Southwestern Mediterranean Sea Research Articles

Age and Growth of European Pilchard Sardina pilchardus (Clupeidae) from the Southwestern Mediterranean Sea

Age and Growth of European Pilchard Sardina pilchardus (Clupeidae) from the Southwestern Mediterranean Sea

Assessment of ecological risk and metal contamination caused by the Lihoud River emissary along the Bay of Tangier Littoral in Morocco (Southwestern Mediterranean Sea)

Assessment of ecological risk and metal contamination caused by the Lihoud River emissary along the Bay of Tangier Littoral in Morocco (Southwestern Mediterranean Sea)

Distribution, ecological, and health risk assessment of trace elements in the surface seawater along the littoral of Tangier Bay (Southwestern Mediterranean Sea)

In the current study, an environmental assessment of surface seawater in Tangier Bay was conducted by analyzing physicochemical parameters and trace elements, such as As, Cr, Zn, Cd, Pb, and Cu. The results showed mean concentrations (μg/l) of 22.50 for As, 0.46 for Cr, 8.57 for Zn, 15.41 for Cd, 0.23 for Pb, and 1.83 for Cu. While most trace elements met the guidelines, elevated levels of Cd raised concerns about long-term exposure. Pollution indices, including the contamination factor, degree of contamination, and water quality index, indicate the impact of human activities, dividing sites into arsenic‐cadmium contamination, wastewater influence, and low pollution levels. Statistical methods, such as ANOVA, revealed no significant differences in trace element levels across the bay. PCA and HCA revealed that Cr, Cu, and Zn originated from common anthropogenic sources, whereas Pb and Cd originated from distinct sources. As indicates that natural geological processes influence its origin.

Uneven response of phytoplankton-bacteria coupling under Saharan dust pulse and ultraviolet radiation in the south-western Mediterranean Sea

The microbial carbon (C) flux in the ocean is a key functional process governed by the excretion of organic carbon by phytoplankton (EOC) and heterotrophic bacterial carbon demand (BCD). Ultraviolet radiation (UVR) levels in upper mixed layers and increasing atmospheric dust deposition from arid regions may alter the degree of coupling in the phytoplankton-bacteria relationship (measured as BCD:EOC ratio) with consequences for the C-flux through these compartments in marine oligotrophic ecosystem. Firstly, we performed a field study across the south-western (SW) Mediterranean Sea to assess the degree of coupling (BCD:EOC) and how it may be related to metabolic balance (total primary production: community respiration; PPT:CR). Secondly, we conducted a microcosm experiment in two contrasting areas (heterotrophic nearshore and autotrophic open sea) to test the impact of UVR and dust interaction on microbial C flux. In the field study, we found that BCD was not satisfied by EOC (i.e., BCD:EOC >1; uncoupled phytoplankton-bacteria relationship). BCD:EOC ratio was negatively related to PPT:CR ratio across the SW Mediterranean Sea. A spatial pattern emerged, i.e. in autotrophic open sea stations uncoupling was less severe (BCD:EOC ranged 1–2), whereas heterotrophic nearshore stations uncoupling was more severe (BCD:EOC > 2). In the experimental study, in the seawater both enriched with dust and under UVR, BCD:EOC ratio decreased by stimulating autotrophic processes (particulate primary production (PPP) and EOC) in the heterotrophic nearshore area, whereas BCD:EOC increased by stimulating heterotrophic processes [heterotrophic bacterial production (HBP), bacterial growth efficiency (BGE), bacterial respiration (BR)] in the autotrophic open sea. Our results show that this spatial pattern could be reversed under future UVR × Dust scenario. Overall, the impact of greater dust deposition and higher UVR levels will alter the phytoplankton-bacteria C-flux with consequences for the productivity of both communities, their standing stocks, and ultimately, the ecosystem's metabolic balance at the sea surface.

Late Miocene palaeobiogeography of the Mediterranean-Atlantic Region: An analysis based on shelf ostracod assemblages of the Northwestern Morocco

A Late Miocene palaeobiogeographic framework of the Mediterranean-Atlantic Region, based on a comparative study of the continental shelf ostracod assemblages collected from a section in the Saïss Basin, northwestern Morocco, is proposed. Distribution data and statistical analyses allowed to identify six palaeoecoregions. Five of these units, that is South European Atlantic Shelf, Saharan Upwelling, Adriatic Sea, Western Mediterranean and Alboran Sea, were identified according to the ecoregions used in modern biogeographic frameworks. For the easternmost part of the Proto-Mediterranean basin a “Late Miocene Southeastern Mediterranean” palaeoecoregion is proposed. The Tortonian-Messinian palaeobiogeography of the so-called “Miocene European-West African Province”, preceding the onset of the Messinian Salinity Crisis, was under the influence of the relationships between Atlantic and Proto-Mediterranean biota in tropical, subtropical and possibly warm temperate waters.

Distribution and species composition of planktonic cnidarians in the Algerian coastal waters (SW Mediterranean Sea)

This study presents an inventory of planktonic cnidarian species collected in Algerian coastal waters (1°10’00” W - 6°52’37” E and 35°37’42” N - 37°01’16” N) from May 2012 to February 2017. The aim was to analyse the qualitative distribution and identify the main components of this gelatinous fauna in different coastal ecosystems in western, central and eastern regions. A total of 60 taxa of planktonic cnidarians were identified, including 57 hydrozoans and three scyphozoans. The class Hydrozoa were represented by 34 hydromedusae, primarily belonging to orders Anthoathecata (15 taxa) and Siphonophorae (23 taxa). Several reported hydrozoan species were recorded for the first time in the southwestern Mediterranean Sea. These new regional records include the hydromedusae Kantiella enigmatica, Leuckartiara octona, Proboscidactylaornata, Cunina globosa, Lovenella cirrata, Cirrholovenia tetranema, Phialella quadrata, Homoeonema platygonon and the siphonophore Forskalia contorta. Seasonal fluctuations were observed in biodiversity, with the highest species richness in spring and summer. The most common species are the calycophorous siphonophores Abylopsis tetragona, Muggiaea atlantica, Lensia subtilis Muggiaea kochii and the holoplanktonic medusae Aglaura hemistoma, Rhopalonema velatum, Solmundella bitentaculata, Liriope tetraphylla and Pelagia noctiluca. The species’ occurrence frequency exhibited seasonal and spatial variability. These distribution patterns were primarily determined by meroplanktonic hydrozoans (Anthoathecata and Leptothecata).

Seasonal dynamics and annual budget of dissolved inorganic carbon in the northwestern Mediterranean deep-convection region

Abstract. Deep convection plays a key role in the circulation, thermodynamics, and biogeochemical cycles in the Mediterranean Sea, which is considered to be a hotspot of biodiversity and climate change. In the framework of the DEWEX (Dense Water Experiment) project, the seasonal and annual budgets of dissolved inorganic carbon in the deep-convection area of the northwestern Mediterranean Sea are investigated over the period September 2012–September 2013 using a 3D coupled physical–biogeochemical–chemical modeling approach. At the annual scale, we estimate that the northwestern Mediterranean Sea's deep-convection region was a moderate sink of 0.5 mol C m−2 yr−1 of CO2 for the atmosphere. The model results show the reduction of oceanic CO2 uptake during deep convection and its increase during the abrupt spring phytoplankton bloom following the deep-convection events. We highlight the major roles in the annual dissolved inorganic carbon budget of both the biogeochemical and physical fluxes, which amount to −3.7 and 3.3 mol C m−2 yr−1, respectively, and are 1 order of magnitude higher than the air–sea CO2 flux. The upper layer (from the surface to 150 m depth) of the northwestern deep-convection region gained dissolved inorganic carbon through vertical physical transport and, to a lesser extent, oceanic CO2 uptake, and it lost dissolved inorganic carbon through lateral transport and biogeochemical fluxes. The region, covering 2.5 % of the Mediterranean, acted as a source of dissolved inorganic carbon for the surface and intermediate water masses of the Balearic Sea and southwestern Mediterranean Sea and could represent up to 22 % and 11 %, respectively, of the CO2 exchanges with the Atlantic Ocean at the Strait of Gibraltar.

Distribution and behaviour of striped dolphins in the southwestern Mediterranean Sea based on whale-watching data

The striped dolphin (Stenella coeruleoalba) is a cosmopolitan cetacean and the most commonly sighted dolphin in the Mediterranean Sea. It usually appears in groups of very different sizes, ranging from less than ten to more than 500 individuals, although it is usually found in groups of between 21 and 50 individuals. In the western Mediterranean, and more specifically in the Gulf of Mazarrón, S. coeruleoalba was the most frequently sighted cetacean during the 1042 whale-watching trips. The goal of this study was to establish the spatial and temporal distribution of striped dolphin sightings along the Gulf of Mazarrón between 2004 and 2014. Spatial patterns were analysed using a Random Forest based Species Distribution Model to estimate the presence of the species. Twentythree variables (three geographic, one temporal, eight geomorphometric and twelve oceanographic) were used as predictors. Out of the 1042 cruises, 872 records of striped dolphins were obtained. Some variations in the grouping patterns of these mammals were observed during the years 2006–2007, with an average shift in the size of the groups to fewer individuals (3−10). This variation is probably related to an epizootic event of morbillivirus occurring during those years, which was responsible for an abnormal rate of strandings of striped dolphins and long-finned pilot whales (Globicephala melas). The Random Forest model allowed to select 6 predictors related to morphometry and sea currents, suggesting the importance of specific habitat in offshore areas between 1000 and 3000 m depth in the continental slope.

Opportunity observation of an Algerian Eddy to the south of Cape Palos (southwestern Mediterranean Sea)

Large anticyclonic eddies can detach from the Algerian Current, forming open-sea Algerian Eddies. These mesoscale structures have been intensively studied by means of sea surface temperature and altimetry data, and using numerical models. However, few studies describe an in situ sampling of their whole vertical structure. Furthermore, the area extending from Cape La Nao (western edge of the Balearic Channels) to the Almería-Orán Front has received very little attention, and it could be considered that there is a gap in our present oceanographic knowledge of this part of the western Mediterranean. An Algerian Eddy lasting for several months was detected in December 2021 to the south of Cape Palos. In order to analyse this eddy, an opportunity sampling was designed taking advantage of the periodic monitoring campaign RADMED 0222. This sampling revealed that the eddy had a baroclinic character, affecting the whole water column. These results suggest that this eddy was generated at the Algerian Current, finally affecting an area close to the eastern Spanish coast. The presence of these structures in this region of the western Mediterranean could alter the southward progression of the Northern Current and even the presence and structure of the Almería-Orán Front.

Transport and distributions of naturally and anthropogenically sourced trace metals and arsenic in submarine canyons

Continental margins play a key role in the cycling of natural and anthropogenic trace metals (TMs) as pathways at the interface between landmasses and deep ocean basins but also as sinks. Knowledge of how short-lived forcings alter the export dynamics of TMs is essential for our understanding of their fate in that setting. Here we report time series of particulate metal fluxes in three submarine canyons —namely Escombreras, Almeria and the Garrucha-Almanzora system— of the South-Western Mediterranean Sea. Our research focuses on combining multi-elemental TMs (Al, Fe, Ti, Co, Cu, Mn, Ni, Pb and Zn) and As (a metalloid) contents of settling particles collected near the bottom by automated particle traps during one year, and seafloor sediment samples from below the traps. We assess the role of storms and bottom trawling in the off-shelf transport of particulate TMs and As, and the natural and anthropogenic contributions of TMs by using enrichment factors (EFs).The TM export fluxes and composition changed over the study period, from March 2015 to March 2016. TM fluxes increase in early spring 2015 in association with short-lived storm events and during calm months in the Garrucha-Almanzora Canyon system, likely due to sediment resuspension triggered by bottom trawling. In terms of composition, TMs in the sinking fluxes appear to be closely associated with lithogenic (Al, Fe and Ti) and authigenic (Mn) particles’ proxies. During storm events, the mass of settling particles in Escombreras and Almeria canyons was impoverished in Al, Fe, As, Co, Cu, Mn and Ni compared to other periods. The Garrucha-Almanzora Canyon system behaves differently as the above-described differences, are not observed there. Moreover, the TM composition of the sediments —with higher contents of Fe, Ti and several other TMs— in this canyon is barely tied to the composition of the settling particles. Finally, Cu and Zn contents, together with Pb in the northernmost Escombreras Canyon, are best explained by referring to anthropogenic sources. This work provides insights into the profound influence of the natural and anthropogenic forcings controlling the distributions and seasonal dynamics of particulate TMs and As in submarine canyons.

The contrasted phytoplankton dynamics across a frontal system in the southwestern Mediterranean Sea

Abstract. Numerical simulations have shown that finescale structures such as fronts are often suitable places for the generation of vertical velocities, transporting subsurface nutrients to the euphotic zone and thus modulating phytoplankton abundance and community structure. In these structures, direct in situ estimations of the phytoplankton growth rates are rare; although difficult to obtain, they provide precious information on the ecosystem functioning. Here, we consider the case of a front separating two water masses characterized by several phytoplankton groups with different abundances in the southwestern Mediterranean Sea. In order to estimate possible differences in growth rates, we measured the phytoplankton diurnal cycle in these two water masses as identified by an adaptive and Lagrangian sampling strategy. A size-structured population model was then applied to these data to estimate the growth and loss rates for each phytoplankton group identified by flow cytometry, showing that these two population parameters are significantly different on the two sides of the front and consistent with the relative abundances. Our results introduce a general method for estimating growth rates at frontal systems, paving the way for in situ exploration of finescale biophysical interactions.

Temporal patterns of gelatinous zooplankton distribution and environmental drivers in the south-western Mediterranean Sea.

This study aims to investigate the distribution of gelatinous zooplankton in relation to environmental parameters along the coastal regions of Algeria in the south-western Mediterranean Sea. A total of 48 species were recorded from nine sampling stations located in the central (Sidi Fredj) and western (Habibas Islands) sectors of the Algerian coast. The results showed that the seasonal distribution of gelatinous species exhibits significant variations. Amongst cnidarians, P.noctiluca, M.atlantica and A.tetragona are the most abundant species. Chaetognaths are primarily represented by F.enflata and P.friderici. Tunicates display high diversity, with T.democratica, O.longicauda and D.nationalis as the most abundant species. Lastly, in molluscs, H.inflatus and L.trochiformis are the most abundant species. The nMDS and ANOSIM analysis reveal significant differences in the ecological community structures between the Habibas Islands and Sidi Fredj. Redundancy analysis results show relationships between different marine species and environmental variables, such as temperature, chlorophyll a and salinity. The studied species exhibit positive or negative correlations with these variables, suggesting an influence of these factors on their abundance and distribution. This study enhances our understanding of the factors that govern the distribution and dispersal of gelatinous zooplankton in the Mediterranean Sea and has significant implications for predicting changes in the distribution of these species under future environmental scenarios.

Influence of environmental factors on biodiversity, abundance and the distribution pattern of dinoflagellates and ciliates during spring and summer in coastal waters of Algeria (southwestern Mediterranean Sea)

Abstract The objective of this study was to assess the distribution patterns of dinoflagellates and ciliates communities during planktonic bloom and post-bloom development periods, in relation to environmental parameters. Their distribution was studied during spring and summer 2012, in coastal waters of Algeria at six sampling stations (four sampling layers). Overall, 116 species were identified, including 98 dinoflagellates. The species richness of microzooplankton was higher in summer (81 species: 67 dinoflagellates, seven tintinnids and seven ciliates) than in spring (76 species: 72 dinoflagellates, three naked ciliates and one tintinnid). Significant difference in total abundances was observed between spring (median = 145 ind l−1) and summer (median = 90 ind l−1) but no significance (P > 0.05, Mann–Whitney test) in Shannon–Wiener (H′spring: 3.31 bits ind−1; H′summer: 3.70 bits ind−1) and evenness (Espring: 0.77; Esummer: 0.84) indices. The ciliate average abundance was higher in summer (11.3 ind l−1) than in spring (1.95 ind l−1), whereas dinoflagellate average abundance was lower in summer (127.92 ind l−1) than spring (190.19 ind l−1). Non-metric multidimensional scaling was used to identify different sample assemblages. It showed that temperature and salinity influenced the distribution pattern in the canonical correspondence analysis followed by chlorophyll a, silicate and nitrate concentrations. Our framework provides insight regarding trait trade off with implications for feedbacks to ecosystems, aiming to bridge the gap of plankton community ecology in Algeria. It elaborates a taxonomic list of dinoflagellates and ciliates in the marine pelagic ecosystem and performs their ecological characterization in their environment.

Monitoring of a sediment plume produced by a deep-sea mining test in shallow water, Málaga Bight, Alboran Sea (southwestern Mediterranean Sea)

In this study different experimental designs for monitoring of sediment plumes produced by deep-sea mining are presented. Plumes of sediment stirred up from the seabed by mining machines are considered to represent a major environmental pressure which may extend far beyond the actual mining area. Two industry field tests with the scaled mining vehicle Apollo II of Royal IHC conducted in a relatively shallow setting offshore southern Spain provided valuable insights for anticipated monitoring of nodule mining activities in the deep Pacific. Although the tests were performed in only 300 m water depth, much less than the depth where future deep-sea mining will take place, the weakly stratified bottom water, tide-dominated near-bed currents with mean magnitude of around 5–10 cm s−1, and gently sloping seabed covered with fine muddy sediment provide a good analogue to operational conditions in the deep sea. The plume of suspended sediment mobilised by the mining vehicle was monitored with turbidity sensors deployed on a ship-operated CTD system and on a static array of moored sensors and monitored visually using a ship-operated ROV. It was found that the generated sediment plume extended no >2 m above the seabed close to the source (<100 m) but increased in height at greater distance. Furthermore, turbidity values decreased rapidly with increasing distance to the source. Even though plume monitoring suffered interference from bottom trawling activities in neighbouring areas, a distinct turbidity signal generated by the mining equipment could still be distinguished above background turbidity at 350 m away from the source. From the experience gained in shallow water, recommendations are made on how a combination of sensors operated from moving and moored platforms may be a suitable and successful strategy for monitoring man-made sediment plumes in the deep sea.

Stock status of the bogue Boops boops (Walbaum, 1792) in Algerian West Coast (Southwestern Mediterranean Sea)

The current study offered management-related information on the fisheries biology of Boops boops. A subsample of 2068 specimen ranging between 9 to 35 cm total length was obtained from 11 commercial catches on Western Algerian coast. To better understand the growth and mortality, distribution frequency of total length, total length–body weight relationship and age were analyzed for B. boops. The estimated parameters of the Von Bertalanffy growth were L∞=30.045 cm, K = 0.117 year−1 and t0=−2.919 year. The B. boops have a long life span (9 years) with growth performance index estimated at 2.03, and the study area seemed supported mainly by nine age groups. The instantaneous rates of total mortality (Z) was 0.418 year−1, natural mortality (M) was estimated at 0.24 year−1. Rates of fishing mortality (F) and exploitation (E) were 0.178 and 0.43 year−1, respectively. Regarding the fishing effort, the stock of bogue appears overexploited. The yield per recruit (Y’/R = 24.86 g) is very close to the MSY. This result indicates that the bogue’s stock is in a state of full exploitation. The MSY model is more prudent than MSE model in predicting the model.

Jellyfish diversity, trends and patterns in Southwestern Mediterranean Sea: a citizen science and field monitoring alliance

Abstract Jellyfish (Cnidaria and Ctenophora) are an essential component of marine ecosystems and represent a potential effect, positive or negative, for several economic activities. Jellyfish blooms and non-indigenous species (NIS) introduction can shift ecological community structure and energy transfer. Despite their importance in ecosystem function and services, knowledge of jellyfish fauna diversity and phenology are largely lacking for the southern Mediterranean Sea. Here, we present an overview of the current state of jellyfish biodiversity in Tunisia. Based on a comprehensive literature survey, historical records and unpublished data, 66 jellyfish species, including 11 non-indigenous species, have been recorded in Tunisia between 1994 and 2020. Additionally, based on field surveys and a citizen science sighting program between 2004 and 2020, we present data on the spatial distribution of 13 conspicuous jellyfish taxa and the phenology of the eight most frequently detected species (Aurelia solida, Cotylorhiza tuberculata, Pelagia noctiluca, Phyllorhiza punctata, Rhizostoma pulmo, Rhopilema nomadica, Olindias muelleri and Velella velella) in Tunisian two ecoregions, the Western Mediterranean and the Tunisian Plateau. The 17 year survey showed a shift in R. pulmo, A. solida and P. noctiluca phenology. Additionally, citizen science revealed an increase in the spatial range of P. punctata (NIS) and the distinct phenologies of populations in the two ecoregions.

Peracarid Assemblages in a Human-Disturbed Location from South-Western Mediterranean Sea: Role of Surface Orientation and Phytal Structure of the Habitat

The goal of this investigation was to assess the role of a number of environmental variables on the traits of a peracarid population inhabiting turfs of Ellisolandia elongata in a Mediterranean disturbed location. Samples were collected from a vertical and a horizontal surface to assess the role of orientation, and several structural traits of the turf (number of fronds, their height and width, available phytal area for colonization, and dry weight) were measured. The samples were characterized by their species composition, relative abundance of constant species, total abundance of individuals, and α-diversity (species richness, Shannon index and Pielou’s evenness). Generalized linear models were used to relate environmental variables with ecological descriptors, whereas a cluster plot and a principal correspondence analysis, both based on the Bray-Curtis coefficient, were conducted to detect the presence of distinct assemblages. Results showed orientation to be a key factor governing the assemblages, since samples from vertical and horizontal surfaces formed definite groups based on Bray-Curtis similarities. Moreover, GLMs pointed out that orientation influenced relative abundance of the majority of constant species, as well as species richness and Shannon index. Phytal height had a positive effect on the relative abundances of Hexapleomera bultidactyla, Coxischyrocerus inexpectatus and Hyale stebbingi, and a negative one on Shannon index and evenness, whereas available phytal area influenced positively evenness and negatively the relative abundance of Elasmopus pocillimanus. Thus, it can be inferred that although species composition depended on surface orientation, the spatial structure of the turf was the main driver for assemblage complexity.

Copepod communities of the Algerian coast (southwestern Mediterranean Sea, 2°-7°E) and their interaction with hydro-climatic variability

Abstract The biodiversity of copepods as examined in the present study is based on an oceanographic survey conducted on two ecosystems along the Algerian coast (central and eastern) in the early autumn of 2014. During that investigation, the thermocline was present at shallow depth, extending between 30 and 80 m. A total of 49 species were recorded in the samples, belonging to 20 families, among which six families, Calanidae, Eucalanidae, Metridinidae, Oithonidae, Paracalanidae, and Temoridae, appeared to be dominant. The total abundance of zooplankton at the 13 stations was less than 600 ind/m3. Two geographic groupings have been distinguished: stations in the central sector (group 1) are in opposition to the other stations in the eastern sector (group 2). The quantitative levels of copepods are less abundant in the east (234.15 ind/m3) than in the centre (334.99 ind/m3). In the central regions, the trophic organization is dominated by omnivores (73.02%), with an inferior co-dominance of carnivores (14.02%) and herbivores (12.95%). In the east, copepods with omnivorous behaviour are strongly represented (82.14%), followed by phytophagous (13.85%) and carnivorous (4.01%) species. The distribution of the abundances over the copepod communities was found to be linked to the temperature, salinity, North Atlantic Oscillation (NAO) index, and phytoplankton biomass.

Occurrence, contamination level and ecological risk assessment of dissolved and particulate trace elements in rivers entering the southwestern Mediterranean Sea

Metal pollution in rivers should not be overlooked before their entry into the sea. However, there are few studies for estimating such contamination in rivers entering the Algerian coastal waters. Semimonthly quantification of dissolved and particulate metals, near the mouths of two industrial-tainted rivers, El Harrach and Mazafran rivers, was carried out during a period of one year. All the trace metals analyzed are originating from anthropogenic sources (EF > 1.5), with higher contamination of dissolved Pb, Cd, Zn and Ni and a slight degree of contamination of particulate Cu and Zn (0 < Igeo < 1). Particulate metals show a stable complex with the particulate phase (e.g. 2 < LogKd < 6). The risk assessment results indicate that particulate Pb and Zn have a 33% likelihood of toxicity for adverse biological effects. A significant toxicity effect (ΣTUi >4) of the combined particulate metals (Pb, Cd, Cu, Zn, Cr, Ni and As) was primarily due to the higher particulate Cd, Zn, and Cr availability.

Comparing the Currents Measured by CARTHE, CODE and SVP Drifters as a Function of Wind and Wave Conditions in the Southwestern Mediterranean Sea.

Instruments drifting at the ocean surface are quasi-Lagrangian, that is, they do not follow exactly the near-surface ocean currents. The currents measured by three commonly-used drifters (CARTHE, CODE and SVP) are compared in a wide range of sea state conditions (winds up to 17 m/s and significant wave height up to 3 m). Nearly collocated and simultaneous drifter measurements in the southwestern Mediterranean reveal that the CARTHE and CODE drifters measure the currents in the first meter below the surface in approximately the same way. When compared to SVP drogued at 15 m nominal depth, the CODE and CARTHE currents are essentially downwind (and down-wave), with a typical speed of 0.5–1% of the wind speed. However, there is a large scatter in velocity differences between CODE/CARTHE and SVP for all wind and sea state conditions encountered, principally due to vertical and horizontal shears not related to the wind. For the CODE drifter with wind speed larger than 10 m/s and significant wave height larger than 1 m, about 30–40% of this difference can be explained by Stokes drift.