Poleward Current Research Articles

Surface layer circulation derived from Lagrangian drifters in the Bay of Biscay

Based on Lagrangian drifter trajectories collected from 1992 to 2009, a seasonal climatology of the circulation in the Bay of Biscay has been computed. The buoys deployed during several cruises are fitted with drogues at depth between ~15m and ~80m to follow the current in surface layers. After a pre-processing of the dataset, 281 trajectories corresponding to a cumulated time series about 71years long have been obtained to describe the residual seasonal circulation. Main features have been identified: an general northwesterly circulation over the deep ocean, a cyclonic along-slope circulation with a poleward slope current along the Aquitaine, Armorican and Celtic shelves (3.5 to 7cms−1), a marked winter eastward flow along the North Spanish slope (larger than 5cms−1 in average), and weak currents on the continental shelf from April to September (lower than 2.5cms−1 in summer). Further circulation patterns have been identified and quantified as the westward current in spring and summer along the North Spanish slope and shelf with speeds reaching 13.5cms−1. A cyclonic circulation cell has also been confirmed on the Armorican shelf from October to March. Intense poleward currents (about 10–15cms−1) have been observed from October to March on the shelf.The density of trajectories from 2004 to 2009 also allowed exploring the interannual variability of the circulation during two main half-year periods from October to March and April to September. Through this analysis, we described the variability of the Iberian Poleward Current intrusions in the Bay of Biscay during recent years as well as the interannual fluctuations in the development of a poleward autumn current on the Armorican shelf.

Eddies in the Bay of Biscay: A numerical approximation

The results of numerical modelling in the Bay of Biscay have been used to investigate eddy dynamics. The modelling has been carried out using mean climatological and atmospheric forcing as input for hydrodynamic simulations with the ROMS model (Regional Ocean Modeling System). The most significant regional circulation features have been simulated in a mesoscale model configuration, with 6.6 km horizontal resolution. Eddy characteristics (horizontal and vertical structure, origins and migration) have been analysed. During winter, jet-like extensions of the slope current off northern Spain (mainly in the vicinity of Cape Ortegal) and France (near Cap Breton, Cap Ferret and Audierne canyons) developed into anticyclonic and cyclonic eddies, with an upper core of slope water. Most of the eddies identified in the model area were anticyclonic, with diameters at sea surface of between 60 and 130 km and significant effects below 500 m in depth. After their generation, the eddies moved slowly westward, with a mean migration speed of less than 2 cm s −1; they dissipated later in the year in the deep basin of the Bay of Biscay. The mean annual surface kinetic energy, estimated with the model, was around 34 cm 2 s −2. Both the kinetic energy and vorticity variability in the study area depend upon changes (magnitude and direction) in the wind stress, which have effects on the development of: the slope poleward current; the Galician upwelling; and the eddy generation and migration.

Wind relaxations and poleward flow events in a coastal upwelling system on the central California coast

When upwelling favorable winds weaken or relax in coastal upwelling systems, prevailing equatorward currents often weaken and then reverse to form propagating poleward currents. Here the statistics of wind relaxations and poleward flow events along the south central California coast are derived using meteorological and oceanographic times series during 2000–2006. Over the 7‐year record, 169 wind relaxations were observed (about 1 every 2 weeks) and 127 of these were followed by poleward flow events. Thermistor moorings and current profilers along the 15 m isobath at Alegria, Point Arguello, Point Purisima, and Point Sal recorded the poleward flows. The poleward flows propagate northward at 10–30 km d−1 and appear as sequential temperature increases at the moorings. Wind relaxations occur throughout the year but are most frequent in September and least frequent in April when upwelling winds are strong and persistent. Poleward flows follow wind relaxations frequently during May through November and rarely in December and January. Sea level differences between Santa Monica and Port San Luis, California, decreased as winds relaxed, consistent with forcing by alongshore pressure gradients. Temperature distributions at Point Arguello, Point Purisima, and Point Sal were skewed toward higher values because of the poleward flows. The alongshore distance traveled by the poleward flows increased with duration of the wind relaxations and magnitudes of alongshore temperature and sea level differences prior to the relaxations.

Summer diatom and dinoflagellate blooms in Lisbon Bay from 2002 to 2005: Pre-conditions inferred from wind and satellite data

Sea surface temperature (SST) and chlorophyll ( Chl) maps derived from satellite data, together with wind and phytoplankton cell concentration time-series, are used to characterize the wind forcing and the spatio-temporal SST/ Chl variability for periods preceding the summer (June–September) diatom and dinoflagellate blooms. The blooms (concentrations of cells higher than 200 × 10 3 cells l - 1 ) were identified in the weekly samples collected at the long-term phytoplankton study site in Lisbon Bay, from 2002 to 2005. It is shown that bloom detection at the site occurs either during upwelling events, i.e. when offshore northerly wind speed is greater than 5 m s - 1 for more than 5 days, or up to 4 days after wind relaxation. However, no direct relationship was found between the blooms and either the intensity or duration of the wind pulses. The satellite image sequences show that the Chl patterns in Lisbon Bay are strongly linked to the orientation and branching of the upwelling filament rooted at Cape Roca and the westward progression of the warm-core cyclones shed from the slope off Setúbal Bay. High Chl values are typically maintained throughout the upwelling cycle along the inshore boundary of the Roca filament, covering the whole bay during the intense wind phase. During the relaxation phase, there is a Chl increase concurrent with the contraction of the high Chl area to the northern part of the bay, associated with an inshore poleward current. It is proposed that the earlier setup of the offshore stratification conditions and the shedding of a slope eddy favored the bloom of dinoflagellates in July 2004, while the stable position of the main circulation features in July 2002 favored a diatom bloom. The results add further evidence that, in summer, the ocean circulation in the embayments south Cape Roca is dominated by the upwelling dynamics and plays a key role in the development of phytoplankton blooms off central Portugal.

Nearshore currents on the southern Namaqua shelf of the Benguela upwelling system

Nearshore currents of the southern Namaqua shelf were investigated using data from a mooring situated three and a half kilometres offshore of Lambert's Bay, downstream of the Cape Columbine upwelling cell, on the west coast of South Africa. This area is susceptible to harmful algal blooms (HABs) and wind-forced variations in currents and water column structure are critical in determining the development, transport and dissipation of blooms. Time series of local wind data, and current and temperature profile data are described for three periods, considered to be representative of the latter part of the upwelling season (27 January–22 February), winter conditions (5–29 May) and the early part of the upwelling season (10 November–12 December) in 2005. Differences observed in mean wind strength and direction between data sets are indicative of seasonal changes in synoptic meteorological conditions. These quasi-seasonal variations in wind forcing affect nearshore current flow, leading to mean northward flow in surface waters early in the upwelling season when equatorward, upwelling-favourable winds are persistent. Mean near-surface currents are southward during the latter part of the upwelling season, consistent with more prolonged periods of relaxation from equatorward winds, and under winter conditions when winds were predominantly poleward. Within these seasonal variations in mean near-surface current direction, two scales of current variability were evident within all data sets: strong inertial oscillations were driven by diurnal winds and introduced vertical shear into the water column enhancing mixing across the thermocline, while sub-inertial current variability was driven by north–south wind reversals at periods of 2–5 days. Sub-inertial currents were found to lag wind reversals by approximately 12 h, with a tendency for near-surface currents to flow poleward in the absence of wind forcing. Consistent with similar sites along the Californian and Iberian coasts, the headland at Cape Columbine is considered to influence currents and circulation patterns during periods of relaxation from upwelling-favourable winds, favouring the development of a nearshore poleward current, leading to poleward advection of warm water, the development of stratification, and the creation of potentially favourable conditions for HAB development.

Seasonal Evolution of the Upwelling Process South of Cape Blanco*

Abstract Bursts of upwelling-favorable winds lasting 4–20 days occur year-round south of Cape Blanco, a major headland on the Oregon coast. The ocean’s response to these events was studied using moored current, temperature, and salinity data; satellite SST data; and a few across-shelf sections near the mooring site. The mooring was located at 42°26.49′N, 124°34.47′W, 6 n mi off Gold Beach, Oregon, from May 2000 to October 2003. After the spring transition but before upwelling jet separation, equatorward wind stress produced a steady upwelling response much the same as a long, straight coast. Currents and winds had similar spectral characteristics with a peak near 15 days. After jet separation, upwelling-favorable winds forced a much more variable current consisting of a series of thin equatorward jets that evolved and moved offshore across the mooring, with shorter time scales than the wind stress forcing. During autumn, the equatorward wind stress weakened slightly and a transition period occurred, with the flow often poleward along the bottom. During winter, the water column was unstratified during poleward winds and currents with little variation in SST across the shelf. Winter upwelling restratified the water column from the bottom up by drawing cold, salty water onshore along the bottom, with little or no change in SST. This scenario was modulated by strong intraseasonal and interannual variability in the ocean and atmosphere. A wavelet transform analysis of alongshore wind stress and the first empirical orthogonal mode of the alongshore currents revealed strong energy peaks in the 30–70-day band. These signals were particularly clear in the ocean and were not coherent with the local wind stress, suggesting they were due to Kelvin waves of equatorial origin. The shift toward longer (40–45–60 days) periods from 2000 to 2003 was consistent with decreasing (warming) northern oscillation index, suggesting that the period as well as the energy of the intraseasonal waves may be important in transmitting heat poleward during warm years.

First record of the red cornetfish Fistularia petimba (Syngnathiformes: Fistularidae) in Galician waters: a northernmost occurrence in the eastern Atlantic

The red cornetfish Fistularia petimba Lacépède, 1803 is a large fish typically found along soft bottom coastal areas and usually at depths over 10 m (Fritzsche, 1978). The geographic distribution ranges from tropical waters of the Atlantic to the Indian and Pacific oceans. The distribution in the eastern Atlantic extends from Cape Blanc and the Cape Verde Islands to Angola (Fritzsche, 1990). Two unusual records were noted northward this area in Cádiz, on the Mediterranean (Cárdenas et al., 1997) and along the Azores Islands (Azevedo et al., 2004). On 11 May 2006, the vessel ‘Nueva Revoltosa’ based in Laxe, at the north-west coast of Galicia (NW Spain), caught a specimen of the cornetfish that was later identified as Fistularia petimba. The specimen (Fig. 1) was 1400 mm total length and weighed 1580 g. It was caught at a depth of 7 m with trammel nets, near a local beach called ‘Traba de Laxe’ (Fig. 2). Specimen of Fistularia petimba, MLICN 6155 (1400 mm LT and 1580 g) caught in Galician waters (NE Atlantic) Map showing area and station () where the Fistularia petimba specimen was captured The specimen was preserved in ethanol 70% and stored in the fish collection of the Museum Luis Iglesias de Ciencias Naturais of Santiago de Compostela (Galicia, Spain) with the reference number 6155. This record constitutes a new extended northern limit for the distribution of F. petimba in the eastern Atlantic. According to Fritzsche (1976), the main diagnostic meristic counts and measurements are given in millimetres followed in parentheses by the percentage of standard length: dorsal fin rays: 16; anal fin rays: 16; pectoral fin rays: 15; ventral fin rays: 6; branchiostegal rays: 7; no gillrakers; Total length with filament: 1400; Total length without filament: 1241; fork length: 1212; standard length: 1182; head length 429 (36.3); pre-orbital length: 321 (27.2); post-orbital length: 77 (6.5); horizontal eye diameter: 31 (2.6); interorbital length: 30 (2.5); pre-dorsal length: 976 (82.6); dorsal base length: 45 (3.8); pre-anal length: 985 (83.3); anal base length: 39 (3.3); upper jaw length: 45 (3.8); lower jaw length: 52 (4.4); pectoral length: 48 (4.1); ventral length: 28 (2.4); body depth: 53 (4.5) and body width: 64 (5.4). The genus Fistularia Linnaeus, 1758 contains four species of which only two, Fistularia tabacaria and F. petimba, are distributed in the eastern Atlantic. Fistularia petimba differs from F. tabacaria mainly by the presence of imbricate bony plates prior to the dorsal and anal fins (Fritzsche, 1976), which are clearly visible in this specimen. As well as F. petimba, other species of this genus have been recorded outside of their normal distribution area. This is the case in F. tabaccaria, occasionally caught in warm and low salinity Hudson River waters (Young et al., 1982) and Fistularia commersoni recorded in Mediterranean Sea as a Lessepsian species (Golani, 2000). The number of new alien southern fish species of tropical and subtropical origin has rapidly increased in recent years in Galician waters (Bañón et al., 2002; Bañón, 2004). The reasons for the spread of NW African fishes northward of their normal distribution range has been attributed to ocean warming (Swaby et al., 1996; Quéro et al., 1998) together with oceanographic events such as the presence of poleward currents (Koutsikopoulos et al., 1998) and fluctuations in frontal systems (Castro-Hernández, 2001). We wish to thank the skipper and fishermen of ‘Nueva Revoltosa’ and the ‘Cofradía of Laxe’ staff for providing the specimen. We also wish to thank the personnel of the Museum Luis Iglesias de Ciencias Naturais of Santiago de Compostela for the use of their facilities in the specimen analysis.

A process-oriented numerical study of currents, eddies and meanders in the Leeuwin Current System

Abstract While observations provide insight for the basic nature of features in the Leeuwin Current System (LCS), process-oriented studies are useful for systematically investigating the characteristics and dynamical forcing mechanisms for the currents and eddies in the LCS. This process-oriented numerical study investigates the roles of wind forcing, thermohaline gradients and bottom topography on currents and eddy generation in the LCS with a terrain-following primitive equation model, in this case the Princeton Ocean Model (POM), on a beta-plane off the western and southwestern coast of Australia. Results show that the LCS is an anomalous Eastern Boundary Current that generates a surface poleward current predominantly over the shelf break, an equatorward surface current with upwelling next to the coast in localized regions, an equatorward undercurrent, and highly energetic mesoscale features such as meanders and eddies. Thermohaline gradient effects are shown to be the primary mechanism in the generation of a poleward (equatorward) current (undercurrent), eddies and meanders in the LCS. Inshore of the poleward surface flow, next to the coast, wind forcing plays an important role in generating an equatorward coastal current and upwelling. The major role of the wind is to slow the poleward surface flow, enhance eddy spin up and create localized upwelling regions, such as in the north near Shark Bay and just north of Cape Leeuwin. Bottom topography is shown to be an important mechanism for intensifying and trapping currents near the coast, weakening subsurface currents and intensifying eddies off capes. The surface poleward current is predominantly steered by the shelf break, frequently leaving the coast as it follows the 200-m depth contour southward to Cape Leeuwin and eastward into the Great Australian Bight. Overall, the results of this process-oriented study compare well with available observations in the LCS.

A spring poleward current and its influence on microplankton assemblages and harmful dinoflagellates on the western Iberian coast

Along-shore currents can propagate harmful algal blooms (HABs) over long distances in many coastal areas of the ocean. Harmful dinoflagellate blooms on the west coast of Iberia frequently occur when the Iberian poleward current (IPC) establishes on the continental slope. This has led to the suggestion that HABs could be transported northward by the IPC. To examine this possibility, the microplankton composition along the west coast of Iberia was studied in May 1993 coinciding with the presence of the IPC. The microplankton of the IPC was almost exclusively composed of small flagellates, with the notable absence of the harmful species usually associated with coastal waters. The primary influence of the IPC was to confine coastal microplankton populations to the shelf, where a downwelling convergence prevented their export from the coastal environment. Microplankton assemblages on the shelf revealed a north–south gradient related to different stages of succession. Earlier stages of succession in which diatoms were prominent were found on the northern shelf, whereas dinoflagellates were more abundant in the south. The toxic species Gymnodinium catenatum, which was only present in the southern shelf, did not show a northward transport associated with the IPC. It is suggested that the northward spreading of HABs along the west coast of Iberia must be related to the interaction between the IPC, which accumulates coastal populations on the shelf, and the latitudinal progress of microplankton succession that determines species composition. Thus, during the course of the season, HABs are likely to be observed in the south prior to their development in the north.

Oceanographic conditions in North and Northwest Iberia and their influence on the Prestige oil spill

Oceanographic conditions at the time of the Prestige oil spill (November 2002) and following months are analyzed based on a set of hydrographic cruises. The ship sank off one of the flanks of the Galician Bank, an offshore seamount, and a major oil spill drifted to the N and NW Iberian coast mainly driven by dominant winds. Coastal circulation was characterized by freshwater plumes and the poleward slope current, and could have affected the fate of the oil spill and influenced stranding places. Seasonal evolution of oceanographic conditions in this particular year is compared with the long-term average and reveals specific features that need to be taken into account in studies of the impact of the oil spill on populations. Spring conditions commenced earlier than other years in the Southern Bay of Biscay, contrastingly in western Iberia. The lack of subsurface intrusion of subtropical waters suggests a low intense penetration of the poleward current in Spanish Biscay slopes. In western Iberia, the slope poleward current observed in late autumn weakens and is exported off slope during upwelling pulses in the spring, with no strong intrusion of the poleward current on the slope at the time of the spring bloom. A description of current velocities near the wreck on the Galician Bank is obtained after the analysis of a mooring line.

Changes in the mesozooplankton community associated with the hydrography off the northwestern Iberian Peninsula

Abstract Changes in the mesozooplankton community were investigated along the northwestern coast of the Iberian Peninsula during OMEX II cruises in August 1998, under upwelling conditions, and in October 1999, when there was a saline, warm poleward current flowing along the slope. Six stations, located at both the shelf break and in oceanic waters, were sampled for mesozooplankton by day and by night. Samples were grouped in accordance with their species abundance levels, and the resulting groups were compared with salinity, integrated chlorophyll a, and sea surface and average temperatures. The analysis discriminated stations from different cruises into different groups, and detected the influence of hydrographic structures. The abundance of species linked to high concentrations of Chl a and low temperature increased during the upwelling session, whereas species related to warmer water were more abundant during the 1999 cruise. Further, the analysis revealed differences in the mesozooplankton composition between coastal and oceanic sites, which were influenced to different degrees by the upwelling and the poleward current.

Influence of the oceanographic conditions during spring 2003 on the transport of the Prestige tanker fuel oil to the Galician coast

Hydrographic data collected during the cruise HIDROPRESTIGE were combined with meteorological and dynamic data provided by remote sensors and drifting/moored buoys, to describe the surface circulation of the Northern Iberian basin in March–April 2003. Sea surface winds transported the floating Prestige oil slicks from the sinking area to the continental slope off the Rías Baixas in 1/2 month: the surface current intensity was 2% of the wind intensity and it was rotated clockwise 5° from the wind direction. Mesoscale cyclonic and anticyclonic structures west of 10°W increased the residence time of oil patches in the Northern Iberian basin, as compared with the expected southwards flow of the Iberian current (IC). On the other hand, the Iberian poleward current (IPC) formed a marked surface front with coastal waters, preventing the entry of fuel oil into the rías. PAHs in the surface layer during the cruise were <0.5 μg L −1, except in the Galicia bank (∼1 μg L −1; where the Prestige tanker was still leaking) and the vicinity of Cape Fisterra (∼1.5 μg L −1; where the convergence front between the IPC and coastal waters vanished).

On the spatial structure of coastal circulation off Newport, Oregon, during spring and summer 2001 in a region of varying shelf width

A time series of hourly surface current maps in the shelf waters off Newport, Oregon, was made during April–September 2001 using five SeaSonde HF current mappers, during Coastal Advances in Shelf Transport (COAST). The surface currents responded rapidly to the changing winds, in repeated patterns that were strongly affected by bottom topography. An equatorward current jet repeatedly formed in response to upwelling winds, its strength, but not its trajectory, covarying with the meridional wind stress. Near Cape Foulweather (44.8°N), where the shelf begins to widen, the jet rotates, weakens, and trends offshore to the south first along, then across, the isobaths. Below Cape Foulweather, inshore of the jet, a lee region of generally weak currents was commonly observed. The equatorward jet core was most commonly observed near the 80‐m isobath between 45.0°N and 44.4°N, but transited offshore between spring and summer over Heceta Bank. At Newport (44.6°N), it was rarely observed less than 8 km from the coast. A second, inshore, equatorward jet, previously unknown, was observed repeatedly south of Waldport (44.4°N). Sustained downwelling wind episodes produced poleward currents, though less responsively north of Cape Foulweather. Strongest poleward flow was generally trapped near the coast. Surface currents were correlated with the northward wind, except regionally far from shore over Heceta Bank, responding within half a day. Response to the wind varies spatially, being intensified in the narrow shelf (northern) region. The equatorward jet persists through periods of zero wind forcing. Coastal sea level covaries with the meridional wind and the primary mode of ocean current response.

HF radar observations of surface circulation off Bodega Bay (northern California, USA)

Two high‐frequency radar stations (CODAR) were installed along the northern California coast in May 2001. Comparisons of radar data with acoustic Doppler current profiler (ADCP) current data and near‐surface drifter tracks indicate considerable agreement, with minimum RMS differences of order 0.05–0.15 m/s and average drifter‐HF‐radar track separation rates of 5 ± 3 km/d. Radar data resolve the three main sources of intraseasonal current variability in the area: (1) upwelling/relaxation dynamics, (2) tidal and diurnal forcing, and (3) inertial currents. Subtidal fluctuations are the largest component of variability, accounting for 45–75% of the variance. Wind‐driven dynamics are the dominant source of this subtidal variability (67% of subtidal variability). Both upwelling and relaxation periods exhibit consistent patterns of surface velocity, with nearshore currents being slower and more poleward than offshore currents, which are strongly equatorward. Analysis of tidal and inertial variability indicate that current fluctuations are polarized toward clockwise rotation and are generally weaker and more linearly polarized near the coast. M2 tidal current ellipses switch direction of rotation at the shelf break, suggesting the presence of internal tidal waves. Currents in all frequency bands are deflected and accelerated around Pt. Reyes, and there are indications of increased current variability and changes in flow direction near Cordell Bank. The presence of considerable cross‐shore and alongshore gradients in the strength and direction of surface flow patterns, and in particular weak poleward currents over the inner shelf, could have important consequences for plankton retention in the area.

Sporadic upwelling on a downwelling coast: Phytoplankton responses to spatially variable nutrient dynamics off the Gascoyne region of Western Australia

The Gascoyne continental shelf extends from North West Cape (21.3°S) to Shark Bay (26.5°S), Western Australia, and is dominated by the Leeuwin Current (LC), an anomalous oligotrophic eastern boundary current that transports tropical water poleward and generates large-scale downwelling. In summer, shelf dynamics are influenced by wind-generated countercurrents (the Ningaloo and Capes Currents) that flow equatorward and are associated with localized upwelling. Here, we examined phytoplankton responses (biomass levels and distribution, primary production rates, species composition) to small-scale upwelling in this ecosystem which is generally dominated by a poleward current, using physical, chemical and biological oceanographic field data collected during a two-week research cruise through the Gascoyne region in the early austral summer of 2000. We found LC and offshore waters to be associated with low phytoplankton biomass (21.4±6.9 s.d. mg chl a m −2) and low primary production (110–530 mg C m −2 d −1); surface (<50 m) waters were nitrate-depleted (generally<0.1 μM), with a strong nutricline present at the base of the mixed layer. Upwelling associated with the Ningaloo Current (NC) sourced water from this nutricline, and in conjunction with mixing generated by seaward offshoots, resulted in nitrate levels of up to 2–6 μM within the euphotic zone. Biomass in these NC waters (35.9±11.6 mg chl a m −2) was significantly higher than in LC/offshore regions, with primary production in the range of 840–1310 mg C m −2 d −1. Capes Current (CC) water was also highly productive (990 mg C m −2 d −1), and with low silicate levels and a high proportion of centric diatoms, was typical of an aging upwelled water mass. Thus, the dominance of the oligotrophic LC along the Gascoyne region can be offset by these equatorward countercurrents, although we hypothesize that the biological impact of any upwelling on the inner shelf would be a function of: (a) the depth of the LC's nutrient-depleted mixed layer, (b) the strength and duration of upwelling-favorable winds (i.e. the intensity of upwelling), and (c) geographical location, primarily with respect to the width of the continental shelf and resultant proximity of upwelling flows to deep nutrient pools.

Mesozooplankton size-fractionated metabolism and feeding off NW Spain during autumn: effects of a poleward current

Mesozooplankton size-fractionated metabolism and feeding off NW Spain during autumn: effects of a poleward current

Mesozooplankton size-fractionated metabolism and feeding off NW Spain during autumn: effects of a poleward current

Abstract Two consecutive cruises were carried out off NW Spain in October 1999 in an attempt to obtain information on the poleward current that affects the hydrodynamics of this area during autumn and winter. The stations sampled were divided into three zones: coastal (C), between the saline intrusion and the coast; poleward current (P), where salinity at 100-m depth was &gt;35.85; and oceanic (O), outside the poleward current. Phytoplankton biomass, integrated through the photic zone, was similar for the three zones. Conversely, mesozooplankton biomass showed high spatial variation, with the lowest values associated with the high-salinity current. There was a decreasing trend in the relative importance of the biomass of the largest size class (&gt;1000 μm) from coastal to oceanic zones. The lowest percentages of both Chl a and primary production ingested daily by copepods were measured at the poleward current stations. Specific metabolic rates were not significantly different between zones. The total amount of carbon respired and the NH4 and PO4 released were highest in coastal areas and lowest within the poleward current, in agreement with the pattern observed for mesozooplankton biomass. Diel vertical migrations were more marked in the coastal areas, decreasing gradually towards the oceanic stations. The role of mesozooplankton in promoting the flux of carbon, nitrogen, and phosphorus followed the same spatial distribution. Overall, the contribution of mesozooplankton to biogeochemical cycles seems to be minor in the poleward current compared to the other two zones, possibly as a consequence of the low biomass associated with this current.

Changes in the pattern of water masses resulting from a poleward slope current in the Cantabrian Sea (Bay of Biscay)

From September to December 1995, three hydrographic surveys were carried out in the eastern Cantabrian Sea (Bay of Biscay). Changes in the water masses pattern were examined to study the variability and main energetic features in the area. At the beginning of December, an intense Poleward Current (PC), which had come from Portuguese slopes, entered the eastern Cantabrian Sea. This current was the most energetic event in this area in winter. The PC waters increased temperature by about 2 °C (subsurface layers) and salinity by 0.2 (surface layers) in the pattern of water masses in the eastern Cantabrian Sea in winter. The core current was approximately 10 km width and 120 m depth and the water transport, estimated from geostrophic current profiles, was of about 1.3 Sv. A well-defined wavelike front with two significant ridges in the western and eastern sampling area, was observed. The variability and meandering flow of the PC were driven by dominantly baroclinic instabilities, which are due to strong vertical velocity shear. In this synoptic-scale system, the potential vorticity advection, the differential vorticity advection, and the geopotential tendency have shown to be the cause of the ageostrophic motion and the main baroclinic disturbances. One important consequence of the entrance of the PC in the eastern Cantabrian Sea was the profound effect on the pattern of nutrients. The current-induced stratification pattern drives the distribution of nutrients in the different layers and the instabilities and meandering pattern of the PC was an important mechanism of fertilisation offshore.

Hydrographic characterization of a winter-upwelling event in the Ria of Pontevedra (NW Spain)

An unusual winter-upwelling event was observed in the Ria of Pontevedra during a cruise carried out on 27 January 1998. Upwelled water masses inside the estuary are observed to depend on the season due to the annual variation of water circulation at the adjacent shelf. The winter upwelled water corresponds to the water mass transported by the poleward current, which is saltier and warmer, but less dense than the Eastern North Atlantic Central Water observed in spring and summer.

Phytoplankton composition, photosynthesis and primary production during different hydrographic conditions at the Northwest Iberian upwelling system

The major hydrographic conditions along the NW Iberian Peninsula are upwelling and poleward current. Changes in prevailing winds create transitional hydrographic states such as down- welling, stratification and upwelling-downwelling relaxation. Four cruises were undertaken between 44.25 and 41.14° N during spring, summer and winter, when there was a clear demarcation of these hydrographic conditions. We studied the effects of hydrography on phytoplankton species composi- tion, photophysiology, light limitation and primary production (PP) derived from broad-band and spectral models. PP was significantly higher during upwelling (F3, 49 = 99.41, p < 0.001; mean 2595 ± 213 mgC m -2 d -1 ) compared with all other hydrographic conditions. The poleward current had the lowest PP (214 ± 22 mgC m -2 d -1 ). PP calculated using the spectral model (∑ PPPUR) was greater than PP from the broad-band model (∑ PPPAR), especially when southerly winds prevailed, and during stratified conditions when the chlorophyll maximum was deeper, irradiance levels were lower and most of the phytoplankton biomass was below the light limitation depth. During northerly winds, chl a was the single most important variable influencing the calculation of PP and the difference between ∑ PPPAR and ∑ PPPUR was small, since most of the phytoplankton biomass occurred above the light limitation depth, in saturating irradiance. Variations in hydrography were intrinsically linked to changes in phytoplankton species composition, photophysiology and PP. Over the whole region 89% of the variance in maximum photosynthetic rate Pm B was explained by daily mean spectral irradiance, biomass of flagellates and cyanobacteria and 75% of the variance in PP was explained by the along- shore Ekman transport component (Qy), cyanobacteria and diatom biomass. Qy alone explained 52% of the variance in PP over the shelf, which was also coupled with variations in Pm B and the biomass of diatoms and flagellates. At oceanic locations there was no relation between Qy and PP (r 2 = 7%), and PP co-varied with Pm B and the biomass of cyanobacteria.