Articles

Keywords: western Mediterranean circulation, Algerian basin, Lagrangian drifters

The Algerian basin is a region of the western Mediterranean with a highly variable circulation structure, including the eastward transport of Modified Atlantic Water (MAW) in its surface layer. An experiment with satellite tracked Lagrangian drifters was performed in 1996-97 to analyse the mesoscale circulation of the Algerian current. The complete trajectories of 18 drifters indicate that, at basin scale, all the surface flow occured along the coast from the Alboran Sea to the strait of Sicily. At that time, no portion of the inflowing MAW was driven to the central or northern regions.

Keywords: Algerian basin, circulation, density, fronts, primary production

ALGERS’96 was the first MATER (MAST 3 Mediterranean Targeted Project MTP II) campaign in the Algerian basin, carried out on board the Spanish R/V Hesperides in October 1996. A mesoscale meander of the Algerian current, developed near 1ºE, was exhaustively sampled: ADCP, CTD and SCTD/XBT profiles and transects, multibeam echosounding, dissolved oxigen, nutrients, chlorophyll, suspended particulate matter, primary production, bacterial abundance and radioactive tracers, together with satellite-tracked surface drifters and real-time remote sensing. The detailed analysis of all this interdisciplinary data set is giving, for the first time, a three-dimensional characterisation of the phenomenon and allows gaining some important answers on the coupling of physical and biological dynamics in the Algerian basin

Keywords: Mesoscale phenomena, fronts, surface waters, Alboran Sea

The OMEGA Project is an interdisciplinary physically initiative aimed to provide the scientific community with a new tool for computation of vertical from routine CTD and ADCP data. The field studies were completed during October (BIO Hespérides) and December (RRS Discovery) 1996. Preliminary analysis of the data show the existence of significant variability at scales of the order of the internal Rossby radius and also indirect evidences of vertical motions. The data are being analyzed and used to compute the associated three-dimensional circulation through methods and models of different complexity

Keywords: Alexandrium, in situ growth rate; life history; planozygotes; resting cyst; temporary cyst

Alexandrium taylori Balech is a phototrophic marine dinoflagellate. It produced recurrent blooms during the summer months (July and August) of 1994 to 1997 in La Fosca beach (NW Mediterranean). In addition to a motile vegetative form, A. taylori had two benthic forms: temporary cysts and resting cysts. Temporary cysts were a temporally quiescent stage produced from the ecdysis of the vegetative cell in both natural populations and laboratory cultures. Temporary cysts may divide to form motile cells. Resting cysts had a thicker wall than the temporary cysts and had a red accumulation body. Gametes and planozygotes were also observed in laboratory cultures. Alexandrium taylori showed in situ diurnal vertical migration with an increase of vegetative cells in the water column in the morning through midday, with concentrations peaking in the afternoon followed by lower levels at night. Most vegetative cells lost their thecae and flagella, and with them their motility, turning into temporary cysts that settled in the early evening. The number of temporary cysts in the water column rose in the evening and at night. The temporary cysts gave rise to motile cells the following morning. Synthesis of DNA occurred in vegetative cells at night, and a preferential period of cell division occurred at sunrise. The estimated division rate in the field was 0.4–0.5 vegetative cells·day−1. Temporary cysts had twice the DNA of a G1 vegetative cell. The minimum in situ division rate of the temporary cysts was 0.14 day−1. The role of the resting and temporary cyst population in the annual recurrence and maintenance of the A. taylori bloom is discussed.

Nowadays, oceanographic research usually involves the simultaneous participation of many research groups and often from many distinct disciplines. The development of a field experiment consists on using many devices necessary to obtain the most complete description of the ocean system. The result is an enormous amount of data from different nature, as physical and chemical water properties or biological characteristics, that must be analysed and interpreted. To give an example, a usual experiment may consist on a vessel cruise, the deployment of moored equipment and the simultaneous reception of satellite images. Thus, the whole data set can easily reach several gigabytes of information. Once all this original information is stored, researchers must pre-process it to obtain a calibrated and quality controlled data set. Then, in order to get a deep knowledge of the system dynamics it is usually necessary to visualise and analyse this information in different ways. This procedure is commonly expensive, not only in terms of computing resources but also in the time that researchers must expend on it. Other problems appear because the way of doing of scientists involved in a common project. They do not use similar resources which means computer platforms, visualisation software, etc.. Some data is read and stored many times in different formats in order to deal with similar analysis or draws

The deep-living octopod Cirroteuthis magna Hoyle, 1885 is redescribed, based on the only three specimens known of the species: a mature female (holotype) captured in the south Indian Ocean between Prince Edward and Crozet islands at 2557 m and two specimens, one submature female and one mature male, recently captured in the central Atlantic at 1300 and 3351 m depth, respectively. Video images from the capture of the latter specimen were recorded. This species is characterized by its very great size (to 1300 mm TL), making it the largest known cirrate octopod; butterfly-like shell with open wings ; very voluminous eyes with large lenses; arm length 73–79% of the total length; primary web inserted at different levels on the dorsal and ventral ends of the dorso- and ventrolateral arms on both sides, and at the same level on both ends of the dorsal and ventral arms; each arm is independent of the primary web, and is connected with it by a single vertical membrane or intermediate web that is attached along the dorsum of the arm; absence of nodule at the fusion point of both webs. Very large cirri, the first cirri commencing between the 4th and 5th suckers, with three types of suckers on all the arms; cylindro-conical form and those with the acetabulum highly deformable on the first 2/3 of arms and barrel-like on the rest of the arm; absence of particularly enlarged suckers. C. magna is compared with C. muelleri and other related species. Sperm sacs and spermatozoids from C. magna and C. muelleri are described and compared. The Cirroteuthis genus is reviewed and a diagnosis is proposed. This study confirms that the members of the Cirroteuthidae family show several unusual features of great interest.

Keywords: vegetation pattern - serpentine grassland - disturbance - Thomomys bottae - geostatistics - remote sensing - fractal - spatial simulation - NDVI - Fast Fourier Transform

An important practical problem in the analysis of spatial pattern in ecological systems is that requires spatially-intensive data, with both fine resolution and large extent. Such information is often difficult to obtain from field-measured variables. Digital imagery can offer a valuable, alternative source of information in the analysis of ecological pattern. In the present paper, we use remotely-sensed imagery to provide a link between field-based information and spatially-explicit modeling of ecological processes. We analyzed one digitized color infrared aerial photograph of a serpentine grassland to develop a detailed digital map of land cover categories (31.24 m × 50.04 m of extent and 135 mm of resolution), and an image of vegetation index (proportional to the amount of green biomass cover in the field). We conducted a variogram analysis of the spatial pattern of both field-measured (microtopography, soil depth) and image-derived (land cover map, vegetation index, gopher disturbance) landscape variables, and used a statistical simulation method to produce random realizations of the image of vegetation index based upon our characterization of its spatial structure. The analysis revealed strong relationships in the spatial distribution of the ecological variables (e.g., gopher mounds and perennial grasses are found primarily on deeper soils) and a non-fractal nested spatial pattern in the distribution of green biomass as measured by the vegetation index. The spatial pattern of the vegetation index was composed of three basic components: an exponential trend from 0 m to 4 m, which is related to local ecological processes, a linear trend at broader scales, which is related to a general change in topography across the study site, and a superimposed periodic structure, which is related to the regular spacing of deeper soils within the study site. Simulations of the image of vegetation index confirmed our interpretation of the variograms. The simulations also illustrated the limits of statistical analysis and interpolations based solely on the semivariogram, because they cannot adequately characterize spatial discontinuities.

The present paper reports on temporalmesoscale (weeks) changes in physical and biological coupling in the cross-frontal area off the coast of Catalonia in Spain (NW Mediterranean) during the spring transition period. The effect of short-term physical variability on the spatio-temporal heterogeneity of phytoplankton biomass and in the area of overlap of the larvae of fish species dwelling on the shelf and slope is discussed. Our results indicate that the region off the Catalan coast is a dynamically active area during the spring transition period. Short-term spatio-temporal variability in the frontal system brought about significant changes in the extension of shelf and slope waters, giving rise to major changes in the spatial distribution patterns of chlorophyll and fish larvae. The relationship between integrated chlorophyll, surface chlorophyll and DCM (Deep Chlorophyll Maximum) showed important variability in a short time period. The area of overlap of the larvae of shelf and slope fish species was broad when the front was located offshore, compared to nearly complete segregation of shelf and oceanic fish larvae when the front moved inshore near the coast.

Keywords: Norway lobster, Nephrops norvegicus, geostatistics, variogram, kriging, resource assessment.

The application of geostatistics in mapping and assessing demersal resources is reviewed. The basic summary of the methodology is illustrated with a case study on data for Nephrops norvegicus off Barcelona (NE Spain) over commercial fishing grounds. The geostatistical analysis revealed that N. norvegicus is distributed over the continental slope in high-density patches of ~ 7 km, alternating with areas of low density. Mapping by disjunctive kriging helped visualize the spatial distribution of the resource and identified areas where the resource abundance was of commercial level.

Keywords: Daily ration · Decapod crustaceans · Feeding ecology · Deep-sea ecology

Estimates of the daily ration consumed by 9 species of decapod crustaceans were obtained from 2 deep-water samplings over the middle and lower continental slope of the NW Mediterranean. The daily rations obtained varied by an order of magnitude from the slow-moving, crab-like species (<0.1 g dry food per 100 g predator body weight) to the actively swimming mesopelagic shrimps (>1.0 g dry food per 100 g predator body weight). A multivariate analysis allowed us to relate the variability in daily ration estimates to the functional morphology and ecology of the species considered. The functional morphology and ecology of the species studied are defined in terms of body shape, swimming capability, trophic diversity and spatial distribution. We found that body shape (ratio of carapace length to carapace width) and trophic diversity (H') account for most of the variability in the amount of food consumed by decapods. The food consumption pattern established for our decapod crustaceans is compared to a similar pattern reported for deep-water fishes.

A hydrographic and zooplankton survey (neuston and water column) was performed over the Blanes submarine canyon (western Mediterranean) in June 1993. Abundance of fish and crab larvae was very low when compared with previous studies conducted in the same area. The horizontal distribution of fish and crab larvae whose adults inhabit the continental shelf was restricted to the coast. The offshore occurrence of low-salinity waters of continental origin, together with the absence of a component perpendicular to the coast in the system of currents observed throughout the study area, support the observed pattern of distribution. The occurrence of mesopelagic fish larvae and of the oceanic neustonic isopod Idotea metallica in some coastal stations may be associated with episodic inshore-offshore exchanges of water masses identified by drifting buoy trajectories.

A methodology is proposed to estimate sea surface currents from the information given by the sea surface temperature (SST) obtained from satellite images. Currents are estimated from the motion field of a temporal sequence of images using a Hopfield neural network. A cost function is minimized when some rules of correspondence between pixels in successive images are considered. A performance test of our methodology is done using the information from the buoys positions, prevent us to use a system able to escape from the local minima, where the Hopfield neural network uses to get trapped, until reaching a global minima

Keywords: western Mediterranean circulation, Algerian basin, Lagrangian drifters.

The Algerian basin is a region of the western Mediterranean with a highly variable circulation structure, including the eastward transport of Modified Atlantic Water (MAW) in its surface layer. An experiment with satellite tracked Lagrangian drifters was performed in 1996-97 to analyse the mesoscale circulation of the Algerian current. The complete trajectories of 18 drifters indicate that, at basin scale, all the surface flow occured along the coast from the Alboran Sea to the strait of Sicily. At that time, no portion of the inflowing MAW was driven to the central or northern regions.