Semi-closed Sea Research Articles

Temporal changes of 137Cs concentrations in the Far Eastern Seas: partitioning of 137Cs between overlying waters and sediments

Deep-ocean sediments, similarly to seawater, are important reservoirs of 137Cs, an anthropogenic radionuclide with a relatively long half-live found in the Earth system. To better understand the geochemical behaviour of 137Cs in the ocean, we examined the temporal changes of 137Cs activity concentrations in the overlying waters and in sediments from the Far Eastern Seas (Sea of Japan, SOJ, and Okhotsk Sea, OS) during the period of 1998–2021. The 137Cs activity levels showed exponential changes during the observed period. The decay-corrected change rates of 137Cs in deep waters of SOJ exhibited a slow increase, while 137Cs levels in seawater and sediment in OS decreased gradually. This reflects a topographical difference, as SOJ is a semi-closed sea, whereas OS receives continuously inflow of subarctic waters. It was confirmed that 137Cs released after the Fukushima Dai-ichi Nuclear Power Plant accident was rapidly transported into the deep waters of the SOJ. To elucidate the transfer processes of 137Cs from seawater to sediment, we discussed the temporal changes of the partition coefficients (Kd) of 137Cs between the overlying water and the surface sediment. In shallow areas (< 1500 m water depth), Kd values were almost constant within the sampling periods, although the temporal changes in the Kd values occurred in deeper waters (> 2500 m depth). The Kd values increased with increasing depth, which may reflect a pressure effect as a possible mechanism. These findings suggest that chemical processes may be important factors controlling the transport of 137Cs between seawater and sediment, although more complicated phenomena occurred in deep waters and sediments of the SOJ (> 3000 m depth).

The Control of Microplastic Pollution in Semi-Closed Seas: Good-Faith Cooperation and Regional Practice

The impacts of microplastic (MP) pollution in the marine environment have already been illustrated by natural science. The issues resulting from MPs have revealed the problem of the current international governance arrangement, and international agreements are found to be inadequate to curb the growing MP pollution. This article explores reasons for the failure of governance cooperation from legal normativism and positivism. By analyzing the semi-closed sea system in the UNCLOS, this article concludes that the principle of good faith makes it possible to moderate geographic political tensions during cooperation and help reach an agreement on matters of mutual concern. Then, from the perspective of regional ocean governance, propose strategies for states bordering semi-enclosed seas and others to gradually control MP pollution in the marine environment through legal and policy measures.

Water Flow Requirements of Post-smolt Atlantic Salmon (Salmo salar L.) Reared in Intensive Seawater Flow-through Systems: A Physiological Perspective

Environmental challenges related to open sea cage production of Atlantic salmon have sparked interest in developing commercial-scale semi-closed sea systems for post-smolt Atlantic salmon (100–1000 g). Determining the mass-specific water flow required by post-smolts will largely influence the design and dimensioning of such systems. In this experiment, post-smolts were exposed to four levels of specific water flow: 0.2, 0.3, 0.4, and 0.5 L kg fish−1 min−1. All treatments involved flow-through seawater with full oxygenation, a salinity of 34‰, and a mean temperature of 9.3 °C. The stocking density was kept stable at 75 kg m−3. Water pH decreased with reduced flow, while partial pressure of carbon dioxide (pCO2) and total ammonia nitrogen (TAN) in the water increased. The increase in water CO2 was reflected in the blood with increased pCO2, HCO3−, and decreased Cl− in the lowest water flow treatment (0.2 L kg fish−1 min−1), indicating a typical regulatory response to increased water CO2 over the eight-week experimental period. No negative effects on osmoregulation, external macroscopic welfare, or performance indicators were observed, suggesting that within the time period of this experiment, post-smolts can compensate for reductions in water flow down to 0.2 L kg fish−1 min−1. However, to avoid activating and exhausting potentially energy-costly physiological regulatory mechanisms, it is suggested to keep specific water flow above 0.3 L kg fish−1 min−1 in large-scale operations with semi-closed sea systems at intermediate temperatures.

Increasing trends in spectral peak energy and period in a semi-closed sea

This study aims to investigate long-term trends in the Black Sea's spectral wave peak energy and periods. Improved Visualization of the Innovative Trend Analysis and the Mann-Kendal methods was applied to the maximum and mean spectral peak energies and peak periods between 1979 and 2020. Long-term spectral data are obtained from the ERA5 reanalysis and two spectral wave models, SWAN and WWIII. The innovative trend analysis method has the particularity to examine trends in higher and lower value categories. Studies of long-term changes in spectral wave characteristics are rare, and trends in spectral peak parameters are evaluated in this study for the first time in the Black Sea. It was detected that both spectral peak energies and peak periods tend to increase predominantly over most of the time scales. Furthermore, while the change rates for peak energies do not exceed 40% annually and seasonally, change rates exceeding 100% are observed on a monthly basis. Besides, the change rates of the peak periods vary in the ±5% band and usually do not exceed 15%. Moreover, despite a few differences, trend analysis results obtained using SWAN and WWIII models were close to the global ERA5 results. The results may provide insight into the design and durable development of coastal and marine structures as well as the evaluation of wave climate change based on spectral wave data.

Spatial calibration of WAVEWATCH III model against satellite observations using different input and dissipation parameterizations in the Black Sea

The quality of the third generation wave model WAVEWATCH III, referred WW3, using the different source term packages is investigated in the context of wave growth and dissipation in a semi-closed sea, Black Sea. Accuracy in wave height hindcast is tested for five different source term parameterizations ST1, ST2, ST3, ST4, and ST6 in WW3 against the 2020-year multi-mission satellite observations over the entire sea. The aims are to determine the most appropriate formulation and optimum tuneable coefficients for wind waves modelling in the Black Sea. For this purpose, significant wave heights (Hm0) are produced using five different source term packages implemented in the WW3 model. A sensitivity test was applied to define the tuneable parameters that contributes to the accuracy of the models, and subsequently the coefficient values of each parameters was turned to calibrate the WW3 models. The tuneable parameter allowing to obtain the most accurate result in each package are defined for each source term package. Comparing between model results and satellite observations, we found that the default WW3 model with all source terms as well as the calibrated ST2 model underestimate Hm0 in the entire domain and mainly in the eastern Black Sea region and are not suitable physical source term for the Black Sea. In the Black sea, the calibrated ST1, ST3 and ST4 forced with ERA5 wind are recommended for long term wave climate simulation, and the calibrated ST6 is recommended for extreme wave simulation.

Increasing Trends in Spectral Peak Energy and Period in a Semi-Closed Sea

Increasing Trends in Spectral Peak Energy and Period in a Semi-Closed Sea

Spatio-temporal variability of sea surface temperatures in the Red Sea and their implications on Saudi Arabia coral reefs

The Red Sea is a conspicuous water body that hosts important coral reef ecosystems. This semi-closed sea is exposed to warming either by climate change or by anthropogenic stressors. The desalination plants along the Red Sea coast of Saudi Arabia dispose tremendous effluents of hot water from cooling systems, which is expected to impact the neighboring coral reef patches. The main objectives of this study were to delineate the spatial variations and the temporal trends of sea surface temperatures (SST) in the Red Sea using geospatial analysis with focusing on water temperature anomalies adjacent to seawater desalination plants. Monthly sea surface temperatures data were acquired by the Moderate Resolution Imaging Spectroradiometer (MODIS) Aqua satellite for the period 2003-2019. Annual SST changes and decadal trends were obtained from interpolation analysis and robust statistical metrics, respectively. Results showed that the Red Sea has three distinguished temperature zones with the coldest at the north and the warmest at the south. In addition, global warming is remarkably apparent in the northern part of the Red Sea (0.8 °C/decade). About one third of the Saudi corals occur at the northern part, while the remaining coral patches settle in the middle and southern parts of relatively lower SST trends. However, some corals in the middle section of the Red Sea occur proximate to regions of abruptly high SST trends due to the discharge of hot waters from desalination plants, particularly south of Jeddah. The continuous human-induced heating by hot water disposal could be fatal to coral reef ecosystems nearby desalination plants.

Contrasting the assembly of phytoplankton and zooplankton communities in a polluted semi-closed sea: Effects of marine compartments and environmental selection

Understanding the underlying mechanisms of community assembly is a major challenge in microbial ecology, particularly in communities composed of diverse organisms with different ecological characteristics. However, very little is known about the effects of marine compartments in shaping marine planktonic communities; primarily, how they are related to organism types and environmental variables. In this study, we used multiple statistical methods to explore the mechanisms driving phytoplankton and zooplankton metacommunity dynamics at the regional scale in the Bohai Sea, China. Clear geographic patterns were observed in both phytoplankton and zooplankton communities. Zooplankton showed a stronger distance-decay of similarity than phytoplankton, which had greater community differences between locations with further distances. Our analyses indicated that the zooplankton communities were primarily governed by species sorting versus dispersal limitation than the phytoplankton communities. Furthermore, we detected that zooplankton exhibited wider habitat niche breadths and dispersal abilities than phytoplankton. Our findings also showed that environmental pollution affected high trophic organisms via food webs; the presence of heavy metals in the Bohai Sea altered the abundance of some phytoplankton, and thus modified the zooplankton that feed on them.

Quality of the ERA5 and CFSR winds and their contribution to wave modelling performance in a semi-closed sea

ABSTRACT This study aims to investigate the quality of ERA5, a recent reanalysis wind product, and its contribution to wave modelling performance in a semi-closed sea, the Black Sea. This investigation includes a comparison of ERA5 surface wind fields with the ones from the CFSR to assess if this latest reanalysis improved the representation of the surface winds. Wind speeds from both reanalyses were validated with measurements at Gloria, the only sea wind measurement station on the Black Sea. Validations were also conducted using altimeter and scatterometer satellite data after which results were later compared against each other. The second aim of this study investigates whether a wave hindcast model forced with the ERA5 wind fields has improved prediction of wave parameters. A SWAN model with default settings was used to compare the results under the same conditions. Performance analyzes of the default SWAN wave estimates with both ERA5 and CFSR winds were conducted using three offshore buoy measurements and altimeter data of satellites over the Black Sea. Results show that ERA5 winds are more biased compared to CFSR winds at Gloria location. Both CFSR and ERA5 underestimate wind speeds. ERA5 performs better than the CFSR in lower wind speeds and worse in higher wind speeds. However, ERA5 winds have less bias and are more scattered than the CFSR winds against the satellite data. SWAN driven by CFSR winds performs better than the one driven by ERA5 winds against both buoy and satellite data.

Reconstruction of the Contamination and Purification of White Sea Water to Remove Global 90Sr

Contamination with 90Sr of the White Sea water in migration during 50 years is reconstructed with the use of a three-component exponential model. Т1, Т2, and Т3, the periods of half-purification of White-Sea water to remove 90Sr were 0.8, 15.4, and 29 years, respectively. During 50 years of migration, the content of 90Sr in water decreased from 41 to 2–3 Bq/m3. The maximum decrease in the content of 90Sr was observed in 1965–1968. The outflow of 90Sr in 1970 from the White Sea was 37.5, and the inflow with Barents-Sea water, 25.5 TBq. The outflow was partly compensated for by the inflow of 4 TBq of 90Sr with water of the rivers Northern Dvina, Onega, Mezen’. The White Sea water is a weak source of 90Sr for the Barents Sea. The inflow of global 90Sr with the river flow makes slower the replacement of the more contaminated White Sea water with the purer water of the Barents Sea. The determinations of 90Sr in the White Sea water (1965–2015) in terms of the exponential model have been confirmed by the results of experimental observation of 90Sr in the sea bays. During 50 years, 1.5–2 times higher concentrations of 90Sr were preserved in the semi-closed White Sea, compared with the Barents Sea.

Source, fate and budget of Dechlorane Plus (DP) in a typical semi-closed sea, China.

Dechlorane Plus (DP), which has severe effects on marine ecosystems, has been proposed for listing under the Stockholm Convention as a persistent organic pollutant (POPs). This study was the first comprehensive investigation of the concentration and fate of DP in the Bohai Sea (BS) based on determination of river estuary water, river estuary sediment, surface seawater, bottom seawater, and sea sediments samples. The highest water DP levels were found in river estuary in Tianjin in North China due to the huge usage of DP in recent years, and spatial distribution analysis indicates it was mainly affected by regional high urbanization and emission of E-waste. The spatial distribution of DP in the BS was mainly affected by a combination of coastal hydrodynamics and land anthropogenic activities. On the basis of multi-box mass balance, simulations of DP in seawater showed an increase from 2014 to 2025, before leveling off at 184pgL -1 by a constant DP input to the BS. Riverine discharge almost contributed to the total input (∼99%) and dominated the DP levels in the BS. Degradation of DP accounted for 55.3% and 78.1% of total DP output in seawater and sediment, respectively, indicating that degradation mainly affected decline of DP in the environment.

The White Sea: Available Data and Numerical Models

The White Sea is a small shallow semi-closed sea in the North-West of Russia. It is strongly affected by induced tides, so the tidal motion dominates in the sea. Sea ice is seasonal and the water salinity is less than in the neighbouring Barents sea due to strong river discharge. We review the sources of in-situ and satellite data that are available for the sea, and describe those few numerical models, together with the challenges that are faced. We focus on the large-scale circulation and thermohaline fields, but also cover sea ice, river runoff, and pelagic biogeochemical data.

Antioxidant response of the sea urchin Paracentrotus lividus to pollution and the invasive algae Lophocladia lallemandii

Pollution derived from human activities and the arrival of invasive species are common worldwide and affect coastal marine ecosystems negatively, and more especially in a semi-closed sea such as the Mediterranean Sea. The aim of the study was to evaluate oxidative stress biomarkers in the gonadal tissue of the sea urchin Paracentrotus lividus (Lamarck, 1816) sampled in different areas of Sant Antoni de Portmany (Ibiza Island, Spain) with different anthropic activities, and in an area deeply covered by the invasive red algae Lophocladia lallemandii. The densities of P. lividus were higher in the area with the greatest anthropogenic influence, while the area invaded by L. lallemandii showed the lowest density. A significant increase in the activities of the antioxidant enzymes catalase, superoxide dismutase (SOD), glutathione peroxidase (GPx) and glutathione reductase (GRd) and the phase II detoxifying enzyme glutathione S-transferase (GST) was found in the most impacted area by the human activity. Moreover, malondialdehyde (MDA) and nitrite levels were also increased in the most impacted area. Similarly, the presence of L. lallemandii induced oxidative stress in P. lividus evidenced by a significant increase in all analysed biomarkers. In conclusion, changes in oxidative stress biomarkers are a good proxy to evaluate the impacts induced by anthropogenic activities and by the presence of invasive algae to P. lividus.

Specific oceanographic characteristics and phytoplankton responses influencing the primary production around the Ulleung Basin area in spring

The East Sea (Sea of Japan) is a marginal, semi-closed sea in the northwestern Pacific. The Ulleung Basin area, which is located near the subpolar front of the East Sea, is known to have high primary production and good fisheries in spring season. After episodic wind-driven events during the spring of 2017, horizontal and vertical profiles of physical chemical biological factors were investigated at 29 stations located in the Ulleung Basin area. In addition, growth responses of phytoplankton communities to nutrient additions were evaluated by bioassay experiments to understand the fluctuation of phytoplankton biomass. Because of strong northwestern wind, phytoplankton biomass was scattered and upwelling phenomenon might be suppressed in this season. The phytoplankton abundances in the coastal stations were significantly higher than offshore and island stations. In contrast, the nutrient and chlorophyll a (Chl a) concentrations and the phytoplankton biomass were quite low in all locations. Bacillariophyceae was dominated group (75.1% for coastal, 40.0% for offshore and 43.6% for island stations). In the algal bioassays, the phytoplankton production was stimulated by N availability. The in vivo Chl a values in the +N and +NP treatments were significantly higher than the values in the control and the +P treatments. Based on the field survey, the higher nutrients in coastal waters affected the growth of diatom assemblages, however, little prosperity of phytoplankton was observed in the offshore waters despite the injection of sufficient nutrients in bioassay experiments. The growth of phytoplankton depended on the initial cell density. All of results indicated that a dominant northwestern wind led to a limited nutrients condition at euphotic layers, and the low level of biomass supply from the coasts resulted in low primary production. Both supplying nutrients and introducing phytoplankton through the currents are critical to maintain the high productivity in the Ulleung Basin area of the East Sea.

SYNCHROTRON BASED MICRO-MAPPING AND XAFS INVESTIGATION OF Fe-Mn AND EPIGENETIC INTERGROWTH WITHIN THE CAMBRIAN SHALLOW MANGANESE DEPOSITS, SOUTH JORDAN

The current work investigates the Cambrian Manganese ore deposits from Wadi Dana at central Wadi Araba region. This investigation aims to unravel the chemistry and micro-textures of the mineral paragenetic sequence for these manganese ore deposits. Particularly the Fe–Mn intimate intergrowth micro texture and the epigenetic Mn mineralization associated with Cu minerals. The combination of the synchrotron-based X-ray fluorescence (syn-XRF) micro-mapping and X-ray absorption fine structure (XAFS) techniques were used. They provide a clear picture of the elemental distribution of Mn, Fe, Cu, and Pb and other elements presented within the ore micro textures. Besides, it determines the exact elemental speciation. The results obtained gave a better understanding of the elemental atomic structures and eventually the depositional environment. Particularly, the syn-XRF micro-mapping reveals the existing of many successive evolution stages in the Mn ores. Besides, the X-ray Absorption Near Edge Structure (XANES) results showed that the micro-rhythmic texture is changing from Mn+4 (Pyrolusite) into Fe+3 (Hematite). This is evidence for the role of Eh as the main controlling factor during the ore formation. Eventually, indicating tentatively the paleoceanography setting of the Mn deposits. This is related to the continuous transgression-regression on the sea level in a semi-closed sea.

SYNCHROTRON BASED MICRO-MAPPING AND XAFS INVESTIGATION OF Fe-Mn AND EPIGENETIC INTERGROWTH WITHIN THE CAMBRIAN SHALLOW MANGANESE DEPOSITS, SOUTH JORDAN

The current work investigates the Cambrian Manganese ore deposits from Wadi Dana at central Wadi Araba region. This investigation aims to unravel the chemistry and micro-textures of the mineral paragenetic sequence for these manganese ore deposits. Particularly the Fe–Mn intimate intergrowth micro texture and the epigenetic Mn mineralization associated with Cu minerals. The combination of the synchrotron-based X-ray fluorescence (syn-XRF) micro-mapping and X-ray absorption fine structure (XAFS) techniques were used. They provide a clear picture of the elemental distribution of Mn, Fe, Cu, and Pb and other elements presented within the ore micro textures. Besides, it determines the exact elemental speciation. The results obtained gave a better understanding of the elemental atomic structures and eventually the depositional environment. Particularly, the syn-XRF micro-mapping reveals the existing of many successive evolution stages in the Mn ores. Besides, the X-ray Absorption Near Edge Structure (XANES) results showed that the micro-rhythmic texture is changing from Mn+4 (Pyrolusite) into Fe+3 (Hematite). This is evidence for the role of Eh as the main controlling factor during the ore formation. Eventually, indicating tentatively the paleoceanography setting of the Mn deposits. This is related to the continuous transgression-regression on the sea level in a semi-closed sea.

Response of lower trophic ecosystem in the Japan Sea to horizontal nutrient flux change through the Tsushima Strait

The Japan Sea is a semi-closed sea with straits such as the Tsushima Strait (Korea Strait). The nutrient transport through the Tsushima Strait impacts on the ecosystem around the coast of Japan. We analyzed hydrographic data from the Tsushima Straits to reveal interannual variations in the volume transport, nutrient concentration, and nutrient transport. The nutrient transport in the strait has large interannual variations of ±50% of average transport. We developed a coupled ocean circulation and ecosystem model, in which the lateral boundary condition was applied based on our observation data, and investigated the biogeochemical responses in the Japan Sea to the variations in horizontal nutrient flux through the Tsushima Strait.We reproduced the lower trophic ecosystem in the Japan Sea and evaluated the contribution ratio of dissolved inorganic nitrogen supplied from the Tsushima Strait to the biogeochemical cycles in the Japan Sea. A primary production of more than 70% in the region of 300–400 km from the coast of Japan is determined by the nutrient inflow through the Tsushima Straits. We investigated the response of the lower trophic ecosystem in the Japan Sea associated with nutrient transport change (±50%) through the Tsushima Strait. We found large impacts on primary production related to horizontal nutrient flux change in the central regions in the Japan Sea and near the East Korea Warm Current region. The increase of chlorophyll a in the central regions is caused by the transportation of nutrients through the sub-surface layer of the Tsushima Straits from 50 m to 100 m. Horizontally transported nutrients in the sub-surface layer are not utilized by phytoplankton during the summer because of stratification, but they are supplied in the surface layer in autumn and winter by vertical mixing. These processes also increase primary production in broader regions (132° E, 37–41° N). The case study on the increased time for nutrient transport showed that the nutrient flux change in summer had the highest impact on the lower ecosystem changes. The present study suggests that the interannual variations in the nutrient transport through the Tsushima Straits impact the productivity of autumn blooms in the broader regions of the Japan Sea.

Distribution of perfluorinated alkyl substances in marine shellfish along the Chinese Bohai Sea coast

Perfluorinated alkyl substances (PFASs) are emerging persistent organic pollutants, which pose a threat to human health primarily by dietary exposure, especially through seafood. Bohai Sea (a semi-closed sea located north of China) is an important shellfish aquaculture area that is possibly highly-polluted with PFASs. In this study, we first evaluated contamination by PFASs in a total of 230 samples of marine shellfish from the Bohai Sea. Samples included five important shellfish species, collected from important aquaculture spots distributed around the Bohai Sea. Samples were analyzed by an ultra-fast liquid chromatography-tandem mass spectrometry method, which could simultaneously detect 23 PFASs in shellfish. Our research verified that PFASs have become a threat to the safety of shellfish products in this area. Furthermore, contamination by PFASs in shellfish changed depending on the components of PFASs, the species of shellfish, and the sampling sites. Many of the 23 target compounds contributed to the high detection ratio (>50%) as follows: perfluorooctanoic acid (PFOA) > perfluorononanoic acid > perfluorodecanesulfonic acid > perfluorooctanesulfonic acid (PFOS). Compared with other dominant components, PFOA not only had the highest detection percentage in shellfish samples (98.3%), but its detection level contributed to 87.2% of total PFASs concentrations, indicating that PFOA is the major threat to the safety of shellfish products. The highest level of PFAS was found in clams (62.5 ng g−1 wet weight of PFOA). The concentration of total PFAS in different shellfish species showed the following trend: clams > mussels > scallops > whelks > oysters. The maximum concentration of total PFAS or PFOA was found in Shouguang. The total concentration of PFOS and its precursor were highest in Cangzhou, possibly due to local industrial activities. The results presented in this paper provide new data on the contamination of marine shellfish along the Bohai Sea coasts in China, and constitute a reference for future monitoring of contamination by emerging contaminants in Bohai coast.

LARGE-SCALE EXPANSION OF HYPOXIA AT MARINE BENTHAL ZONE IN GEOMORPHOLOGICALLY COMPARABLE BAYS-FJORDS OF CRIMEA PENINSULA UNDER DIFFERENT TECHNOGENIC LOADS

Aim. Comparative analysis of the aeration features of the bottom waters during the different seasons of the year between the Sevastopol and Donuzlav Bays as geomorphological similar in configuration semi-closed sea reservoirs of the Black Sea under different anthropogenic loads. Material and methods. Sevastopol and Donuzlav Bays, located in the Western part of the Crimean Peninsula, were studied. The concentration and total consumption of dissolved oxygen in the water column and the bottom layer were determined in situ using a digital LDO analyzer. The redox potential in bottom sediments was measured using a platinum electrode. Results. Aeration of the water column in the Sevastopol seaside is suitable for normal life of pelagic and epibenthic organisms. At the same time, in the Sevastopol and Donuzlav Bays the large-scale anoxia in the surface layer of bottom sediments was found. Also, the muddy sediments of the Sevastopol Bay’ bed can be an oxygen-deficient extreme biotope for most of the year, the minimum concentration of O2 in the bottom water is not higher than 0.11-0.36 mg/l. The benthal zone here was well aerated only at the end of November. Conclusion. The general patterns of benthic hypoxia were identified for such coast objects as Sevastopol and the Donuzlav Bays, which are similar by their contours and recent seabed structures. Benthal zones of both investigated bays-fjords can be attributed to hypoxic high-gradient habitats, that have a whole generality with oxygen-deficient marine waters habitats – estuarine zones, areas of gas seeps, etc. Strong oxygen deficiency, toxic H2S contamination coupled with organic enrichment a typically benthic environment for such biotopes.

Functional-biogeography of the reef fishes of the islands of the Gulf of California: Integrating functional divergence into marine conservation

The Gulf of California (GC) is a semi-closed sea in the Tropical Eastern Pacific and is recognised as a highly diverse marine ecosystem. Despite this status, this region is still poorly studied in comparison to other marine hotspots. To start filling this gap, we attempt to provide a global overview of reef-fish diversity around the numerous islands of the region. We evaluated species richness, the abundance and biomass, and the functional diversity of the fish assemblages for the major islands of the GC. We first highlight that the southwestern part of the central GC is the hotspot of reef-fishes diversity within the GC, in terms of species richness, functional diversity, and fish abundance. We then found out an important functional divergence between fish assemblages of northern and southern regions. The fish biomass of each region is dominated by different species, characterised by different ecological traits (the opposite of functional convergence). This functional divergence may be explained by an important oceanographic heterogeneity along the latitudinal axis of the GC. The northern part shows larger climate fluctuations while the southern part is more tropical and climatically stable. Such functional divergence is a biodiversity facet to take into account when determining the sites to focus conservation action. In the GC, this criterion allows the importance of some sites to be highlighted to preserve the legacy of the reef-fishes, despite their lower diversity levels.