Summary: The response of Posidonia oceanica meadows to global warming of the Eastern Mediterranean Sea, where the increase in sea surface temperature (SST) is particularly severe, is poorly investigated. Here, we reconstructed the long‐term P. oceanica production in 60 meadows along the Greek Seas over two decades (1997–2018), using lepidochronology. We determined the effect of warming on production by reconstructing the annual and maximum (i.e. August) SST, considering the role of other production drivers related to water quality (i.e. Chla, suspended particulate matter, Secchi depth). Grand mean (±SE) production across all sites and the study period was 48 ± 1.1 mg DW per shoot yr⁻¹. Production over the last two decades followed a trajectory of decrease, which was related to the concurrent increase in annual SST and SST_aug. Annual SST > 20°C and SST_aug > 26.5°C was related to production decline (GAMM, P < 0.05), while the rest of the tested factors did not help explain the production pattern. Our results indicate a persistent and increasing threat for Eastern Mediterranean meadows, drawing attention to management authorities, highlighting the necessity of reducing local impacts to enhance the resilience of seagrass meadows to global change threats.

Biotic indices are considered key assessment tools in most national and European policies aimed at improving the quality of coastal waters. At present, several Water Framework Directive (WFD)-compliant biotic indices based on the marine angiosperm Posidonia oceanica have been developed and applied in the Mediterranean Sea. In this study, we investigated the effectiveness of four different P. oceanica indices (POMI, PREI, Valencian CS, and BiPo) in evaluating the ecological status of coastal waters in a case study area of Greece. The evaluation, comparison, and validation of the Ecological Status Class (ESC) assessments obtained by each index were based on a set of eight common sites that encompasses the maximum range of environmental quality in the study area. Four sampling sites separated by tens of km were chosen in each of the two water bodies (WBs) studied. The spatial variations of the features of P. oceanica meadows were examined according to a hierarchical sampling design across four spatial scales, ranging from metres to tens of km, using independent nested analysis of variance. Except for the BiPo index, the reference values for each metric/index were defined by the dataset available for the study area. All biotic indices classified the WBs of the study area in Good ESC category. Only three of the four indices (PREI, Valencian CS, and BiPo) showed high comparability in the assessment of ESC at study site level. It is assumed that the differences found in the remaining index (POMI) are due to the different type of metrics taken into consideration and the different weighting given to them. Our findings suggest that all indices can provide an overall view of the cumulative impact of multiple environmental stressors existing in the study area, and can thus help raise awareness of ecosystem degradation.

The variability in stocks and accumulation rates of organic carbon (C_org), nitrogen (N), and carbonate (CaCO₃) was studied in fifteen Posidonia oceanica meadows spread throughout the South Aegean Sea (Greece). In addition, the abiotic and biotic drivers determining the pattern of variability in the accumulation rates were assessed by exploring the influence of sediment characteristics, seagrass traits, and environmental settings. The meadows supported on average (±STDEV) 14.6 ± 5.0 kg C_org m⁻², 0.47 ± 0.17 kg N m⁻², and 249 ± 210 kg CaCO₃ m⁻² in the top meter of their sediments, with mean accumulation rates over the last 500 years of 33.6 ± 23.6 g C_org m⁻² yr⁻¹, 1.00 ± 0.62 g N m⁻² yr⁻¹, and 405 ± 336 g CaCO₃ m⁻² yr⁻¹ across sites. A redundancy analysis (RDA) explained 70% of the variation in C_org, N, and CaCO₃ accumulation rates, with three sediment characteristics (i.e., sediment C_org:N and C_org:C_inorg ratios and P. oceanica contribution to the sediment C_org pool) emerging as the primary set of factors shaping the accumulation of matter, followed by seagrass traits (i.e., leaf biomass and rhizome elongation) and environmental variables (i.e., suspended organic matter). The high degree of variability within the region emphasizes the need for fine‐scale assessments to understand the local conditions influencing sequestration. Our findings underscored the critical role of seagrass meadows in carbon and nitrogen sequestration in the region, urging conservation efforts to protect these ecosystems and prevent potential losses of stored carbon and nitrogen following seagrass degradation.

The genetic traits of seagrass populations are a crucial aspect of their ecology and evolution, influencing their adaptability and resilience. Here, we studied the genetic diversity, population structure, and connectivity of eighteen Posidonia oceanica meadows extending in the Eastern Mediterranean Sea (Aegean, Ionian and Cretan Seas, Greece), combining twelve microsatellite markers and Lagrangian particle drift modelling. Our findings revealed a strong genetic differentiation between the Ionian Sea and 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. Notably, populations of the North Aegean Sea displayed the lowest genetic diversity and the highest clonality compared to the rest of the populations. The lack of substantial oceanographic connectivity between Ionian and Aegean/Cretan Sea populations supported their genetic differentiation. 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. The genetic information provided here can be incorporated into management strategies aimed at identifying suitable areas as management units in conservation efforts and determining meadows that may serve as donor sites in transplantation initiatives.

Non-indigenous species (NIS) are one of the major threats to the native marine ecosystems of the Mediterranean Sea. Halophila stipulacea was the only exotic seagrass of the Mediterranean until 2018, when small patches of a species morphologically identified as Halophila decipiens were reported in Salamina Island, Greece. Given the absence of reproductive structures during the identification and the taxonomic ambiguities known to lead to misidentifications on this genus, we reassessed the identity of this new exotic record using DNA barcoding (rbcL, matK and ITS) and the recently published taxonomic key. Despite their morphologic similarity to H. decipiens based on the new taxonomic key, the specimens showed no nucleotide differences with H. stipulacea specimens (Crete) for the three barcodes and clustered together on the ITS phylogenetic tree. Considering the high species resolution of the ITS region and the common morphological variability within the genus, the unequivocal genetic result suggests that the Halophila population found in Salamina Island most likely corresponds to a morphologically variant H. stipulacea. Our results highlight the importance of applying an integrated taxonomic approach (morphological and molecular) to taxonomically complex genera such as Halophila, in order to avoid overlooking or misreporting species range shifts, which is essential for monitoring NIS introductions.