The Sedimentary Role of Posidonia Oceanica in Wave-Dynamic Bays. The case of Varkiza Bay

Supervisor: Christos Anagnostou (Hellenic Centre for Marine Research, Athens)
The present work studied the impacts of the Posidonia Oceanica meadows to the sedimentation and geomorphological patterns of the Bay of Varkiza. Firstly, topographic measurements were collected before and after a severe storm, revealing the wave-dynamic aspect of the bay. As a result, the Bay of Varkiza can be considered a perfect wave-dynamic bay and the survey can be generalized to bays that share the same characteristics. Simultaneously to the topographic survey, sand samples were collected at the shoreline of the Bay of Varkiza. These samples helped us produce a shoreline sediment scale distribution which was used as a reference for the samples collected later offshore.  With the use of a research vessel we collected in-situ sand samples of several locations of interest in the bay. The samples’ analysis promoted the biogenic and muddy character of the sediment distribution in areas that fall into the immediate vicinity of the Posidonia Oceanica meadows. These areas can either be dominated by a dense meadow or located in a sparse meadow or simply in the “intermatte”. On the other hand, areas slightly distanced from the meadows, showed similar sediment distribution characteristics to the reference scale distribution of the bay’s shoreline. These areas are dominated by terrigenous material.  Additionally, we produced a seabed sediment classification table and map for the Bay of Varkiza. This table can be broadly used for other wave-dynamic bays as well.  Moreover, the analysis demonstrated that the weight distribution appears to be widespread at areas immediately affected by more than one factors, such as the meadows the wave action etc. Furthermore, following the installation of a Side Scan Sonar on a vessel and the help of various computer software, we produced a handful of maps demonstrating the geomorphology of the Bay of Varkiza, such as the slope, aspect and bathymetry of the bay. These maps helped us clarify the fact that the Posidonia Oceanica can alter the geomorphology of the bay. Finally, we analysed sand samples collected by fellow HCMR researchers in the Bay of Vouliagmeni in a Posidonia Oceanica site located there. This analysis confirmed the results obtained from the Bay of Varkiza and solidified the results.