Showing papers in "Scientia Marina in 2001"

The oceanic fixed nitrogen and nitrous oxide budgets: Moving targets as we enter the anthropocene?*

Abstract: New data force us to raise previous estimates of oceanic denitrification. Our revised estimate of ~ 450 Tg N yr -1 (Tg = 10 12 g) produces an oceanic fixed N budget with a large deficit (~ 200 Tg N yr -1 ) that can be explained only by positing an ocean that has deviated far from a steady-state, the need for a major upwards revision of fixed N inputs, particularly nitrogen fixation, or both. Oceanic denitrification can be significantly altered by small re-distributions of carbon and dissolved oxygen. Since fixed N is a limiting nutrient, uncompensated changes in denitrification affect the ocean´s ability to sequester atmospheric CO 2 via the "biological pump". We have also had to modify our concepts of the oceanic N 2 O regime to take better account of the extremely high N 2 O saturations that can arise in productive, low oxygen waters. Recent results from the western Indian Shelf during a period when hypoxic, suboxic and anoxic waters were present produced a maximum surface N 2 O saturation of > 8000%, a likely consequence of "stop and go" denitrification. The sensitivity of N 2 O production and consumption to small changes in the oceanic dissolved oxygen distribution and to the "spin-up" phase of denitrification suggests that the oceanic source term for N 2 O could change rapidly.

Formation and fate of marine snow: small-scale processes with large- scale implications

Abstract: Marine snow aggregates are believed to be the main vehicles for vertical material transport in the ocean. However, aggregates are also sites of elevated heterotrophic activity, which may rather cause enhanced retention of aggregated material in the upper ocean. Small-scale biological-physical interactions govern the formation and fate of marine snow. Aggregates may form by physical coagulation: fluid motion causes collisions between small primary particles (e.g. phytoplankton) that may then stick together to form aggregates with enhanced sinking velocities. Bacteria may subsequently solubilise and remineralise aggregated particles. Because the solubilization rate exceeds the remineralization rate, organic solutes leak out of sinking aggregates. The leaking solutes spread by diffusion and advection and form a chemical trail in the wake of the sinking aggregate that may guide small zooplankters to the aggregate. Also, suspended bacteria may enjoy the elevated concentration of organic solutes in the plume. I explore these small-scale formation and degradation processes by means of models, experiments and field observations. The larger scale implications for the structure and functioning of pelagic food chains of export vs. retention of material will be discussed.

Marine diversity: the paradigms in patterns of species richness examined*

Abstract: SUMMARY: The two central paradigms of marine diversity are that there is a latitudinal cline of increasing species richness from poles to tropics and that species richness increases with depth to a maximum around 2,000 m and thereafter decreases. However, these paradigms were based on data collected in the late 1950’s and early 1960’s. Here I show that the 1960’s data, are not representative and thus the paradigms need re-examination. New data from coastal areas in the northern hemisphere record species richness as high as the highest recorded in the deep-sea. Whilst this suggests that the cline of increasing diversity from shallow to deep-sea does not exist, however, the database for the deep sea is not sufficient to draw such a conclusion. The basic problem with the data from the 1960s is that samples were taken on ecological scales and yet they are used to answer evolutionary questions. The questions that such data were to answer were why do the tropics have higher species richness than polar regions or why do deep-sea sediments have more species than coastal sediments? Evolutionary questions need data from much larger spatial areas. Recently, data representative of large scales have been collected from coastal areas in the northern hemisphere and show that there is a cline of increasing species richness from the Arctic to the tropics, but there does not yet seem to be a similar cline in the southern hemisphere. A number of hypotheses have been proposed for the observed patterns in biodiversity. In terrestrial ecology the energy-productivity hypothesis has gained wide acceptance as an explanation for the latitudinal gradient. Here I examine this and other hypotheses critically. Finally an analysis of research priorities is made. Assessment is urgently needed of the spatial scales and dynamics of species richness from point samples to assemblages, habitats and landscapes, especially in coastal areas and in the tropics, where the threats to biodiversity are greatest. New technologies are available, such as side-scan sonar, acoustics, and under-water digital video cameras but as yet have been relatively little used. Conservation of marine biodiversity must be based on sound theory, yet marine diversity studies lag well behind those of terrestrial habitats.

Marine benthic faunal successional stages and related sedimentary activity

Abstract: This paper is a brief review of successional stages and activity of benthic soft-bottom communities. Benthic communities was first described by Petersen in the 1910s and further developed by Molander, Thorson and Margalef. Successional stages of benthic communities chance in a predictable way in relation to environmental disturbance and food availability. Food supply to the bottom can occur as a vertical flux, but transport through lateral advection is more important in some areas. While at the bottom, the infauna processes the food in many different ways, and the feeding modes can be categorised into more than 20 functional groups, but fewer are present in brackish water. This categorisation is based on animal mobility and where and how they ingest the food. Animal activity in the sediment, bioturbation, has a significant effect on redox conditions and diagenetic processes. Structures in the sediment due to infaunal presence and activity can be observed in situ by sediment profile imaging, and the biogenic structures and redox conditions can be parameterised and have been shown to correlate to benthic community successional stages. The largest threat to benthic faunal biodiversity is the spread of near-bottom oxygen deficiency in many enclosed are stratified coastal areas.

Seasonal patterns of wind-induced upwelling/ downwelling in the Mediterranean Sea

Abstract: SUMMARY: The historical file of wind observations from maritime weather reports is summarized to identify the characteristic seasonal distributions of wind-induced Ekman upwelling and downwelling in the Mediterranean Sea. Both coastal upwelling/downwelling and wind-stress curl-driven “open ocean” upwelling/downwelling are treated in a unified description. Vigorous upwelling zones are found in the eastern Aegean Sea, off the west coast of Greece, and in the Gulf of Lyons. The southern coast of the Mediterranean is found to be primarily a downwelling area, although significant coastal upwelling does appear in the Gulf of Sidra during the spring and summer seasons, and along the Algerian coast during summer.

The sedimentary record of dinoflagellate cysts: looking back into the future of phytoplankton blooms*

Abstract: SUMMARY: Marine systems are not as well understood as terrestrial systems, and there is still a great need for more primary observations, in the tradition of the old-time naturalists, before newer methods such as molecular genetics and modeling can be fully utilized. The scientific process whereby the smaller, detailed “building blocks” of observation are ultimately linked towards better understanding natural systems is illustrated from my own career experience, especially with regard to the dinoflagellates and plankton blooms. Some dinoflagellates produce a fossilizable resting stage (cyst) in their life cycle, and dinoflagellate cysts have become one of the most important groups of microfossils used in geological exploration (e.g. oil and gas). This has stimulated both paleontological and biological research producing detailed “building blocks” of information, currently scattered throughout the respective literature. Here, I attempt to bring together the present day perspective, from biology, with the past, from paleontology, as the most comprehensive basis for future work on the group. This shows the cysts to be the critical link needed for focusing future molecular genetics studies towards a more verifiable view of evolutionary pathways, and it also suggests new integrated methods for studying past, present, and future blooms. The large, rapidly growing field of harmful algal bloom studies is producing many different “building blocks”, but plankton blooms as episodic phenomena are still poorly understood. This is largely due to the general lack of long-term datasets allowing identification of the changing environmental factors that permit certain species to bloom at unpredictable intervals of time. Cysts in sediments are useful environmental indicators today, e.g. reflecting aspects of climate and pollution, and provide information directly relevant to some dinoflagellate blooms. They therefore may be used for obtaining retrospective information from the sedimentary record on the history of blooms, in turn suggesting information relevant for future blooms and the way we study them.

Spirals on the sea

Abstract: Spiral eddies were first seen in the sun glitter on the Apollo Mission 30 years ago; they have since been recorded on SAR missions and in the infrared. The spirals are globally distributed, 10-25 km in size and overwhelmingly cyclonic. They have not been explained. Under light winds favorable to visualization, linear surface features with high surfactant density and low surface roughness are of common occurrence. We have proposed that frontal formations concentrate the ambient shear and prevailing surfactants. Horizontal shear instabilities ensue when the shear becomes comparable to the coriolis frequency. The resulting vortices wind the liner features into spirals. The hypothesis needs to be tested by prolonged measurements and surface truth. Spiral eddies are a manifestation of a sub-mesoscale oceanography associated with upper ocean stirring; dimensional considerations suggest a horizontal diffusivity of order 103 m2 s-1.

Algal cover and sea urchin spatial distribution at Madeira Island (NE Atlantic)

Abstract: This study describes sea urchin spatial distribution in relation to environmental factors, and the relationship between Diadema antillarum density and algal abundance. Twenty-three transects around Madeira Island were surveyed by scuba divers, and sea urchin density and algal cover were determined in situ. Sampling sites along these transects were characterised in terms of distance from the tide line, water depth, substratum type, bottom declivity and water turbulence. Diadema antillarum was the dominant sea urchin species. Paracentrotus lividus and Arbacia lixula occurred at shallower depths (2-6 m), contrasting with the distribution of Sphaerechinus granularis , which occurs among D. antillarum (4-20 m). Surveys found two alternative types of communities on rocky shores: 1) a community with high algal cover and low numbers of sea urchins, along the north and south-west coasts and; 2) a community with little algal cover and high densities of sea urchins, along the south-east coast. Macroalgal cover and D. antillarum densities were inversely correlated (adjusted R 2 =75.6%; n = 429; p D. antillarum on rocky substrates. We propose a multiple non-linear regression model (using backward stepwise analysis) to explain D. antillarum abundance on the rocky shores: D. antillarum/m 2 (??)= 0.121 - 0.209 distance from shore (in m) (??) + 2.052 water depth (in m) (??) - 1.778 water turbulence level (??) - 0.007 water turbulence level4 (??); where ?? indicates data are square-root transformed (adjusted R 2 = 60.99%; n = 454; p< 0.05).

Delivery and fate of fluvial water and sediment to the sea: a marine geologist's view of European rivers

Abstract: Despite their relatively small drainage areas, European rivers reflect a wide variety of hydrologic regimes, although with very few exceptions they have been strongly affected by human activity. Scandinavian rivers (particularly those draining Iceland and western Norway) can have high runoff, and, except for those draining Iceland, all have very low suspended and dissolved sediment loads. Northern and western European rivers have somewhat lower runoff, among the lowest suspended sediment yields in the world, and anthropogenically enhanced dissolved solid loads. Annual discharge of many of these rivers appears to vary inversely with the North Atlantic Oscillation index. Rivers discharging from the southern Alps into the Mediterranean Sea have relatively high runoff, high suspended sediment yields (reflecting younger, more easily erodable rocks as well as generally smaller drainage basins), and high dissolved yields, although presumably with somewhat less human influence. European rivers and their estuaries tend to reflect the terrestrial environments of their drainage basins (i.e. climate, landscape geomorphology, geology), but they also display strong anthropogenic signatures. Sediment erosion increased dramatically in the last several millenia in response to deforestation, farming and mining. In the past 50 years, however, increased soil conservation and local reversion of agricultural land to forest, as well as river diversion and dam construction, have decreased the suspended sediment loads of many European rivers. Improved mining and manufacturing techniques, as well as more effective use of fertilizers and improved waste treatment, almost surely will result in lower dissolved solids and nutrient fluxes to the coastal environments, which presently are the highest in the world. The long-range effects of changed land use on estuarine and coastal environments remain to be seen, although decreased sediment loads in the past 20-40 years have already resulted in increased shoreline erosion. Decreased nutrient fluxes almost certainly will affect water quality in European coastal waters, and decreased silicate delivery by some dammed rivers may result in proliferation of new (and perhaps harmful) estuarine and coastal ecosystems. Everything points to further changes as European rivers and their drainage basins continue to change in the coming years.

Larval trophodynamics, turbulence, and drift on Georges Bank: A sensitivity analysis of cod and haddock

Abstract: Using an individual-based model approach we consider trophodynamic effects on the growth and survival of larval cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) on Georges Bank during late winter/early spring. These studies represent an extension of results described in Werner et al. (1996; Deep-Sea Res. II), wherein the effect of turbulence-enhanced larval-prey contact rates increased the effective prey concentration resulting in growth of cod larvae consistent with observed rates in the field. We reformulated the feeding of the larvae to include existing relationships between maximum prey-length and larval-length and we examined: (i) larval search behaviour and its effect on encounter with prey, (ii) the ability of larvae to pursue and capture prey in a turbulent environment, and (iii) the effect of turbulence on the dispersion of larvae in the vertical. We find that search behaviour, the effect of turbulence on pursuit and capture, and vertical dispersion decrease the predicted larval growth rates compared to those observed in the earlier study. These results suggest that larval feeding behaviour, and especially the ability of larvae to pursue encountered prey, could be an important input to larval growth and survival models. The inclusion of turbulence in determining the position of passive larvae in the water column allows the larvae to sample the entire water column, contributing to a decrease in the variance of the size of the larvae over time. The ability of larvae to swim and aggregate in the vertical will be necessary to reproduce distributions observed in the field.

Nivmar: a storm surge forecasting system for Spanish waters

Abstract: In this paper, a storm surge prediction system for the Spanish Waters is presented. The system, named Nivmar, is based on the ocean circulation Hamsom model and on the harmonical prediction of tides computed from data measured by the tide gauge network Redmar, managed by Puertos del Estado. Nivmar is executed twice a day, running Hamsom forced by meteorological fields derived from the INM (Instituto Nacional de Meteorologia) operational application of Hirlam atmospheric model. Data from Redmar tide gauges is used to to forecast the tidal elevations, to validate the system and to perform data assimilation, correcting systematic errors in the mean sea level due to physicals processes that are not included in the ocean model (i. e. steric height). The forecast horizon is 48 hours. In order to validate the system with measured data from Redmar a very stormy 5 months period was selected. Results from this test (November 95 to March 96) are presented. Data from this experiment shown that Nivmar is able to correctly predict sea level in the region. A simple data assimilation scheme for sea level is described and results from its application are studied. Finally, special focus is made in future plans and potential developments and applications of the system.

The macrofauna associated with the bryozoan Schizoporella errata (Walters) in southeastern Brazil

Abstract: The invertebrate fauna associated with colonies of the bryozoan Schizoporella errata was studied at two sites differing in hydrodynamism in southeastern Brazil. Patterns of species distribution, abundance, and diversity were similar at both sites. Polychaetes dominated the assemblages, both in number of individuals and species, with 70 species distributed in 20 families. Decapod crustaceans were also abundant, mainly anomuran crabs and caridean shrimps. Cluster analysis separated the samples from each site, and further analysis indicated that an interaction between water movement and trophic structure could explain these differences. At the site with low water movement, more muddy sediments were found inside the colonies, and a greater proportion of deposit-feeders was found. At the other site, higher hydrodynamism resulted in less sediments within the colonies, and favoured the occurrence of more filter-feeders. The trophic structure of faunal assemblages can help understand the structure of assemblages associated with biogenic substrata, as already demonstrated for benthic communities.

Natural History: the sense of wonder, creativity and progress in ecology

Abstract: This essay addresses the question of blending natural history and ecological wisdom into the genuine creativity exemplified by Prof. Ramon Margalef. Many have observed that modern biology is a triumph of precision over accuracy, and that ecology has sought maturity by striving toward this model in which the precision value of the tools has supplanted important questions. In pursuing a model of hard science, ecology has struggled with Popperian approaches designed to create a thin patina of real science over the vast seas of uncertainty so admired by the naturalists. We start with a discussion of the importance of natural history in ecology and conservation, and the present state of natural history in academic ecology. We then discuss the respect for natural history in human cultures, and conclude that an infatuation with authority has obfuscated the important truths to be found in nature. We consider some general processes associated with creativity, and finally we ask how natural history influences creativity in ecology. We conclude that the soaring creativity exemplified by Ramon Margalef is based on a joyful almost spiritual understanding of natural history and the courage to avoid authority.

Caught in the food web: complexity made simple?

Abstract: Several historically separate lines of food-web research are merging into a unified approach. Connections between microbial and metazoan food webs are significant. Interactions of control by predators, defenses against predation, and availability of organic and inorganic nutrition, not any one of these, shape food webs. The same principles of population ecology apply to metazoans and microorganisms, but microorganisms dominate the flux of energy in both marine and terrestrial systems. Microbial biomass often is a major fraction of total biomass, and very small organisms have a very large ratio of production and respiration to biomass. Assimilation efficiency of bacteria in natural systems is often not as high as in experimental systems, so more primary production is lost to microbial respiration than had been thought. Simulation has been a highly useful adjunct to experiments in both population theory and in studies of biogeochemical mass balance, but it does not fully encompass the complexity of real systems. A major challenge for the future is to find better ways to deal with the real complexity of food webs, both in modeling and in empirical observations, and to do a better job of bringing together conceptually the dynamics of population processes and biogeochemistry.

The seasonal planktonic cycle in coastal waters of the Canary Islands

Abstract: The short-term temporal variation in the phytoplankton and mesozooplankton cycles was studied in a coastal area off east Gran Canaria Island. A small phytoplankton bloom, split into two peaks, appeared during late winter (end of February and March), coinciding with the lowest temperatures in the water column. A clear inverse relationship was observed between the biomasses in mesozooplankton and phytoplankton during the bloom period. The peaks in primary production and phytoplankton biomass were uncoupled in time, suggesting that biomass could depend on consumer control (grazing), and primary production on resource control (nutrients). Mesozooplankton grazing represented less than 20% of the primary production, an indication that small zooplankton and protozoans controlled the phytoplankton populations, dominated by picoplanktonic cells (> 60% of the primary production). The ratio between depth-integrated primary production and community respiration (P/R) covaried with primary production (P), showing that changes in P control the trophic status of the system. At P > 400 mgC m -2 day -1 the P/R ratio is > 1, switching the system from heterotrophy to autotrophy, a situation that takes place during the phytoplankton growth period.

A case for tolerance in marine ecology: let us not put out the baby with the bathwater*

Abstract: After working in marine ecological research and lecturing at universities for several decades, the authors are aware that almost none of the concepts and paradigms established at the time they were students or in the decades before have survived unchanged up to the present day. This appears quite natural taking into account the enormous progress made by marine science due to refined methods, extended range of research e.g. into the deep sea and the polar regions, increased number of researchers and funding, and greatly improved and accelerated exchange of scientific results. What is striking, however, against this background is the almost messianic fervour with which many of our professional guild (and not only the younger ones lacking experience) call for the immediate “abolishment” or “dismissal” of old ideas because their creators were wrong in thinking the way they did. In these authors` opinion, there is a basic misunderstandig about the role of ecological concepts in scientific argumentation. Very few concepts are so foolish that they can be dismissed entirely without any loss; the vast majority contain some truth that may fit at least certain situations, and may thus serve as a brickstone in constructing ecological theory. An attempt will be made to expand on that point in this contribution. What is more important: many of the concepts nowadays considered (partially?) erroneous have stimulated scientific discussion enormously, in some cases up to the present day and have, by their mere existence, guaranteed that ecological work was found necessary to continue in the respective fields. Finally, observing the tremendous pace at which ecological progress is being made these days in some disciplines and at the same time, the degree of uncertainty we are - despite all efforts - still facing in others, it is not difficult to anticipate that many of the new ideas replacing the old concepts nowadays will end up shortly on the garbage dump of science history as well. So tolerance should be the motto: let us not put out the baby with the bathwater! Give the old concepts their credit, look eagerly for alternatives and try to provide the best possible evidence for your results, but be modest. There may be various solutions to one ecological problem, and the way to handle this situation is differentiation, not abolishment.

Seasonal grazing and food preference of herbivores in a Posidonia oceanica meadow

Abstract: Seasonal grazing of the fish Sarpa salpa (L), the urchin Paracentrotus lividus Lamarck and the isopods Idotea spp was compared with the C/N ratio of adult and intermediate leaves and epiphytes of Posidonia oceanica (L) Delile, collected at three different depths Despite seasonal differences in grazing, herbivores showed preferences throughout the year for adult leaves with more epiphyte and higher N contents The maximum grazing on adult and intermediate leaves was observed in September and in June for fish and in March for urchins, whereas it was irregular for isopods Grazing by the three herbivores was not related to their preference for leaves or epiphytes, notwithstanding the seasonal differences in their C and N contents We concluded that herbivores show no preference for food type throughout the year and that seasonal consumption of P oceanica is related mainly to herbivore behaviour

Seasonal and interannual variability of the Canary Current

Abstract: Seasonal and interannual variability of the Canary Current on its passage through the Canary archipelago and between 20-35°N and 10-20°N are studied with long series of historical tide gauge, sea surface temperature and hydrographic data. The variability of winds is more seasonal in the north of the area, where they are weak and perpendicular to the coast in winter, than in the south, where winds are roughly parallel to shore all year though strongest in summer. Temperature and salinity data maps show a curved structure indicative of the subtropical gyre, which varies seasonally from North to South and from the open ocean to the coast. Dynamic height analyses show this seasonal variation of the gyre to depths of 200 m and indicate the existence of a meander-like diversion around the Canary Islands. The sea level in the Canary Islands also indicates a strong seasonal variation of the north Atlantic subtropical gyre. The southward geostrophic surface flow, derived from sea surface slope, was strongest during spring and summer at the eastern islands while it was strongest in winter at the outermost islands. The spatially averaged flow between the innermost and outermost islands shows the strongest southward flow in winter. Interannual variation of the flow between these islands, investigated during two periods, 1950-1956 and 1960-1973, reveals a mainly southward flow, although several show a northward tendency. The maximum equatorward velocity of the current in both periods is around 5 cm s-1, indicating a weak current.

Checklist and annotated bibliography of decapod crustacean larvae from the Southwestern European coast (Gibraltar Strait area)

Abstract: SUMMARY: Studies on decapod larvae entailing identification of specimens collected from plankton should be based on sound bibliographic sources that contain morphological descriptions of the species cited in the area. In order to facilitate this bibliographic search, we present a survey of the literature in which decapod larvae from the Gibraltar Strait area (encompassing most of the Mediterranean and northeastern Atlantic decapod crustacean species) are partially or completely described. Among these, the most accuratly identified larval specimens are pointed out. For each original case the taxon assigned by the author, types and numbers of stages described and the origin of the studied larvae are noted. Morever, a checklist of 377 species is given for Gibraltar Strait decapod crustaceans. Of all the species recorded in the area, larval descriptions are known, at least partially, for 220 species (58,35 %), but for 36 (9,54 %) morphological descriptions are sufficiently accurate for a correct identification from plankton specimens.

Zooplankton abundance in subtropical waters : is there a lunar cycle?

Abstract: Based on historical data of abundance, we report evidence of changes in zooplankton abundance in the 0-200 m layer related to the moon cycle confirming that this phenomenon is produced in the marine environment, similarly to the one described for freshwater ecosystems. A clear decrease in the abundance of copepodites plus copepods was observed from the second to the fourth quarter of the moon when the seasonal variability was suppressed. During the full moon phase the large zooplankton and micronekton of the deep scattering layers (DSL) would not reach the upper mixed layer in order to avoid predation because of the relatively high level of illumination. Thus epipelagic zooplankton abundance increases as the effect of a lower predatory pressure. Conversely, during the new moon phase the diel migrants reach the surface waters and epiplankton abundance considerably decreases. Recent oceanic sediment trap data in subtropical waters indicate that the particle flux increases at about 30 days period. Thus, the effect of diel vertical migrants could promote not only the variability in their resources and the intensity of the active flux, but could also drive the variability in the gravitational flux.

Metazoan parasites of grey mullets (Teleostea: Mugilidae) from the Mistras Lagoon (Sardinia, western Mediterranean)*

Abstract: A total of 127 specimens of grey mullets (Teleostea: Mugilidae) from the Mistras Lagoon (Sardinia - western Mediterranean) were examined for metazoan parasites. Five species of grey mullets were found: Chelon labrosus, Liza aurata, L. ramada, L. saliens and Mugil cephalus. Forty-one species of parasites were identified: 14 monogeneans, 5 crustaceans, 18 digeneans, 3 nematodes and 1 acanthocephalan; 26 of them being reported from Sardinia for the first time. Findings of faunal interest are: Ergenstrema sp. (Monogenea) in L. saliens; Caligus pageti (Crustacea) in L. aurata and L. saliens; Saturnius sp. (Digenea) in C. labrosus, L. aurata and L. saliens; Capillaria sp. (Nematoda) in C. labrosus, L. aurata and M. cephalus. The dynamics and distribution of ectoparasites were partially influenced by salinity, while the interpretation of the results for endoparasites was more complex. The data obtained show the richness of the parasite fauna of grey mullets from the Mistras Lagoon, providing additional environmental information on this body of water.

Seasonal distribution and spawning of small tunas ( Auxis rochei and Sarda sarda ) in the Northwestern Mediterranean

Abstract: SUMMARY: Bullet tuna, Auxis rochei and Atlantic bonito, Sarda sarda are the most abundant small tuna species in the Mediterranean Sea. Both species are commercially exploited by a small-scale inshore fishery off the Catalan coast. Catches of A. rochei are highly seasonal, with maximum values in the warmest months (June to September), which is also the spawning period. Larvae of A. rochei were collected between July and September and were particularly abundant in July, when the mean sea surface temperature was 25.4oC. In summer, adults of this species migrate from the Atlantic to the Western Mediterranean, including the Catalan Sea, to spawn, but they are not found in this last region during the colder months. S. sarda is taken all year round in the Catalan Sea, with maximum catches from September to March, and minimum levels during the spawning period (May to July). Larvae of this species were only observed in July and larval abundance was considerably lower than that of larval A. rochei. Unlike A. rochei, S. sarda is resident over the continental shelf in the Western Mediterranean all year long, but at the beginning of summer, coinciding with the arrival of the other species, it moves to other areas in order to spawn.

Seasonal pattern of zooplankton biomass in the Argentinian shelf off Southern Patagonia (45º - 55º S)

Abstract: Fil: Sabatini, Marina Elena. Consejo Nacional de Investigaciones Cientificas y Tecnicas; Argentina. Instituto Nacional de Investigaciones y Desarrollo Pesquero; Argentina

Advection of continental water as an export mechanism for anchovy, Engraulis encrasicolus, larvae

Abstract: SUMMARY: The presence of a surface layer of a less saline water of continental influence (CIW) along the continental slope off the Catalan coast (NW Mediterranean) was observed in June 1995. This CIW was formed in the Gulf of Lions from Rhone runoff and carried by the current associated with the shelf-slope front. This study analyses the spatio-temporal behaviour of that water of continental influence in relation to the shelf-slope front, and its effect on the distribution of fish larvae, in particular the larvae of the anchovy, Engraulis encrasicolus. Displacements of the front are observed to follow an oscillation with a period of eight days and an amplitude of 10 km. Anchovy larvae were associated with the presence of CIW, whereas the oceanic species Hygophum benoiti appeared in the region of the slope when the front, and the associated CIW layer, moved close inshore during its oscillation. The size frequency distribution for the anchovy larvae trapped within the CIW suggests that the larvae had been spawned further north, near the Gulf of Lions. It is concluded not only that larvae were transported by the current but also that their development had taken place entirely inside the CIW. The possible benefit to the survival of anchovy larvae of being kept inside that water, though they have been carried far from their spawning areas, is discussed in the basis of fluorescence and microzooplankton data.

Water masses, circulation and transport in the eastern boundary current of the North Atlantic subtropical gyre*

Abstract: CTD sections carried out in September 1998 are used to describe the water masses, geostrophic circulation and mass transport in the easternmost branch of the Canary Current. The surface water mass ( 9 kg s -1 . A tongue of relatively fresh water, consisting of Antarctic Intermediate Water (AAIW), was found approximately in the 600-1100 m depth layer. This tongue was 200 km wide, stretching from the African coast almost to Gran Canaria Island, and transported a net mass of 1.1 x 10 9 kg s -1 northward. This system of currents is what constitutes the real eastern boundary current of the North Atlantic Subtropical Gyre.

Improved methods for generating and estimating turbulence in tanks suitable for fish larvae experiments

Abstract: An experimental design has been developed for the purpose of testing the response of larval fish feeding to turbulence. The main purpose of the design is to reproduce natural turbulence within the range of scales relevant to the feeding processes. The design is based on grid-generated turbulence in a circular tank with a diameter of 1.5 m and a water depth of 0.90 m. The spatial distribution of the turbulence was measured. The data analysis of our experiment indicate that we successfully simulated natural turbulence covering six orders of magnitude for the energy dissipation rate (4*10-11 - 2*10-4 W/kg) and over three orders of spatial scales from the Kolmogorov microscale (0.3 mm -13 mm) to the size of the tank. It is concluded that the tank is well designed for studies of zooplankton - larval fish interactions, as the turbulence is generated on a range of scales similar to the separation distance between the zooplankton particles found in nature. The short and relatively slow movement of the grid in addition to the relatively large volume of water should make it possible for the larvae to avoid extreme shears, and minimises mechanical impact on larvae behaviour compared to other known turbulence generating systems. A method is developed to calculate the turbulent energy dissipation rate in a zero mean velocity field, assuming the turbulent cascade follows a -5/3 law in the inertial subrange. This method is also expanded for use on noisy time series. Comparisons with five other methods have been conducted. The comparisons show that Taylors frozen turbulence hypothesis can be used even when the ratio between the turbulent and the mean velocity is approximately 1.

Dynamical characteristics of the Cape Verde frontal zone

Abstract: SUMMARY: We analysed historical data (Fraga and Manriquez, 1974; Manriquez and Fraga, 1978) to investigate the relation between the position of the Cape Verde frontal region and the geostrophic velocity field in summer and winter. The position of the front is well determined in the plotting of the salinity and nutrient distributions in isopycnic coordinates. The geostrophic velocity shows sharp gradients as a function of density and distance on sections normal to the coast. These gradients are reasonable well correlated with the frontal position and indicate the existence of strong meridional interleaving. The flow field relative to 400 m illustrates this interleaving as fluctuations on a westward mean flow. The surface temperature distribution undergoes very large latitudinal displacements but may help to locate the presence of north and south surface water.

Evolution of longshore beach contour lines determined by E.O.F. method

Abstract: Detailed topo-bathymetric levellings were performed biannually for four years at Victoria Beach (Cadiz, Spain) after a beach renourishment carried out in Spring 1991. The subsequent time series were analysed using the Empirical Orthogonal Functions (EOF) method. The evolution of some characteristic longshore contour lines, such as the Highest High Water Level and the Lowest Low Water Level, is studied. The mean coastal line is related to the first spatial EOF mode. Furthermore, an objective criterion for distinguishing between a generalised recession and cyclic accretion-erosion processes due to seasonal sea-swell changes is described, and a uniformly clockwise turn of the shoreline to minimise longshore transport is identified.

Residual circulation and thermohaline distribution of the Ría de Vigo: A 3-D hydrodynamical model

Abstract: A three-dimensional, non-linear, baroclinic model is described and tested for the first time to study the residual circulation and the thermohaline distribution of the Ria de Vigo (NW Spain) at short time scales and under different wind stress regimes. Two markedly different realistic scenarios were chosen: northerly upwelling-favourable winds and southerly downwelling-favourable winds. The numerical experiments carried out indicate that the hydrodynamic regime of the Ria de Vigo is mostly a consequence of wind events. As could be expected, moderately strong North winds reinforce the normal (positive) Ria circulation, while winds blowing from the South, when sufficiently strong, reverse the typical circulation pattern and reduce the characteristic outgoing velocities and the flushing time inside the Ria. The temperature and salinity fields generated by the model in both situations were compared with observations and found to be in qualitatively good agreement, supporting the 3D velocity field distribution.

Settlement and juvenile habitat of the European spiny lobster Palinurus elephas (Crustacea: Decapoda: Palinuridae) in the western Mediterranean Sea

Abstract: Settlement characteristics, like timing, depth, microhabitat and density of European spiny lobster Palinurus elephas are described for the very first time. Regular SCUBA-diving surveys were conducted from July 1998 to January 2000 on rocky bottoms of three different geologic origins to assess substratum-dependent differences in recruitment density. Settlement of pueruli took place in June-July, a few weeks after sea surface temperature started to rise. The highest density of juveniles was found at 10-15 m depth. Most spiny lobsters settled in limestone rocks, into empty holes of the date mussel Lithophaga lithophaga, which provided daytime refuge. As they grew, individuals were increasingly found in larger holes and crevices of the rock surface. Sizes were estimated from photographs taken at night when the animals were actively foraging. The smallest observed individuals measured 7.5-8 mm carapace length (CL), but they reached 15-18 mm CL at the end of October. The consequences of our results for the management of the spiny lobster populations in the northwestern Mediterranean are summarily discussed.