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Since recreational diving activities have increased in recent decades, resulting in additional environmental pressure on the coastal zone, the deployment of artificial reefs as a conservation strategy to divert mass ecotourism from fragile natural reefs has been proposed and realized in many areas of the world. Twelve units of a patented naturoid artificial reef technology developed by the Hellenic Centre for Marine Research (HCMR) were deployed in 2015 in the Underwater Biotechnological Park of Crete (UBPC) in order to create an experimental diving oasis and investigate the potential of achieving this aim for the over-exploited coastal ecosystems of this part of the Eastern Mediterranean. Assessment of the degree of establishment of artificial reefs and their ability to mimic natural ecosystems is often monitored through biological surveys and sampling. The measurement of the chemical, physical, and hydrodynamic characteristics of the water mass surrounding artificial reefs is also essential to fully understand their comparison to natural reefs. In particular, the flow field around reefs has been shown to be one of the most important physical factors in determining suitable conditions for the establishment of a number of key species on reef habitats. However, the combination of biological establishment monitoring and realistic flow-field simulation using computational fluid dynamics as a tool to aid in the design improvement of already existing reef installations has not been fully investigated in previous work. They are often reported separately as either ecological or engineering studies. Therefore, this study examined a full-scale numerical simulation of the field flow around individual already installed naturoid reef shapes, and part of their present arrangement on the sea bottom of the UPBC combined with the field-testing of the functionality of the installed artificial reefs concerning fish species aggregation. The results show that the simulated flow characteristics around the HCMR diving oasis artificial reefs were in good general agreement with the results of former studies, both for flows around a single deployed unit and for flows around a cluster of more than one unit. The results also gave good indications of the performance of individual reef units concerning key desirable characteristics such as downstream shadowing and sediment/nutrient upwelling and resuspension. In particular, they confirmed extended low flow levels (less than 0.3 m/s) and in some cases double vortexes on the downstream side of reef units where observed colonization and habitation of some key fish species had taken place. They also showed how the present distribution of units could be optimized to perform better as an integrated reef cluster. The use of computational fluid dynamics, with field survey data, is therefore suggested as a useful design improvement tool for installed reef structures and their deployment arrangement for recreational diving oases that can aid the sustainable development of the coastal zone.

The catch per unit effort (CPUE), population structure, sex ratio, and reproductive aspects of the invasive blue crab Callinectes sapidus Rathbun were studied in Monolimni Lagoon and the adjacent coastal waters in the estuarine area of the Evros River (Northeast Aegean Sea, Eastern Mediterranean). The CPUE varied both seasonally and spatially; higher values were recorded in summer and fall in the lagoonal stations characterized by shallow depth (<0.5 m) and a dense meadow of the angiosperm Ruppia maritima, whereas significantly lower CPUE was recorded in the adjacent coastal area. The bottom temperature was positively correlated with CPUE. Μodal progression analysis estimated a three-modal size–frequency distribution for both sexes corresponding to the 0+, 1+, and 2+ age classes. Females attained a significantly larger size (carapace width (CW)) than males. Crab size decreased from the marine area to the lagoonal stations. The sex ratio was estimated at 2.45:1 (♂/♀). Males dominated in all months, except for October, when an equal sex ratio was observed. The mean size at maturity (L50) of females was estimated at 124 mm CW. Females with mature ovaries were found from February to October. Ovigerous females were observed from May to October in the coastal area, where a spawning peak occurred in September. In the Evros River estuary, the American blue crab exhibits a life cycle that seems to conform to the general complex life-cycle pattern of the species along the mid-Atlantic coast.

Recreational SCUBA diving is currently a nature-based USD multibillion tourism industry across the globe. However, degradation of many recreational diving destinations all over the world due to “soft” ecotourists necessitates the adoption of innovative management measures. Hellenic Centre for Marine Research (HCMR) developed an innovative technology for the creation of artificial underwater ecotourism attractions (“oases”) to divert visitors away from sensitive marine natural areas of high ecological and aesthetic value. This innovative technology includes specially constructed artificial reefs in an attempt to simulate the functional and morphological characteristics and the aesthetics of the natural rocky reefs. In this study, a pilot survey was conducted in three diving centres of Crete Island, one of the most important tourist destinations in the Mediterranean Sea, involving the participation of 144 SCUBA divers from all over the world. The survey aimed at investigating SCUBA divers’ profiles and perceptions concerning recreational diving activities and artificial reefs technology. Findings of this study indicate that large naval shipwrecks combined with innovative man-fabricated constructions simulating natural rocky reefs meet the preferences of the majority of the participants of the survey and they can be used as an alternative tool for relevant marine ecotourism sustainable applications.

The coastal ocean is one of the most important environments on our planet, home to some of the most bio-diverse and productive ecosystems and providing key input to the livelihood of the majority of human society. It is also a highly dynamic and sensitive environment, particularly susceptible to damage from anthropogenic influences such as pollution and over-exploitation as well as the effects of climate change. These have the added potential to exacerbate other anthropogenic effects and the recent change in sea temperature can be considered as the most pervasive and severe cause of impact in coastal ecosystems worldwide. In addition to open ocean measurements, satellite observations of sea surface temperature (SST) have the potential to provide accurate synoptic coverage of this essential climate variable for the near-shore coastal ocean. However, this potential has not been fully realized, mainly because of a lack of reliable in situ validation data, and the contamination of near-shore measurements by the land. The underwater biotechnological park of Crete (UBPC) has been taking near surface temperature readings autonomously since 2014. Therefore, this study investigated the potential for this infrastructure to be used to validate SST measurements of the near-shore coastal ocean. A comparison between in situ data and Moderate Resolution Imaging Spectroradiometer (MODIS) Aqua and Terra SST data is presented for a four year (2014–2018) in situ time series recorded from the UBPC. For matchups between in situ and satellite SST data, only nighttime in situ extrapolated to the sea surface (SSTskin) data within ±1 h from the satellite’s overpass are selected and averaged. A close correlation between the in situ data and the MODIS SST was found (squared Pearson correlation coefficient-r2 > 0.9689, mean absolute error-Δ < 0.51 both for Aqua and Terra products). Moreover, close correlation was found between the satellite data and their adjacent satellite pixel’s data further from the shore (r2 > 0.9945, Δ < 0.23 for both Aqua and Terra products, daytime and nighttime satellite SST). However, there was also a consistent positive systematic difference in the satellite against satellite mean biases indicating a thermal adjacency effect from the land (e.g., mean bias between daytime Aqua satellite SST from the UBPC cell minus the respective adjacent cell’s data is δ = 0.02). Nevertheless, if improvements are made in the in situ sensors and their calibration and uncertainty evaluation, these initial results indicate that near-shore autonomous coastal underwater temperature arrays, such as the one at UBPC, could in the future provide valuable in situ data for the validation of satellite coastal SST measurements.

The Gulf of Heraklion is one of the most well-studied coastal marine ecosystems in the eastern Mediterranean. It is an oligotrophic area, exposed to wave action where muddy sediments prevail and its coastal zone is relatively unaffected by major riverine inputs. However, it faces pressures deriving from urbanisation, tourism, small-scale fisheries and climate change. Amongst its biological components, macrobenthos is an important component of soft-bottom habitats of the continental shelf and considered to be an indicator of environmental and human-induced disturbance. Nevertheless, long-term data are limited, thus restricting our understanding of their temporal trends. This is of particular importance for the Mediterranean Sea which is characterised as a hot spot of biodiversity and climate change impacts on its biota. The macrοbenthic faunal communities were investigated in the continental shelf of Heraklion Bay within the framework of the European funded project entitled MARine Biodiversity and Ecosystem Functioning leading to Ecosystem Services (MARBEFES), with the objective to enhance the understanding of marine biodiversity, ecosystem functioning and the ecosystem services they provide. Additional studies, with the same objectives, were carried under national and local monitoring programmes. Samples were collected from a specific transect of stations, named H2 (10, 20, 30, 40, 50, 100 and 200 m depth), located in the wider marine area of the wastewater treatment plant of Heraklion City. The transect was sampled during three different sampling surveys and periods: June 2010, June 2015 and August 2024.A total of 30,280 individuals were examined belonging to the taxonomic groups of Annelida, Crustacea, Mollusca, Echinodermata, Nemertea and varia. The identification of the key taxonomic classes (Polychaeta, Bivalvia, Gastropoda, Scaphopoda and Malacostraca) revealed 231 macrobenthic taxa. Out of the 231 taxa, only 30 were common amongst all the three sampling periods, while Polychaeta was the most abundant macrobenthic group in all sampling periods.

Objectives We report a transcriptome acquisition for the bath sponge Spongia officinalis , a non-model marine organism that hosts rich symbiotic microbial communities. To this end, a pipeline was developed to efficiently separate between bacterial expressed genes from those of eukaryotic origin. The transcriptome was produced to support the assessment of gene expression and, thus, the response of the sponge, to elevated temperatures, replicating conditions currently occurring in its native habitat. Data description We describe the assembled transcriptome along with the bioinformatic pipeline used to discriminate between signals of metazoan and prokaryotic origin. The pipeline involves standard read pre-processing steps and incorporates extra analyses to identify and filter prokaryotic reads out of the analysis. The proposed pipeline can be followed to overcome the technical RNASeq problems characteristic for symbiont-rich metazoan organisms with low or non-existent tissue differentiation, such as sponges and cnidarians. At the same time, it can be valuable towards the development of approaches for parallel transcriptomic studies of symbiotic communities and the host.

The emblematic sponge Spongia officinalis is currently threatened by recurrent mortality incidents in its native habitats. Elevated temperature has been indicated as a major triggering factor, but the molecular mechanisms recruited for the organism’s response to thermal shifts are yet unknown. Here, we experimentally tested the effect of exposure to temperatures of varying intensity and span on its gene expression profile, replicating gradients encountered in the species’ native habitat. Analysis revealed major shifts in the organism’s transcriptomic profile induced by temperatures corresponding to the standard seasonal maximum, triggering processes related to signal transduction, inflammation and apoptotic pathway. Further elevation of temperature corresponding to local extremes activated further the immune response of the sponge along with protein ubiquitination. Following prolonged exposure, activation of endoplasmic reticulum stress related to accumulation of misfolded proteins and signs of resilience were observed. In the latter condition, categories such as cellular response to stress, wound repair, and diminution of pathological inflammation as also genes related to cell regeneration and cell growth were upregulated. Our results highlight the acknowledged sensitivity of S. officinalis to environmental shifts, providing an insight into the molecular mechanisms involved in the process. Furthermore, they suggest innate capacity for resilience at the current thermal extremes, implying a combination of factors and not temperature per se as the lethal agent. This sheds light on the mechanisms of pressure induced by the ongoing ocean warming trend to coastal sessile invertebrates.

This study investigates the trophic diversity of fishes living in a meadow of Caulerpa prolifera on a bimonthly basis between May 2006 and April 2007 in a semi-enclosed coastal marine ecosystem of the Mediterranean Sea (Elounda Bay, Crete Island). The study area is shallow and protected from waves, and it is covered by a C. prolifera bed, characterized by high organic input and a highly diverse macrobenthic community. Feeding patterns of the fish, investigated on the basis of stomach content analyses, were described in terms of numerical abundance and frequency of occurrence of prey taxa. A total of 1642 fish individuals, belonging to 17 species, were examined. In total, 45,674 prey individuals were identified belonging to 110 prey taxa, most of which were Malacostraca including their larvae and Copepoda (41,175 individuals identified to 71 taxa). Four different trophic groups were identified: herbivorous, pelagic, benthic (hyperbenthic) and piscivorous. Trophic diversity patterns of the fish species studied were also compared to the relative availability of macrobenthic and zooplanktonic taxa during the same period in the study area. The coexistence of many different, mostly benthic but also pelagic, fishes and their juveniles implies their high trophic flexibility, which is probably important for their survival in this particular habitat. Results of the present study provide basic knowledge on trophic diversity and interactions in the marine ecosystem and, therefore, some evidence as to the protection value of this particular habitat, which is essential for the implementation of a multispecies approach to decision-makers and managers of fisheries sources of the region.

There is growing interest about marine bivalve aquaculture globally, not only for the market value of the goods produced, but also for the socio-economic and environmental services that this activity can provide. Arca noae is an endemic Mediterranean bivalve of commercial value, whose previously undescribed population in Elounda Bay we studied in terms of its structure and reproduction, while constructing a timeseries of the basic environmental parameters of the bay, thus, gaining fundamental knowledge for the potential future exploitation of the species in the area. We found a variable spatial distribution of arks in the study area, with local high peaks in the population density, consisting of smaller size individuals, in comparison to other areas. Because of protandry of the species, human pressure on this population could have a strong negative effect, by targeting the limited numbers of large females in the study area. The reproduction pattern was similar to the reports from other Mediterranean locations. The abiotic conditions in Elounda Bay differed from those in the adjacent coastal zone, confirming that the Bay is a unique semi-enclosed marine area in the island of Crete.

The colonization of artificial structures by benthic organisms in the marine realm is known to be affected by the general trophic patterns of the biogeographical zone and the prevailing environmental traits at the local scale. The present work aims to present quantitative data on the early settlement progress of macrofaunal benthic assemblages developing on artificial reefs (ARs) deployed at the Underwater Biotechnological Park of Crete (UBPC) in the oligotrophic Eastern Mediterranean. Visual census and subsequent image analysis combined with scraped quadrats were used to describe the establishment of the communities and their development over three consecutive campaigns, spanning 5 years post-deployment. Macroalgae consistently dominated in terms of coverage, while sessile invertebrates displayed different patterns over the years. Polychaeta and Bryozoa were gradually replaced by Cnidaria, while Porifera and Mollusca displayed an increasing trend over the years. Motile benthos was mainly represented by Mollusca, while the abundance of Polychaeta increased in contrast to that of Crustacea. For both sessile and motile assemblages, significant differences were observed among the years. The results of this study indicate that ecological succession is still ongoing, and further improvement in the monitoring methodology can assist towards a more accurate assessment of the community composition in complex AR structures.