Skeletal growth, morphology and skeletal parameters of a temperate, solitary and zooxanthellate coral along a depth gradient in the Dardanelles (Turkey)

摘要

Studying coral populations along depth gradients is important to find out if deeper habitats can act as microrefugia from bleaching and/or mortality events associated with high temperature and irradiance. Skeletal biometry, growth, and parameters of the solitary, zooxanthellate, Mediterranean endemic scleractinian Balanophyllia europaea were determined at depths of 1, 11, and 21?m in the Dardanelles (Turkey), to (1) compare the population parameters in the Eastern Mediterranean Sea with the well-characterized populations in the NW Mediterranean Sea, that are threatened by ocean warming and acidification; and (2) assess the variation of studied parameters along a depth gradient in the Dardanelles. Biometric relationships were similar to those previously described on Italian coasts. At shallow depths, average coral height decreased, likely because of (1) the higher current velocity and wave action that is expected to limit vertical skeletal growth and/or (2) the need to increase photoprotection in high light conditions. Only a very slight increase of skeletal bulk density with depth (and consequent slight decrease of porosity) was detected, while age–length relationship, linear extension rate and net calcification rate were homogeneous among depths. The homogeneous net calcification rate with depth may depend on the balance between the response of zooxanthellae photosynthesis to temperature and light. In fact, shallow depths are likely to experience higher temperatures that negatively affect photosynthetic efficiency of B. europaea , thus reducing available energy for calcification. On the other hand, lower light availability with depth is expected to decrease photosynthesis and these two effects may compensate each other. Unexpectedly, the observed net calcification rate in the Dardanelles was almost double than the higher value reported in the northwestern Mediterranean Sea. Further analyses are required to analyse if energetic trade-offs between skeletal growth an