Juan M. Vargas

Upwelling mechanisms in the northwestern Alboran Sea

Abstract From April 1996 to July 1997, a series of hydrographic surveys were carried out in the Northwestern part of the Alboran Sea to investigate the upwelling that is an almost permanent feature in this area. Simultaneously a mooring line was deployed in the north part of the eastern section of the Strait of Gibraltar to monitor the variability of the Atlantic Jet (AJ). Two mechanisms are shown to be relevant for the upwelling dynamic in the region: the southward drifting of the AJ and wind stress. A linear relation between the angle under which the Jet enters the Alboran Sea and the distance from the coastline to the front associated with the Jet has been found. This angle that has been estimated from the low passed time series of current velocity measured by the uppermost instrument of the moored line has been then used to identify the onshore–offshore excursions of the Jet. Both upwelling mechanisms are identified from hydrographic data, because each of them has associated a different type of water mass, and they take place in different locations. Wind-driven upwelling dominates in coastal zones, on the shelf, while upwelling associated with southward drifting of the AJ prevails further offshore. The amount of sub-surface water brought up to the surface by each one is of the same order. However, wind-driven upwelling contributes to the fertilization of this region in a major extent because water upwelled by wind is richer in nutrient concentration.

About the seasonal variability of the Alboran Sea Circulation

Data from a mooring line deployed midway between the Alboran Island and Cape Tres Forcas are used to study the time variability of the Alboran Sea from May 1997 to May 1998. The upper layer salinity and zonal velocity present annual and semiannual cycles characterised by a minimum in spring and autumn and a maximum in summer and winter. Temperature has the opposite behaviour to that of salinity indicating changes in the presence of the Atlantic water within the Alboran Passage. A large set of SST images is used to study these cycles. The decrease of salinity and velocity in our mooring location in spring and autumn seems to be related to the eastward drifting of the Western Alboran Gyre (WAG). The increase of salinity and velocity is caused by the Atlantic current flowing south of the Alboran Island and its associated thermohaline front. Conductivity– temperature–depth (CTD) data from two cruises along the 3jW are coherent with current meters and SST interpretations. During the period analysed, summer months are characterised by the stability of the two-gyre system, while in winter, the circulation is characterised by a coastal jet flowing close to the African shore. We use sea level differences across the Strait of Gibraltar for studying the variability of the Atlantic inflow. We discuss the changes in the Alboran Sea circulation and its relation with the variability of the inertial radius of the Atlantic inflow. Though our results are speculative, we find a possible relation between the disappearance of the two-gyre system and a reversal of the circulation in Gibraltar. Longer time series are needed to conclude, but comparison with previous works makes us think that the seasonal cycle described from May 1997 to May 1998 could be the most likely one for the Alboran Sea upper layer. D 2002 Elsevier Science B.V. All rights reserved.

Tide at the eastern section of the Strait of Gibraltar

From October 1995 to April 1996, three mooring lines were deployed at the eastern entrance of the Strait of Gibraltar. The spatial coverage of the mooring array allows for a good description of the tides. They exhibit a dominant semidiurnal nature and a noticeable baroclinic structure that matches the one of the mean exchange. Tidal currents in the upper layer are irregular and usually too weak to reverse the mean upper layer flow that keeps on flowing east. Lower layer flow reverses with semidiurnal periodicity because of the smallness of the mean flow and the appreciable amplitude of the regular semidiurnal oscillation of tidal currents in this layer. Tidal transports can be satisfactorily compared with previous estimates of Bryden et al. [1994] if we allow for strong internal divergences associated with the internal tide. No significant eddy flux of water transport (tidal rectification) is observed at the eastern section, contrary to the almost 50% of the total layer transport found by Bryden et al. [1994] in Camarinal Sill section. Time-dependent hydraulic theory provides a good scenario for interpreting these two independent sets of observations despite the fact that the composite Froude number does not reach the critical values predicted in the hydraulic models most of the time.

Low-frequency variability of the exchanged flows through the Strait of Gibraltar during CANIGO

Time series of the exchanged flows through the Strait of Gibraltar at the eastern section have been estimated from current-meter observations taken between October 1995 and May 1998 within the Canary Islands Azores Gibraltar Observations (CANIGO) project. The inflow exhibits a clear annual signal that peaks in late summer simultaneously with a deepening of the interface. The cycle seems to be driven by the seasonal signal of the density contrast between the surface Atlantic water that forms the inflow and the deep Mediterranean water of the outflow. The outflow and the depth of the interface have predominant semiannual signals and a smaller annual one whose phase agrees with that of the density contrast as well. Local wind stress and atmospheric pressure difference between the Atlantic and the Western Mediterranean to less extent have clear semiannual signal, so that the possibility that the semiannual cycle of the outflow and of the depth of the interface are forced by them was analyzed. The composite Froude number in this section is well below the critical value, suggesting submaximal exchange. Therefore, the conditions in the Alboran basin influence the exchange and some evidence that the size and location of the Western Alboran Gyre contribute to the observed signals, both annual and semiannual, is provided. r 2002 Elsevier Science Ltd. All rights reserved.

Seasonal and wind-induced variability of Sea Surface Temperature patterns in the Gulf of Cadiz

The evolution of thermal structures in the Gulf of Cadiz is analysed with a set of 325 weekly composite Sea Surface Temperature (SST) images derived from NOAA-AVHRR sensor, and covering a time span of 7 years, from 1993 to 1999. A spatial Empirical Orthogonal Function (EOF) analysis has been performed in order to identify the main SSTspatial patterns. The first EOF mode explains 60% of the temperature variance of the images, and shows a quasi-permanently warmer than the mean region in the southern part of the area of study. The second mode (13% of variance), has a strong temporal variability, and is the main responsible for the cooling and warming of the shelf waters in southwestern Iberia. These two modes explain together most of the seasonal variability of SSTover the basin, particularly the variation and strength of the upwelling area located southeast of Portugal. The third mode explains 6% of variance and is well correlated with the local zonal wind. Two wind-induced upwelling can be clearly identified in this mode. The first one, located at the southwestern end of the Strait of Gibraltar, takes place during easterlies events. The second one, related to westerlies, is located to the east of Cape Santa Maro´a, and is associated with a southeastward transport of cold surface waters from that Cape. D 2002 Elsevier Science B.V. All rights reserved.

Physical–biological coupling in the Strait of Gibraltar

This study presents a joint analysis of the distributions of some biogeochemical variables and their relation to the hydrodynamics of Gibraltar Strait. It is a synthesis paper that brings together many results obtained during CANIGO project. We show the role of hydrodynamics as a forcing agent for the plankton community structure in the Strait, with emphasis on the two physical processes that we propose as key factors for the coupling: interface position and oscillations, and mixing processes along the Strait. As a general pattern, autotrophic plankton biomass increases at the Strait from southwest to northeast, a tendency that coincides with a gradual elevation of the interface depth in the same direction. The different mechanisms of mixing that take place in the Strait are briefly reviewed: The occurrence of the internal hydraulic jump is an important mechanism of mixing constrained to the spring tide situations, but other processes such as the generation of arrested internal waves of wavelength around 1 km are proposed as a complementary mixing mechanism, particularly during neap tides situations. Both mechanisms, the elevation of the pycnocline and these mixing events, can enhance biological productivity and biomass accumulation on the northeastern sector of the Strait, since phytoplankton cells are there packaged in a water mass with sufficient light and nutrients and smaller advective velocity. There is a clear north–south difference in the biological response to these upwelling episodes in the eastern section, with high nutrient and low chlorophyll in the south and the opposite in the north. The deeper interface and the greater water speed are the proposed reasons for this lower nutrient uptake on the southeastern sector. Finally, the temporal scales of variation of the mixing events, the influence of its periodicity on the productivity of the area and the influence of these upwelling episodes in the nearest Albor! an Sea are discussed. r 2002 Elsevier Science

Tidal motions and tidally induced fluxes through La Línea submarine canyon, western Alboran Sea

Detailed observations from two mooring lines deployed in La Linea submarine canyon, western Alboran Sea, are presented. This is a narrow canyon in the sense that its width is always less than the internal radius of deformation. Tidal currents within the canyon are polarized in the along-canyon direction according to its narrow nature. They have considerable amplitude (values of around 0.5 m/s are often observed) and are forced by the internal pressure gradients associated with the baroclinic tide that is generated in the surroundings. Subsequent amplification of onshore baroclinic currents within the canyon accounts for the large amplitude observed. Cross-shelf exchange through the canyon due to tidal motions is different from zero despite the close to zero mean of tidal currents. The explanation is based on the asymmetry of water properties flowing up-canyon and down-canyon (some sort of tidal rectification). Regarding the energy flux, the canyon seems to be an adequate conduit to carry energy to the shore. Estimations made from our observations indicate that energy input onto the shelf per unit length parallel to the shore at the canyon head is enough to maintain mixing on the shelf at intermediate depths.

Distribution patterns of indigenous freshwater fishes in the Tagus River basin, Spain

Classification and ordination methods used to examine the internal complexity of the Mediterranean Tagus River catchment based on fish distribution revealed that it is not a homogeneous biogeographical unit. The indigenous fishes analyzed in this study are distributed through the basin forming geographical communities (chorotypes), some of which are associated with environmental factors like river morphology, water quality or geographical location. Nevertheless, 40% of the variation in species occurrence remains unexplained by either environmental or geographical variables, suggesting that historical factors may influence the freshwater fish distribution patterns. Three main biogeographical areas, delimited by significant boundaries, were identified. Two of them are identified as the upper and the middle-lower basins of the Tagus River catchment; the third corresponds to the Alagón River and seems to be linked to historical factors of the catchment.

Correlates of bushmeat in markets and depletion of wildlife

We used data on number of carcasses of wildlife species sold in 79 bushmeat markets in a region of Nigeria and Cameroon to assess whether species composition of a market could be explained by anthropogenic pressures and environmental variables around each market. More than 45 mammal species from 9 orders were traded across all markets; mostly ungulates and rodents. For each market, we determined median body mass, species diversity (game diversity), and taxa that were principal contributors to the total number of carcasses for sale (game dominance). Human population density in surrounding areas was significantly and negatively related to the percentage ungulates and primates sold in markets and significantly and positively related to the proportion of rodents. The proportion of carnivores sold was higher in markets with high human population densities. Proportion of small-bodied mammals (<1 kg) sold in markets increased as human population density increased, but proportion of large-bodied mammals (>10 kg) decreased as human population density increased. We calculated an index of game depletion (GDI) for each market from the sum of the total number of carcasses traded per annum and species, weighted by the intrinsic rate of natural increase (rmax ) of each species, divided by individuals traded in a market. The GDI of a market increased as the proportion of fast-reproducing species (highest rmax ) increased and as the representation of species with lowest rmax (slow-reproducing) decreased. The best explanatory factor for a markets GDI was anthropogenic pressure-road density, human settlements with >3000 inhabitants, and nonforest vegetation. High and low GDI were significantly differentiated by human density and human settlements with >3000 inhabitants. Our results provided empirical evidence that human activity is correlated with more depleted bushmeat faunas and can be used as a proxy to determine areas in need of conservation action.

Small-scale temporal variations in biogeochemical features in the Strait of Gibraltar, Mediterranean side—the role of NACW and the interface oscillation

On the Mediterranean side of the Strait of Gibraltar, the distribution of physical, chemical and biological variables Ž . temperature, salinity, nutrients, chlorophyll a, lipids, particles size and plankton abundance was examined. Sampling was carried out between the surface and 150 m at a fixed station over a 24-h time series. The patterns observed were related to the overlaying of different processes. The Atlantic-Mediterranean interface acts as a strong pycnocline and its vertical oscillation accounts for the gross distribution of nutrients, particles, and living biomass. Injection of North Atlantic Central Ž. Ž . Water NACW into the upper layer occurs at the sill each semidiurnal tidal cycle every 12 h . As a consequence, in the upper Atlantic layer the NACW was observed every 12 h in the trough of the interface oscillation, whereas Surface Atlantic Ž. Water SAW dominated in the crest at the fixed station. The initially nutrient-rich NACW was associated with eutrophic signatures such as high chlorophyll, large cells and low turbidity; The nutrient depleted SAW was associated with oligotrophic signatures such as low chlorophyll, small cells and high turbidity. The distribution of lipid biotracers at the Ž. depth of the chlorophyll maxima 10-40 m depicted a similar trend with abundant chloroplast lipids and a low lipolysis index in NACW-enriched waters, and a high lipolysis index and abundant zooplankton tracers in SAW especially at night. During the eastward advection of Atlantic water, the nutrient content of NACW is likely to be assimilated by phytoplankton. A scenario is proposed for explaining changes in phytoplankton maxima composition during the time series observations, taking into account the timing of the NACW injection at the sill, the diurnal cycle and zooplankton grazing. Although more studies over a longer temporal scale are necessary to validate this scenario, our observations show the scale of daily variations in the physicalrbiological coupling in the Strait and the implications for nutrient and matter exchanges between the Atlantic and Mediterranean. q 2001 Elsevier Science B.V. All rights reserved.