth and regional distribution are a direct consequence of Marmara’s unique oceanography and they are highly threatened by regional anthropogenic pressures. An extensive and persistent mucilage that was first noticed in the winter of 2020 covered the entire Marmara basin, peaked in the spring of 2021, and then disappeared from the sea surface in July 2021. Mucilaginous fragments lasted on the sea bottom until the end of August. Sedimentation and mucilage affect sessile benthic organisms in similar ways. The major adverse effects are (i) coverage and entanglement by mucilaginous fragments that limit/ prevent respiration and nutritional activities, (ii) establishment of hypoxic and anoxic conditions on mid to long-term, (iii) formation of necrotic areas, followed by (iv) increased overgrowth risk, and (v) infections by pathogenic/ opportunistic microorganisms carried by mucilaginous fragments. During the mucilage in 2021, corals were entangled in mucilaginous fragments, resulting in high numbers of partially dead and dead colonies. In Princes Islands monitoring stations, populations of the two gorgonians S. klavereni and P. macrospina decreased by 82% and 99% respectively in size by the cumulative impacts from the two disturbances, excessive sedimentation in 2015/2016 and mucilage in 2021. Eventually, the risk of regional extinction for P. macrospina in the Sea of Marmara needs to be evaluated. An assessment of the current distribution and health status of these gorgonians throughout the Sea of Marmara seems urgently required. Moreover, it is strongly advised to safeguard healthy colonies from mechanical damage by fishing activities and anchoring in the Sea of Marmara, where the recovery of which is aided by newly put into effect regulations, i.e., the 2021 Action Plan and SEPA (Special Environmental Protection Area) status.
