Northwestern Alboran Sea Research Articles

Spatial Distribution and Potential Impact of Drifted Thalli of the Invasive Alga Rugulopteryx okamurae in Circalittoral and Bathyal Habitats of the Northern Strait of Gibraltar and the Alboran Sea

The arrival of a new invasive alga, Rugulopteryx okamurae, in the Strait of Gibraltar (SoG) in 2015 marked an unprecedented milestone in the North African and, later, in the European marine ecosystems. Nowadays, it is colonising vast infralittoral areas and significantly modifying some habitats and associated communities of the southern Iberian Peninsula. In recent expeditions, a high amount of free drifted thalli of this alga has been detected in different circalittoral and bathyal habitats of the northern SoG and the Alboran Sea. The present study combines quantitative data of this alga obtained with the use of a remotely operated vehicle (ROV) and a bottom otter trawl. The coverage–entanglement level of the drifted thalli on circalittoral and bathyal benthic invertebrates (e.g., not covering, covering only the basal part, covering one-third of the invertebrate, etc.) was also annotated from picture frames taken in locations with abundant drifted thalli. In underwater images, drifted thalli were mainly detected in circalittoral and bathyal bottoms of the northern SoG and the north-western Alboran Sea, between 50 to ca. 450 m depth. Nevertheless, abundant drifted thalli were also detected in bottom otter trawl samples from circalittoral bottoms of the north-central and north-eastern Alboran Sea. Small benthic organisms (e.g., encrusting sponges, hydrozoans, etc.) generally displayed low coverage–entanglement levels of drifted thalli. Nevertheless, large sessile and colonial benthic organisms with a complex three-dimensional morphology (e.g., gorgonians, colonial scleractinians) reached high levels of R. okamurae thalli entangled in different parts of their colonies. The drifted R. okamurae thalli entangled in these colonial suspension feeding organisms may hinder their feeding capability in the long term, resulting in habitat deterioration in the near future.

Analysis of internal soliton signals and their eastward propagation in the Alboran Sea: exploring the effect of subinertial forcing and fortnightly variability

Internal waves are commonly observed in oceans and lakes where high stratification exists. In the present study in the Strait of Gibraltar, we analyse internal soliton signals recorded in different locations in their eastward propagation from their release point (Camarinal Sill) to the continental slope of the northwestern Alboran Sea. Moreover, the effect of subinertial forcing on the release of solitons is also explored. The internal soliton activity was assessed from different approaches: (i) in-situ data (i.e., current and temperature measurements or High-Frequency Radar), (ii) numerical modelling, and (iii) an analytical approach. The arrival of solitons over the continental slope of the north-western Alboran Sea showed fortnightly variability in both number (occurrence) and amplitude during spring tides when compared with during neap tides. The observed arrival times of the solitons oscillated between 14 (spring tides) and 20 h (neap tides). Nevertheless, to provide a comprehensive explanation for the fluctuations in travel times, it is necessary to consider the subinertial variability driven by atmospheric forcing, which impacts both the flow in the Strait of Gibraltar and the mesoscale patterns in the Alboran Sea (e.g., the Coastal Cyclonic Gyre).

Posidonia oceanica (L.) Delile at Its Westernmost Biogeographical Limit (Northwestern Alboran Sea): Meadow Features and Plant Phenology

Meadows of the seagrass Posidonia oceanica inhabit most infralittoral bottoms of the Mediterranean Sea and are considered one of the main climax stages of the infralittoral environment. This seagrass has its western distributional limit along the coast of the Alboran Sea. Taking into account the decline of P. oceanica meadows and the global scenario of ocean warming, it becomes essential to know the structure, temporal dynamics, sexual reproduction and conservation status of this seagrass, across its geographical distribution, including the distribution boundaries where the meadows withstand limiting environmental conditions. In the present work, we studied the structure, phenology and flowering events of four P. oceanica meadows located in the northwestern Alboran Sea (close to the Strait of Gibraltar). Results indicate a decreasing trend of patch size, bathymetric range and number of leaves per shoot towards the Strait (and the Atlantic Ocean), as well as an increasing trend of shoot density and leaf height. Phenological parameters of the northwestern Alboran Sea P. oceanica meadows presented temporal dynamics similar to meadows from other locations within the biogeographical distribution of this seagrass, with similar or even less annual variability in the former. Although most of the studied P. oceanica meadows seem to present a good health status (BiPo index ~0.6) with high shoot densities and some flowering events, some of them showed evidence of regression.

The Guadiaro-Baños contourite drifts (SW Mediterranean). A geotechnical approach to stability analysis

Two Quaternary plastered contourite drifts, with terraced and low-mounded morphologies, make up the continental slope and base-of-slope in the northwestern Alboran Sea, respectively, between the Guadiaro and Baños turbidite systems, close to the Strait of Gibraltar.Considering their significant lateral extent, the link between the contourite drift deposits and landslides may be particularly important for hazard assessment. The physical properties, composition and geometry of contourite drifts have been proposed as key factors in slope stability, although this relationship still needs to be better constrained. In this work, new in-situ geotechnical data (cone penetration tests; CPTu) has been combined with morphostratigraphic, sedimentological, and (laboratory) geotechnical properties to determine the stability of the Guadiaro-Baños drifts.For the depositional domains of both drifts, the resulting sedimentary and geotechnical model describes low-plasticity granular and silty sands on the erosive terraced domain that evolve seawards to silty and silty-clay deposits with a higher plasticity and uniform geomechanical properties. For the shallower coarse-grained contourite sediments, the cohesion (c') and internal friction angle (ϕ') values are 0–9 kPa and 46–30°, respectively, whereas for the distal fine contourites the undrained shear strength gradient (∇Su) is 2 kPa/m. These properties allow us to establish high factors of safety for all the scenarios considered, including seismic loading. Slope failure may be triggered in the unlikely event that there is seismic acceleration of PGA > 0.19, although no potential glide planes have been observed within the first 20 m below the seafloor.This suggests that the contourite drifts studied tend to resist failure better than others with similar sedimentary characteristics. The interplay of several processes is proposed to explain the enhanced undrained shear strength: 1) the geometry of the drifts, defined by an upper contouritic terrace and lower low-mounded shapes; 2) recurrent low-intensity earthquakes with insufficient energy to trigger landslides, favouring increased strength due to dynamic compaction; and 3) cyclic loading induced by solitons/internal waves acting on the sediment.

Revising the Effects of Local and Remote Atmospheric Forcing on the Atlantic Jet and Western Alboran Gyre Dynamics

AbstractThe main objective of the present study is to gain insight into the response of the Atlantic Jet (AJ) to the atmospheric forcing, in order to understand the hydrodynamic processes behind its disconnection from the Western Alboran Gyre (WAG) and the eastward displacement of the latter. An analysis was carried out based on observations of high frequency radar (HFR) derived currents and sea level, numerical model simulations and satellite imagery, with particular focus on the events of intense and endured easterlies. Among the main factors that characterize the atmospheric forcing linked to intense and persistent easterly wind events, the conjugation of the direct local wind forcing effect on the Atlantic Inflow (AI) on the eastern side of the Strait of Gibraltar and the response of the coastal waters in the northwestern (NW) corner of the Alboran Sea to the wind forcing over the Alboran Sea are of particular interest. In addition, the effect of the elevation surges in the Strait coming from the western Mediterranean Sea on reducing the AI intensity must be taken into account. These surges appear in coincidence with the establishment of persistent and intense easterlies over the Alboran Sea. The alteration of the AJ‐WAG system in response to this forcing causes important changes in the phytoplankton distributions in the Alboran Sea. After the events of persistent and intense easterlies the phytoplankton accumulations in the NW Alboran Sea are displaced offshore and the phytoplankton transported with the AJ is injected toward the central zone of the WAG.

A first description of the summer upwelling off the Bay of Algeciras and its role in the northwestern Alboran Sea

A local upwelling process is observed in the mouth of the Bay of Algeciras showing a singular pattern, partially disengaged of the neighboring coastal upwelling variability. The physical mechanisms which drive this local upwelling are not clear, but the Ekman pumping seems to be the most important forcing factor. The surface chlorophyll-a variability associated with this local upwelling is uncorrelated with the concentration registered in the inner Bay of Algeciras or in the surrounding coastal and shelf waters. Secondary, the emerged waters in the mouth of the Bay of Algeciras flow eastwards into the northwestern Alboran Sea dragged by the northern edge of the Atlantic Jet, forming a narrow and elongated nutrient-enriched area (hereafter called filament) usually detached from the coast. A sampling station located at 14.6 km from the coastline and 68 km far from the mouth of the Bay of Algeciras was employed to monitor the presence of the filament in the Alboran Sea. The filament is most frequently presented during the summer (in fact, it was detectable in more than 50% of the days of august). When the filament is distinguishable, its chlorophyll-a concentration doubles in average the values registered in neighboring locations, and in annual average also higher nitrate and phosphate surface concentrations (about 30%) are registered at the filament location. These results indicate that the filament provides nutrients to the surface waters of Alboran not only when it is optically detected. Our analyses indicate that the trajectory of the filament follows the northern edge of the western gyre of Alboran although its width and speed are variable depending on the Atlantic Jet properties and the related sub-mesoscale activity.

Decapod assemblages associated with shallow macroalgal communities in the northwestern Alboran Sea: Microhabitat use and temporal variability

Decapod assemblages associated with algal fronds and the underlying substratum in two different photophilous macroalgal beds dominated by the brown algae Halopteris scoparia were studied in the northwestern Alboran Sea, between July 2007 and April 2008. A total of 35 decapod species were found in the macroalgal beds, most of them inhabiting both strata and with Hippolyte leptocerus, Pilumnus hirtellus, Sirpus zariquieyi, Acanthonyx lunulatus, Athanas nitescens and Achaeus gracilis as the dominant species. Assemblages on algal fronds and sediment displayed significant variations mainly due to differences in the abundance values of some dominant species (e.g. H. leptocerus) and/or the presence of certain species exclusively in one strata (e.g. Pisa nodipes in algal fronds, Atelecyclus rotundatus and Sicyonia carinata on the sediment stratum). Higher abundance, species richness and Shannon-Wiener diversity index values were registered in the sediment stratum, with a higher contribution of adults-large individuals than of juvenile-small individuals. The temporal variability of the studied assemblages showed maximum abundance values in November, when algal development is minimal. This decoupling between temporal patterns of decapod assemblages and macroalgal dynamic could be related to the lifestyles (recruitment events, movements of species between adjacent habitats and microhabitats) and trophic guilds of dominant species, fish predation pressure and the structural complexity of the habitat. A similar trophic structure was observed for both strata, however there was a predominance of grazers in the algae stratum and of predators and scavengers in the sediment stratum. The high diversity and abundance of predator decapods, the relatively balanced distribution of most trophic groups, and the overall high values of species richness and evenness, could indicate a healthy status of at least two of the eleven “Good Environmental Status” indicators (biodiversity and food webs) during the study period, according to the Marine Strategy Framework Directive.

Biodiversity and biogeographical patterns of molluscan assemblages in vegetated and unvegetated habitats in the northern Alboran Sea (W Mediterranean Sea)

This paper evaluates the influence of habitat heterogeneity, inter-habitat connectivity, and biogeograhical patterns in the context of a biodiversity hotspot in an Atlantic-Mediterranean marine transition area. We analyzed some biological and ecological aspects of molluscan assemblages inhabiting different habitats located in the northwestern Alboran Sea, in order to elucidate the inter-habitat connectivity among coastal habitats, and to understand the contribution of each habitat to the integrity of the infralittoral ecosystem. The assessment of molluscan assemblages was performed by combining information regarding (1) feeding guilds, to analyze trophic strategies; (2) microhabitat preference, to study the link between habitat heterogeneity with biodiversity; and (3) biogeographical affinities, to identify distributional patterns of molluscs. A total of 354 molluscs were identified, with vegetated habitats harbouring the highest number of species. A wide variety of trophic strategies were observed, with infaunal filter feeders, specialized carnivores living on sessile invertebrates and microalgal grazers dominating the assemblages. Most of the species have a wide distribution while few strictly Mediterranean species have been collected. The presence of subtropical species highlights the uniqueness of this area in European seas. The high β-diversity value obtained is indicative of high habitat heterogeneity and a high amount of uncommon species, some of them displaying their sole European populations. The different assemblages enrich the global taxocoenosis through complementarity, with an important inter-habitat connectivity reflected by the different temporal ordination of samples according to both micro-habitat affinities and feeding guilds. This work highlights the balanced contribution of soft and vegetated bottoms as reservoirs of biodiversity.

Posidonia oceanica (L.) Delile in its westernmost biogeographical limit (northwestern Alboran Sea): Meadows characterisation, phenology and flowering events

Mateo-Ramirez A, Urra J, Rueda J, Marina, Banares-Espana E, Garcia Raso E. (2016) Posidonia oceanica (L.) Delile in its westernmost biogeographical limit (northwestern Alboran Sea): Meadows characterisation, phenology and flowering events. Front. Mar, Sci. Conference Abstract: XIX Iberian Symposium on Marine Biology Studies. doi: 10.3389/conf.FMARS.2016.05.00055

Seasonal dynamics of molluscan assemblages associated with littoral soft bottoms of the NW Alboran Sea (Western Mediterranean Sea)

Molluscan assemblages inhabiting subtidal soft bottoms in the northwestern Alboran Sea were studied to analyse temporal changes in structure and composition. A total of 117 samples were collected seasonally using a small rock dredge at 10 sampling stations, yielding 210 mollucan species. The assemblages occurring on the different soft bottoms were well-sorted fine sands assemblage (SFBC), mixed coarse sands assemblage (MX), muddy bioclastic sands assemblage (DE) and coastal bioclastic sands assemblage (DC), and all displayed high species richness values throughout the year. Different seasonal dynamics were observed and ecological indices were influenced by dominant species, especially the bivalve Spisula subtruncata at the shallow stations and the gastropods Calyptraea chinensis and Nassarius reticulatus at the deeper stations. SFBC displayed maximum abundance (N), species richness (S) and Shannon–Wiener diversity (H′) in autumn, DE and MX displayed maximum N and S in spring but maximum H′ in autumn (DE) and winter (MX), and finally DC displayed stable values of ecological indices throughout the year with minima in spring. Seasonal changes in the deep assemblages (25 m) were not significant and therefore less acute than those of shallower ones (5, 15 m), probably due to a lower influence of the environmental conditions and a higher habitat complexity. Seasonal patterns were somewhat different from those observed in more northern areas of Europe, where maximum abundance and species richness is usually concentrated at the end of summer. This original information will provide a baseline to understand the dynamics of benthic communities in this area.

Highly diverse molluscan assemblages of Posidonia oceanica meadows in northwestern Alboran Sea (W Mediterranean): Seasonal dynamics and environmental drivers

The seasonal dynamics of the molluscan fauna associated with the westernmost populations of the Mediterranean seagrass Posidonia oceanica, has been studied throughout an annual cycle in the northwestern coasts of the Alboran Sea. Samples were collected seasonally (5 replicated per season) using a non-destructive sampling technique (airlift sampler) on quadrats of 50 × 50 cm at 2 sites located 7 km apart. Several environmental variables from the water column (temperature, chlorophyll a), the sediment (percentage of organic matter) and the seagrass meadows (shoot density, leaf height and width, number of leaves per shoot) were also measured in order to elucidate their relationships with the dynamics of the molluscan assemblages. In these meadows, a total of 17,416 individuals of molluscs were collected, belonging to 71 families and 171 species, being Rissoidae, Pyramidellidae and Trochidae the best-represented families, and Mytilidae, Nassaridae and Trochidae the dominant ones in terms of abundance. The assemblages were dominated by micro-algal grazers, filter feeders and ectoparasites (including those feeding on sessile preys). The species richness and the abundance displayed significant maximum values in summer, whereas evenness and diversity displayed maximum values in spring, being significant for the evenness. Both abundance and species richness values were positively correlated to seawater temperature and percentage organic matter, only for the latter, and negatively to leaf width. Significant seasonal groupings were obtained with multivariate analyses (MDS, Cluster, ANOSIM) using qualitative and quantitative data that could be mainly related to biological aspects (i.e. recruitment) of single species. The molluscan assemblages are influenced by the biogeographical location of the area (Alboran Sea), reflected in the absence or scarcity of most Mediterranean species strictly associated with P. oceanica (e.g. Tricolia speciosa, Rissoa ventricosa) and by the occurrence of subtropical fauna (e.g. Ungulina cuneata). The patchiness of the meadows, intercalated in other habitats like photophilous algae, and the strong hydrodynamism resulting from the unusually shallow setting, may promote the movement of species between these habitats and the seagrass and therefore enhance the species richness. This large faunistic list for the molluscan fauna associated with P. oceanica is therefore different than those observed in other meadows, and highlights the conservation value of these fragmented meadows located in their western distributional limit.

Atmospheric-induced variability of hydrological and biogeochemical signatures in the NW Alboran Sea. Consequences for the spawning and nursery habitats of European anchovy

The north-western Alboran Sea is a highly dynamic region in which the hydrological processes are mainly controlled by the entrance of the Atlantic Jet (AJ) through the Strait of Gibraltar. The biological patterns of the area are also related to this variability in which atmospheric pressure distributions and wind intensity and direction play major roles. In this work, we studied how changes in atmospheric forcing (from high atmospheric pressure over the Mediterranean to low atmospheric pressure) induced alterations in the physical and biogeochemical environment by re-activating coastal upwelling on the Spanish shore. The nursery area of European anchovy ( Engraulis encrasicolus) in the NW Alboran Sea, confirmed to be the very coastal band around Malaga Bay, did not show any drastic change in its biogeochemical characteristics, indicating that this coastal region is somewhat isolated from the rest of the basin. Our data also suggests that anchovy distribution is tightly coupled to the presence of microzooplankton rather than mesozooplankton. Finally, we use detailed physical and biological information to evaluate a hydrological–biogeochemical coupled model with a specific hydrological configuration to represent the Alboran basin. This model is able to reproduce the general circulation patterns in the region forced by the AJ movements only including two variable external forcings; atmospheric pressure over the western Mediterranean and realistic wind fields.

The Baraza Slide: model and dynamics

The Baraza Slide is located in the northwestern Alboran Sea, western Mediterranean region, between 590 and 830 m water depth, and its morphology, seismic facies and sedimentary structure are analyzed based on multibeam bathymetry and very high to medium resolution single-channel seismic records. During the Pleistocene and Late Quaternary this landslide has undergone repeated slope failures characterized by a succession of two main types of mass movement, the first of the mass-flow type and the second of the slide type. This study also reveals that the western sector of this landslide could still be active. The relatively high slope gradients, a sedimentary column characterized by the presence of under consolidated layers and earthquake shaking related to tectonic activity on the margin are the factors governing the genesis and post-mobility behavior of the Baraza mass movement. The recognition and analysis of these factors suggest that the Baraza Slide should be considered as a geo-hazard in the tectonically active Alboran Sea.

Primary production in the Strait of Gibraltar: Carbon fixation rates in relation to hydrodynamic and phytoplankton dynamics

Primary production was studied at nine sites distributed within the Strait of Gibraltar (Southern Spain) and North-Western (NW) Alboran Sea by analyzing photosynthesis-irradiance (P–I) relationships and integrated primary production rates in relation to the different types of Deep Chlorophyll Maxima (DCM) detected in the area. The characteristics of the DCM were examined by several methods, including flow cytometry, quantification of transparent expolymer particles and fluorimetric measurements that were applied in order to assess the photo-physiological state of the phytoplankton assemblages with respect to their species composition and water column structure (hydrology). The photosynthetic parameters (derived from P–I relationships) and integrated primary production (range 6–644 mg m −2 d −1) responded greatly to the diverse DCM identified and thereby the spatial variability of the primary production observed in the region was found to depend upon the occurrence of the different types of phytoplankton accumulations, which were themselves indicative of the previous history of the water column. The net contribution of the primary production to the air–sea CO 2 exchange process was also evaluated in the area. Results indicated that this region behaved as a net sink for the atmospheric CO 2, with the intensity of the flux being strongly modulated by the wind intensity.

Modelling the Gibraltar Strait/Western Alboran Sea ecohydrodynamics

The ecohydrodynamics of the Gibraltar Strait and the Western Alboran Sea is investigated using a 3-D, two-way nested, coupled hydrodynamic/plankton ecosystem model, exploiting the MEDATLAS climatological database. A high-resolution model (~1 km) of the Gibraltar/Western Alboran region embedded within a coarse-resolution model of the West Mediterranean (~5 km) is implemented. The model seasonal climatology of the 3-D circulation and the flow characteristics at the Gibraltar Strait and the Alboran Sea are discussed, and their impact on the plankton ecosystem evolution is explored. An important ecohydrodynamic feature produced by the model is a permanent upwelling zone in the northwestern part of the Alboran Sea in agreement with observations. Model results show that both horizontal and vertical current intensity of the Atlantic Jet increases progressively at the strait to obtain maximum values in the northeastern Mediterranean entrance, inducing an upward displacement of the nitracline. The nutrient-rich water transport through the strait along with the generation of cyclonic vorticity in the northwestern Alboran Sea result in the accumulation of nutrients there and thus induce a permanent fertilisation of this area.

Meteorologically-induced mesoscale variability of the North-western Alboran Sea (southern Spain) and related biological patterns

Hydrographic mesoscale structures in the North-western Alboran Sea show a high variability induced by a number of different factors. One of the most important is the differences in atmospheric pressure over the Mediterranean basin when compared to the Gulf of Cádiz. This difference modulates the zonal wind field in the Alboran Sea and the intensity of the Atlantic inflow through the Strait of Gibraltar, also affecting the formation and extension of the Western Alboran Gyre (WAG). When westerly winds are dominant, lower atmospheric pressure in the Mediterranean enhances the inflow of Atlantic waters causing the Atlantic Jet to be located in the vicinity of the Spanish shore, creating a well-defined frontal zone in front of Estepona Cove. In this situation, the coastal upwelling is enhanced, leading to a minimum in sea surface temperature and a maximum of surface nutrient concentrations located in the coastal area. The vertical position of the chlorophyll maximum found in these circumstances appeared to be controlled by the nutrient availability. On the other hand, when easterly winds prevail, higher atmospheric pressure in the Mediterranean leads to a reduced inflow and the oceanographic and biological structures are clearly different. The Atlantic Jet moves southward flowing in a south-eastern direction, changing the structure of the currents, resulting in an enhanced cyclonic circulation extending throughout the North-western Alboran Sea basin. These physical alterations also induce changes in the distribution of biogeochemical variables. Maximum nutrient and chlorophyll concentrations are located further off the coast in the central area of the newly created cyclonic gyre. During these easterlies periods coastal upwelling stops and the distribution of phytoplankton cells seems to be mainly controlled by physical processes such as advection of coastal waters to the open sea.

Phytoplankton pigment distribution in the northwestern Alboran Sea and meteorological forcing: A remote sensing study

A set of weekly-composite SeaWiFS (Sea-viewing Wide Field of view Sensor) imagery (from 1998 to 2004) has been used to asses the temporal and spatial variability of the surface chlorophyll distribution in the Northwestern Alboran Sea. Empirical Orthogonal Function (EOF) analysis of the images series shows the existence of three different regions in which surface chlorophyll presents a homogeneous temporal behavior in the neighborhood of the Strait of Gibraltar. The temporal patterns of surface chlorophyll concentrations in each region have been characterized by studying the role of several forcing factors, mainly the zonal component of the wind field. The first identified region occupies the coastal area in front of Estepona and Malaga and shows the highest chlorophyll concentration in the entire region of study in absence of easterly winds. A second region is located far offshore, in the zone usually occupied by the Atlantic jet entering the Alboran Sea, showing high concentrations of chlorophyll when the easterlies blow. Finally, a third region has been situated in the normal location of the Western Alboran Gyre (WAG). The zonal wind field arises as a main forcing function in the modulation of the intensity and location of the upwelling processes in the area, while the entrance of the Atlantic jet seems to be the main agent feeding of all these processes.

Seasonal and inter-annual changes in the planktonic communities of the northwest Alboran Sea (Mediterranean Sea)

The response of the northwestern Alboran Sea pelagic ecosystem to temporal changes in hydrological conditions has been examined for the time period of 1992–2002. In addition, the bottom–up linkages between the lower trophic levels and the growth and nutritional status of sardine larvae were examined using quarterly data from 1992 to 2002 within the frame of the monitoring Project ECOMALAGA. The study area was characterised by the almost permanent presence of an upwelling which was intensified in the spring period. Consequently, an annual peak of nutrients was usually found during this season when the nitrate concentration averaged 1.35 μM. Accordingly, chlorophyll- a concentration and cell abundance of micro- plus nano-phytoplankton increased in that season (1.51 μg L −1 and 446 cell mL −1 compared to 0.85 μg L −1 and 225 cell mL −1 obtained from summer to fall). Despite these seasonal changes, the analysis of the taxonomic composition of the phytoplankton communities did not reveal a clear annual succession pattern. Contrastingly, peaks of zooplankton abundance were obtained in summer (1964 ind m −3) due to the increased presence of brachiopods with respect to copepods (which dominated from fall to spring). Significant inter-annual changes were obtained in the phytoplankton and zooplankton communities. Thus, dinoflagellate and coccolitophorid abundances relative to diatom abundances tended to increase from 1997 to 2002. This trend matched the progressive reduction of the upwelling intensity. These inter-annual changes significantly affected the larval growth of Sardine pilchardus and their nutritional condition, as higher growth rates in terms of body length coupled to higher somatic mass increases (expressed by DNA content) occurred in spring, matching with the higher chlorophyll- a concentration. Furthermore, the highest larval growth was obtained in 2001, coinciding with the change observed in the composition of phytoplankton community.

Respiratory ETS activity of plankton in the northwestern Alboran Sea: seasonal variability and relationship with hydrological and biological features

Respiratory electron transport system (ETS) activity was measured in plankton samples (<200 mm) collected in the NW Alboran Sea. Sampling was carried out during seasonal cruises (summer and autumn 2003 and winter and spring 2004) in 12 stations located in transects off the coast of Malaga (southern Spain). This work reports for the first time seasonal variations of the Arrhenius activation energy (Ea) as well as being the first study to address CO2 balance in the NW Alboran Sea. These variations were related to changes in the phytoplankton community assemblage, which could ultimately be caused by the seasonal variability of hydrological conditions. ETS activity was significantly higher in summer, coinciding with a higher chlorophyll a (Chl a) concentration and relatively high levels of particulate organic matter. The ETS:Chl atotal ratios were low during the four seasons, suggesting a high contribution of autotrophic phytoplankton to the respiratory activity of planktonic community. Respiratory CO2 production (RCP) calculated from ETS activity ranged from 4.6 to 28.1 mg C m ‐3 day ‐1 during the four cruises. Chl a-specific RCP was lower than the maximum photosynthetic rates reported in the literature for the studied area, suggesting that primary production (PP) and respiration in the water column might be unbalanced.

Distribution of TEP in the euphotic and upper mesopelagic zones of the southern Iberian coasts

The spatial and temporal distributions of transparent exopolymer particles (TEP) were studied for the Gulf of Cádiz, by analyzing its abundance in the euphotic and upper mesopelagic zones. Both layers of the water column should reflect a distinct pattern in TEP distribution, as production of TEP is mainly linked to phytoplankton activity, which is sited in the euphotic zone, while losses of TEP due to aggregation-sinking and degradation occur from the surface to further below in the water column. Concentration of TEP in the euphotic zone reflected the varied oceanographic characteristics of the studied region. It was higher in coastal upwellings, fronts and such areas of topographic forcing, suggesting that TEP were being produced by active growing phytoplankters. A comparison with a well-known ecosystem, the coastal upwelling in the northwestern Alboran Sea, showed that the TEP:Chl a ratio at stations with clear eutrophic characteristics was several orders of magnitude lower than in the area southwest of the anticyclonic current of the Gulf. This ratio in the highly dynamic system of the Strait of Gibraltar seems to have a much clearer relation with the stage of a bloom and, at a basin scale, to the trophic condition of the ecosystem than TEP concentrations alone. Throughout the area studied in the Gulf of Cádiz, the role of TEP for aggregation processes was analyzed based on two aggregation models. Both approximations pointed to the importance of TEP in the export of matter from the euphotic zone to deeper waters in the Gulf of Cádiz.