Ecological Changes in the Sea of Marmara

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DOI :10.26650/B/LSB21LSB37.2024.023.022   IUP :10.26650/B/LSB21LSB37.2024.023.022    Full Text (PDF)

Prince Islands in the Sea of Marmara: Safe Haven or War Zone for Octocorals ?

Nur Eda Topçu Eryalçın, Cansu Saraçoğlu, İzzet Noyan Yılmaz, Bayram Öztürk

 Prince Islands are an archipelago facing the Anatolian coasts of Istanbul megacity. The southern coasts of the islands present rich octocoral assemblages. Soft corals (those without an axial skeleton) and gorgonians (those with a flexible axial skeleton) are absent on the sea bottom within the brackish Black Sea originated upper layer. Soft corals and gorgonians are present below the halocline layer, from 20 – 22 m to deeper parts, depending on the species. Four gorgonian species can commonly be found in this area: the Mediterranean yellow gorgonian Eunicella cavolini and the red gorgonian Paramuricea clavata mainly inhabit steep/inclined hard substrate, while Paramuricea macrospina and Spinimuricea klavereni also colonize horizontal substrates. In the last years, the area received excessive sediment loads, initially stemming from a construction project (2015-2016) and later experiencing a large scale mucilage event in 2021. We monitored the distribution and density of soft corals and gorgonians at nine sites in 2013 – 2014, 2015 – 2016 and then in 2021-2022 with a transect technique. We surveyed the demographic pattern of the yellow gorgonian in 2015 – 2016 and 2021-2022 with a quadrat technique. Abundances of soft corals (Alcyonium palmatum and Paralcyonium spinulosum) in Prince Islands decreased by 91.60 % in 2015. In 2021, following the severe mucilage, soft corals witnessed another decrease by 76.74 %. Abundances of healthy gorgonian in Prince Islands decreased by 88.03 % in 2015 and then by 51.11 % in 2021 – 2022 resulting in a cumulative decrease by 94.15 % compared to 2013 abundances. Compared to the pre-disturbance conditions, there were 36   times more injured gorgonians in 2015 than 2013 and 10- times more than 2013 following the mucilage (average of 2021 – 2022 data). The number of dead colonies were 147 times more than 2013 in 2015 and 23- times more than 2013 following the mucilage (average of 2021 – 2022 data). The decreases of these keystone species in the area are alarming, particularly for soft corals and Paramuricea spp., and may have severe consequences in terms of functional diversity in the Prince Islands area.The two yellow gorgonian populations were dominated by small colonies in both years, however, the demographic pattern was significantly different between the two monitored years. Following the mucilage, large-sized colonies highly decreased while small sizes increased tremendously. In case severe mucilage reappears in the Sea of Marmara, small colonies may face a higher mortality and the population would be at risk of disappearing since the reproductive output of the remaining mid-large size colonies might not suffice another recovery by increased recruitment.The ecological condition of the Sea of Marmara is undeniably concerning, demanding immediate and effective action to prevent the recurrence of mucilage outbreaks and limit sediment input from anthropogenic terrestrial sources. We recommend restriction zones to protect octocorals and other vulnerable benthic organisms from mechanical damage until the environmental conditions improve in this inner sea. Considering the slow progress of natural recovery and the decrease in population sizes, specific restorative actions could be considered for certain species.


Keywords: Soft corals, gorgonians, biomonitoring, demographics, Mediterranean octocorals

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