| Abstract |
In times of global change, coastal ecosystems are exposed to direct and indirect anthropogenic pressures that affect their functions and lead to their gradual degradation. Among the coastal ecosystems exposed to multiple pressures is the endemic seagrass species of the Mediterranean Sea, Posidonia oceanica (L.) Delile. P. oceanica meadows of the Eastern Mediterranean Sea, and the Greek Seas in particular, remain largely unexplored, despite the intense pressures to which they are subjected and the special environmental characteristics of the Eastern Mediterranean Sea, as most relevant research has focused on the Western and Central Mediterranean meadows. In the present PhD thesis, the function of the P. oceanica meadows of the Greek Seas and their vulnerability to environmental changes were investigated by studying the spatiotemporal variation of their genetic and functional traits and their response to global (warming) and local (pollution) pressures. In total, 65 P. oceanica meadows were studied along the species’ distribution range in the Greek Seas (Ionian Sea, North and South Aegean Sea, Cretan Sea). To achieve the objectives of the present thesis, genetic and biochemical laboratory analyses, satellite data and Lagrangian models were combined, to produce the largest time - series of data (about 20 years) for P. oceanica meadows in Greece. In Chapter 2, the genetic diversity, population structure and connectivity of P. oceanica meadows along the Greek Seas (Aegean, Ionian, and Cretan Seas) were studied. For this purpose, twelve microsatellite markers and Lagrangian particle drift modelling were applied. Our findings revealed a genetic differentiation of P. oceanica populations, with a distinct separation between the populations of the Ionian Sea and the populations of the Aegean and Cretan Seas, suggesting limited genetic exchange between these two groups. High gene flow was observed within the meadows of the Aegean and Cretan Seas, indicating a well-connected group of populations. However, populations of the North Aegean Sea showed spatial differences to those of the South Aegean and Cretan Seas, with the former displaying overall lower genetic diversity and higher clonality. The results of the Lagrangian particle dispersal simulations supported the genetic differentiation between Ionian and Aegean/ Cretan Seas populations. However, the Lagrangian simulations did not fully support gene flow patterns in the Aegean Sea, suggesting that in addition to contemporary processes, historical events may have contributed to the formation of the observed genetic pattern. In Chapter 3, the rhizome production patterns of P. oceanica meadows of the Mediterranean Sea were examined to investigate the dynamic of the meadows in a changing environment. For this purpose, the production data of P. oceanica meadows covering their distribution range in Greece, were combined with the literature already reported in the other Mediterranean countries. In total, data were collected from 60 meadows along the Greek Seas and their rhizome production was reconstructed for the period 1997 - 2018, using lepidochronology. The grand mean (± SE) rhizome production among all sites and throughout the whole study period in Greece was 48 ± 1.1 mg DW shoot-1 yr -1 . The Cyclades Islands had the highest mean rhizome production (58.2 ± 3.1 mg DW shoot-1 yr-1) compared to the grand mean of rhizome production for the whole country. However, the differences between regions appeared to be primarily driven by specific study sites, indicating that the variability in rhizome production did not stem from geographic discontinuities, but rather from local differentiations. Among the Mediterranean countries, the lower rhizome production was found in the meadows of Egypt (30 ± 14 mg DW shoot-1 yr-1) and the highest in the meadows of Tunisia (173 ± 28 mg DW shoot-1 yr--1). The mean rhizome production for the species in the Mediterranean Sea was 83 ± 45 mg DW shoot-1 yr-1, ranging from 17 to 273 mg DW shoot-1 yr-1 . Rhizome production in the Western and Central Mediterranean was higher compared to the Eastern Mediterranean, with the highest values found in Tunisia and in Sicily, reflecting the differences in nutrient availability and evolutionary processes across the Mediterranean basins. In Chapter 4, the vulnerability of Greek P. oceanica meadows to the increase in sea surface temperature (SST) of the Eastern Mediterranean, was examined to assess the vulnerability of the country’s meadows to the climate crisis. For this purpose, the rhizome production data of the Greek meadows for the period 1997 – 2018 were used (Chapter 3). For the same period, the SST was reconstructed using satellite data of the annual and maximum (i.e., of August, SSTaug) SST using the Copernicus database. The effect of warming on P. oceanica rhizome production was determined considering the role of other production drivers related to water quality (i.e., nutrient, and light availability and transparency using satellite data of chlorophyll a, Secchi depth and suspended particulate matter, respectively). Rhizome production over the last two decades followed a trajectory of decrease, which was related to the concurrent increase in SST and SSTaug. Specifically, SST > 20°C and SSTaug > 26.5°C was related to the rhizome production decline, while the rest of the factors examined did not help explain the rhizome production pattern. It becomes apparent that warming is a constant and increasing threat to the seagrass meadows of the Eastern Mediterranean, underlining the need for management measures to address it. In Chapter 5, the effects of possible pollution from fish farming on P. oceanica meadows were studied to assess their vulnerability to local pressures. The effect was studied in two meadows in proximity to fish farm activities in North Aegean Sea. In each site, two stations were chosen, one close (Station ‘Cage) and one in safe distance (Station ‘Control’) from the fish farm cages. Seagrass rhizome production and the concentrations of ten trace elements (TEs) (As, Cd, Co, Cu, Mn, Mo, Ni, Pb, V, Zn) in the shoots (leaf blades, sheaths, rhizomes, roots, dead sheaths) were reconstructed for a decade (2012 – 2021), using lepidochronology. Seagrass production at the ‘Cage’ stations was up to 50 % lower than at the ‘Control’ stations. The concentrations of most TEs were higher in the shoots of the ‘Cage’ stations than in the ‘Control’ stations, especially in the one fish farm. The contamination factor (CF) of the ‘Cage’ stations was mostly moderate, while an increasing contamination trend was found for certain potential phytotoxic TEs (As, Cu, Cd, Mo, V). The results indicate possible seagrass degradation due to fish farming activities. However, the dissimilarities between study sites highlight the importance of local environmental conditions. The data generated in the present PhD thesis contribute to a more comprehensive understanding of P. oceanica dynamics in the Mediterranean Sea by filling an existing gap in literature concerning P. oceanica meadows of the Eastern Mediterranean. The results underscore the importance of local conditions in shaping the genetic and functional characteristics of P. oceanica meadows but also indicate their response to increasing threats from both global and local pressures, calling attention to policy makers for strategic conservation measures and/or restoration of seagrass meadows to strengthen the resilience of coastal ecosystems.
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