The sources and circulation of waters along the NW Iberian Margin exhibit pronounced seasonal variation because of their coupling with large-scale climatology of the north-eastern Atlantic:

During summer months, the Azores high-pressure cell in the central north Atlantic drives trade winds with southerly components along the coast of western Iberia.  This induces an equatorward slope current and an offshore upwelling of nutrient-rich Eastern North Atlantic Central Water.   By late June or July, the upwelling intensity has increased to the point that major headlands and flow instabilities in the slope current may generate several eddies and fully developed filaments which jet out far into the open Atlantic ocean.  Filaments tend to recur every year at the same sites.  Typically, they first develop in June and occur principally from late July to extend 200-250 km offshore in September, subsequently diminishing through October, which ends the upwelling season (some may remain until December). The most northern filament develops somewhere in the region off Finisterre. The next filament extends far offshore northern Galicia. There are about four others farther south, some positioned by capes but separated by 200 km probably related to instability of the southward upwelling-related flow. Filaments have potential importance as exporters of coastal water and its contents (including large quantities of organic matter produced on the shelf) to the ocean, carrying a seasonally intense cross-slope flux.

The stable forcing of the upwelling system in the spring and summer may be stopped by low-pressure passages, which alter the wind pattern and may cause periods with relaxation or even downwelling.

During the non-upwelling period in winter, current directions are reversed and the river fluxes of nutrients and terrestrial matter are at their maximum. Therefore, land sources via freshwater from river flushing events and plumes onto the coastal zone, have direct effects across the Iberian shelf, especially in the north.

In winter, a general northward water flow extends from the surface down to the level of intermediate water (about 1000 m depth), along the north African and Portuguese coasts and into the Bay of Biscay.

The intermediate waters are characterised by the poleward flow of poorly oxygenated, low-nutrient, warm, saline, Mediterranean Outflow Water (MOW) on the Iberian slope. In deeper waters, there are overall trends of decreasing temperature and salinity from south to north. At depths below 2500 m, the Deep Water with high dissolved silica content and relatively low salinity is transported northwards on the deep eastern boundary.

At small scales turbulence which is commonly described in terms of horizontal and vertical diffusion, irreversibly mixes water and its constituents. The vertical turbulent diffusion, contributes to cross-shelf tidal and wind-driven mixing and sediment resuspension, drives vertical transfer of heat, salt, dissolved chemicals and some small planktonic organisms, and provides drag to the slope current. Despite the difficulty to accurately parameterise mixing in stratified and shelf edge waters because of the diversity of complex and non-linear processes, direct evaluations of the vertical diffusion are available, as required to calibrate the numerical circulation and exchange models in OMEX II-II.

The physical and chemical signatures of water masses contributing to shelf and coastal regions such as the Iberian margin exchanges of energy and material can be defined from horizontal transects and vertical profiles of conservative constituents (mainly salinity and temperature). It can also be dynamically constrained by isotopic transient tracers of gases (¹⁸O₂, ¹³CO₂, ¹⁴CO₂, etc.), and combined with flow fields obtained from current meter moorings and shipborne acoustic current profilers (ADCP).

In the framework of Global Change investigations, the distribution of carbon dioxide pressure (pCO₂) and the net budget of CO₂ exchanges in surface seawater are key issues for identifying and quantifying oceanic sources and sinks for atmospheric CO₂. The coastal water such as in the Iberian margin is most often neglected in global budget calculations, in spite of its intense biological activity or the coastal processes (e.g., upwelling, river input) that can induce high CO₂ fluxes at the air-sea interface.

The distribution of carbon in the ocean is dominated by dissolved inorganic species (carbonates, bicarbonates) which are in dynamic equilibrium accordingly to seawater temperature and pH (acid/base characteristics), formation and dissolution of calcite (CaCO₃), phytoplanktonic CO₂ assimilation to form organic carbon (i.e., photosynthesis), and heterotrophic regeneration back to CO₂ (i.e., respiration and mineralisation). Intermediate and deep ocean waters where heterotrophy dominates are normally super-saturated with CO₂ and rich in oxidised nutrients (mainly, nitrate, phosphates and silicates). The upwelled deep water that reaches the sea surface therefore ventilates CO₂ to the atmosphere, but also supplies new nutrients to the phytoplankton, stimulating photosynthesis which will tend to capture atmospheric CO₂ into marine planktonic biomass. Similarly, nutrient-enriched freshwater inputs also stimulate the formation of organic matter and the CO₂ sink in the coastal water of the Iberian margin.

Thus to precisely budget the flow of carbon in upwelling system, it is essential to simultaneously determine the distribution and speciation of dissolved and particulate, inorganic and organic carbon, together with the CO₂ exchange flux, phytoplankton production and heterotrophic mineralisation of organic matter.

The most well-established method for estimating the CO₂ exchange flux across the air-sea surface is the transfer velocity method, where the flux depends on the difference in partial pressures (DpCO₂) on each side of the air-water interface. It also depends on a transfer velocity, which is a function of wind speed, temperature, sea-state, film thickness, humidity, etc.

Because anthropogenic CO₂ produced by fossil fuel burning is isotopically lighter than natural oceanic CO₂, it has recently become possible to precisely track the penetration, residence times and fate of anthropogenic CO₂ in the ocean by following the isotopic ¹³CO₂ signal.

NUTRIENTS

The nutrients are various essential mineral elements that are required in small quantities in the biological processes, particularly for the photosynthesis. Especially, in most oceans, nitrate (NO₃) is considered to be the growth-limiting nutrient for phytoplankton. However, for complete nitrogen speciation and mass balance, it is important to measure all the forms of assimilable nitrogen, i.e., nitrate (NO₃), nitrites (NO₂), ammonium (NH₄), urea and dissolved organic nitrogen (DON) which peaks in surface waters during summer.

Any new source of NO₃ from rivers, lateral flux, upwelling, etc., will induce a net increase in phyto-biomass (i.e., new production). On the other hand, phytoplankton metabolism in NO₃-depleted waters is maintained by the assimilation of reduced nitrogen forms (i.e., less oxidised) such as NO₂, NH₄, urea and dissolved organic nitrogen that are present in surface waters, but with no net increase in planktonic biomass (i.e., regenerated production).

Thus NO₃ levels are critical in controlling the balance between new and regenerated production. Nitrate concentrations reported for the Vigo River in winter are around 10 - 25 M NO₃ compared with open sea (salinity above 35 psu) concentrations of 2-4 M in the winter and decreasing below 0.5 M in the summer months. Offshore winter NO₃ levels in the Bay of Biscay are about 7-8 M, and decrease progressively to the south to below 1 M NO₃ off Oporto.

The Iberian margin is a biogeochemical reactor with contrasted ecological situations. It ranges from opportunistic communities supporting new production due to nitrate inputs by upwelling events, to stratified impoverished system dominated by regenerated production and the microbial loop, and to slope communities under the influence of the low-oxygenated Mediterranean water.

Furthermore, the activity of grazers, the distributions of fish larvae and benthic invertebrates, and ultimately the whole structure of the food web in that area, depend on the water masses stability and/or dynamics. The superimposition of short-term events such as upwelling (from few days to few weeks) on the seasonal structure, results in significant spatial heterogeneity (e.g., filaments, patches) in the biological structure within the plankton communities of the Iberian margin.

Communities of phytoplankton (small organisms that photosynthesise organic matter using CO₂, nutrients and light) are constrained and structured by the interplay of main limiting factors, i.e., light and nutrients, within meso-scale features (e.g., filaments, eddies, fronts) via physical trapping and dispersal processes. Main phytoplanktonic groups include diatoms, dinoflagellates, cyanobacteria and prochlorophytes.

By monitoring chlorophyll and carotenoid biomarkers in the water column, in sediment traps, or using in situ filtration pumps, it is possible to assess which phytoplankton groups contribute to production and export flux. It also permits the study of the biogenic material fate, via such routes as zooplanktonic grazing and bacterial lysis.

Measurements of uptake and regeneration for various nitrogen forms, along with investigation of total primary production and the determination of the "f-ratio" (ratio of new production to sum of new and regenerated productions) will give valuable information about the factors which control the production of different size fractions of phytoplankton.

Large celled diatom-dominated communities mainly support new production during spring and upwelling events, i.e., when upwelled deeper waters supply phytoplankton with new nutrients (namely, nitrates). With strong upwelling, the shelf plankton populations will be advected to the open ocean in the filament structures. The proportion of this NO₃-fuelled new production that will be rapidly deposited offshore versus being grazed by zooplankton (see below) for transmission up the food web is of strategic importance to the future fisheries potential at the Iberian margin. Diatoms are also found in near-shore waters that are influenced by export of nutrient rich water from rivers.

Diatoms are siliceous organisms that produce sticky transparent exopolymer particles (TEP). The resulting cell aggregates may sink rapidly out of the photic layer and regulate diatom population dynamics. It was hypothesised that the cause of diatom aggregates formation was a deterioration of living conditions such as specific nutrient limitation, especially for dissolved silicates since diatom cells usually aggregate, sink and leave the photic zone before phosphate and nitrate become depleted. From the perspective of the phytoplankton cells, it may be a prerequisite for successful upwelling species to form aggregates as soon as nutrients are depleted. They can thus sink into the source waters in deeper layers, where cells are in a position to provide a seeding population waiting for this water to reach again the illuminated surface waters.

On the other hand, when the stratification is established (i.e., when the surface water is markedly warmer and lighter than the underlying water masses), a subsurface chlorophyll maximum, attributed to small diatoms, dinoflagellates, small flagellates and ciliates, is developed. The small size of these cells means they will not sink so readily and tend to stay in the nitrate/silicate-impoverished surface waters. Dinoflagellates, which do not require silicates, support "regenerated" production during periods of relaxation ( i.e., when the primary production is supported by local "regenerated" nutrients e.g. reduced nitrogen forms), and also when intrusions into the shelf by oceanic populations supply regenerated nutrients.

The smaller phytoplankton species can be responsible for more than one half of the annual production in many shelf areas, i.e., the regenerated production can exceed the new production over an annual cycle. Under such conditions, as phytoplankters remain in surface waters, the food web includes microzooplanktonic grazers and induces limited vertical export of small particles. Therefore, physical disaggregation, chemical degradation and microbial remineralisation mostly occur within the surface water masses.

Zooplankton are some animals living in open waters that are mainly grazing phytoplankton and producing particles such as faecal pellets and detritus. When the generation cycle of a specific zooplanktonic population is short, its response to increased food (i.e., mainly phytoplankton) availability during periods of upwelling is potentially rapid in terms of vertical particle fluxes. However, according to their size and ecology, two different pools must be considered at the Iberian margin:

Microzooplankton are tiny (below 200 m in size) organisms that are common in marine surface waters and are known to dominate grazing fluxes in the pelagic zone of many oceanic areas. They may limit the vertical export of particulate organic matter (POM) from the surface waters into deep ocean, by producing small faecal pellets, which are more likely to be retained in the surface layer. Moreover, due to their small size, the microzooplankton are unable to vertically migrate across the stratification "barrier" between surface and deeper water masses. Under such conditions, phytogenic carbon could not fuel ocean margin sediments via microzooplankton grazing.

Mesozooplankton (about 200-2,000 m, mainly copepods) produce large faecal pellets with fast sedimentation rates (~ 100 m/day). Mesozooplankton faecal material can, in some cases, dominate export from the upper ocean during rapid diatom blooms, while copepod abundance is minimal in winter. Moreover, mesozooplankton vertical migration is an effective dispersant for grazing by-products in the deep waters. Under such conditions, phytogenic carbon could fuel ocean margin sediments by mesozooplankton grazing.

The copepod egg production provides an easily accessible short-term indicator of the carbon transfer through the various trophic levels in the pelagic food web during upwelling and relaxation periods outside the shelf of NW Spain.

At the Iberian margin, the zooplankton ecology is especially studied using the Continuous Plankton Recorder (CPR) that provides extensive information on the abundance and distribution of zooplankton species. CPR surveys (1958-present) provide historical data about zooplankton species and allow determining inter-annual variability in the Iberian margin area. It also reveals meso-scale correlation between zooplankton and ocean structures, and provides an index of oceanic fertility at the Iberian margin and in the context of historical and climate trends.

The recognition of the importance of pico- (protozoa, viruses) and nano-plankton (bacteria) in the sea led to a reformation of the classic concept of a linear food chain based on phytoplankton, zooplankton and fishes. A more dynamic and complex structure must be investigated that includes a microbial food web, with dissolved organic matter cycling via bacteria and protozoa. Hydrodynamical singularities, such as fronts, eddies or upwelling events in the Iberian margin area, play a major role in favouring export production over in situ regeneration and, as a result, the prevalence of short food webs over the long ones which include the microbial loop. Whereas during relaxation periods, the microbial loop plays an essential role in providing regenerated nutrients to phytoplankton. Since the overall efficiency of an ecosystem decreases with increasing number of trophic steps, the microbial loop constitutes a rather inefficient system of energy transfer. However, it is not yet clearly stated whether bacteria are a net carbon sink or an active biogeochemical link within the pelagic food web. Indeed, bacteria are a major source of regenerated nitrogen in highly productive waters where their biomass often forms a significant proportion of the total plankton.

The particulate organic carbon (POC) that is operationally defined by its recovery on glass fibre filters with about 0.7 m porosity, is an essential carrier in the oceanic CO₂ sink processes. It is mainly composed of detrital (phytodetritus, humics, terrestrial organics) and living (bacteria, viruses, protozoa, phytoplankton) organic carbon. In the Iberian coastal and upwelling systems, the major inputs of terrestrial and river-derived material can be quantitatively distinguished from marine organic matter by tracking the molecular-specific (e.g., phytoplankton pigments, humics, etc.) and isotopic signatures of POC.

Over 50 chlorophylls and carotenoids biomarkers of phytoplankton can be accurately determined using high performance liquid chromatography (HPLC) separation. But the chemo-taxonomy of phytoplankton carbon and phytodetritus can also be measured from carotenoids and chlorophyll degradation products that are biomarkers of various phytoplanktonic groups.

In addition, in complex, multi-source environments like the upwelling-shelf-river system of the Iberian margin, analyses of lithogenic (aluminium) and bio-mineral (calcite, silicates) components of particles provide information about the relative terrestrial versus marine contributions to particles.

The largest pool of organic carbon in the sea is the dissolved organic matter, with a dissolved organic carbon (DOC) concentration ranging from 50 M C (deep water) to 120 M C (surface water). Moreover riverine DOC (100-250 M C) is conservatively (i.e., by simple dilution) delivered to the coastal zone. DOM is composed of a large number of small-to-high molecular weight compounds that originate directly from planktonic exudation and metabolism or from sloppy feeding and particle degradation. Using reliable and accurate high temperature catalytic oxidation techniques (HTCO), it is now possible to track the small changes (less than 2%) in DOC profiles. Pronounced DOC productions in surface water are observed following upwelling and during the relaxation phase when phytoplankton blooms become senescent. DOM also fuels bacterial production and mineralisation to CO₂ in the deep sea. The DOM fate in the oceans generally depends on its advective exchange across ocean margins during upwelling events and on its availability to bacteria and phytoplankton assimilation in regenerated-production conditions related to relaxation/downwelling periods.

In a recent evaluation of marine primary productivity, it was estimated that the ocean margins, covering 11% of the ocean surface, account for 29% of the global primary productivity and for over 90% of the global organic matter sedimentation.

However, in the Iberian margin area, the net export of organic carbon from river discharge of terrestrial material to the shelf and margin, the vertical export of recently and locally photosynthesised material in the shelf and the lateral export of resuspended fine sediments to the continental slope, and the ocean bottom must still be determined. Given the trophically contrasted situations in the Iberian margin, major changes are expected in the patterns of aggregation/dispersion of particles and in the timing, quality and quantity of vertical export to be recorded, for instance, in the sediment traps.

Dominated by the summer shelf upwelling events and by sporadic filament exports, diatom blooms and terrestrial inputs to the narrow shelf, the Iberian Margin is a good potential site of "Carbon Depocentre", i.e., as a net sink for atmospheric CO₂. Filaments also provide a means by which upwelled nutrient-enriched waters, that support intense local biological productivity, could induce sedimentation over an abyssal site, bypassing the shelf and slope sediments. In addition, the seasonal switch to a downwelling northward slope currents and enhanced river inputs in winter will alter the extent and location of vertical export along the Iberian margin.

Various physical and ecological constraints such as nutrient availability, assimilation efficiency, succession or co-existence of various phytoplankters and associated grazers with different life cycles and feeding strategies, determine in the Iberian margin the potential particle sedimentation.

The vertical flux generally decreases logarithmically with depth due to continuous particle dissolution or bacterial degradation and through direct feeding in the mid-water column. Indeed, the major fisheries are often observed where particle production in upwelling events and filaments moving offshore, allow large fish schools.

Ultimately, the fate of material settling from whatever source to the sea floor is largely determined by the coupling of pelagic and benthic systems. Particularly, the efficiency of benthic utilisation of organic matter, and the possible resuspension and lateral transport in bottom (BNL) and intermediate (INL) nepheloid layers across the shelf edge and margin to the deep sea are to be considered. Indeed, resuspension processes occur at continental margins, where internal waves, eddies reaching the seabed and slope currents with potential high energy for bottom friction, can erode particles from shallow shelf and slope sediments. The shelf edge is also susceptible to sediment resuspension and transport under the influence of long period forerunner swells from major North Atlantic storms.

Field studies on benthic-pelagic coupling indicate that vertical sedimentation as determined by sediment-trap analyses is not sufficient to balance material and carbon budgets and in fact, lateral advection of organic matter is to be invoked. A combination of long-term moored sediment traps, current meters, nephelometers and in situ pumps are to be deployed for tracking the different sources of biogenic and lithogenic material along with water column measurements of planktonic productivity and biomass, current fields and source-specific biogeochemical markers. Moreover, independent geo-chronological measurements on mid-water particles allow the calculation of particle residence times.

The quality and the quantity of the organic matter input, microbial remineralisation, assimilation by the benthic community, burial and diagenesis processes at the sediment-water interface and in the surface sediments are highly variable. Sediment transport, accumulation or resuspension and particle aggregation, scavenging or in disaggregation are the main processes and fluxes in the benthic boundary layer (BBL) that are involved in such variations.

The amount, origin, composition, freshness and seasonal variability of sedimentary organic matter reaching by sedimentation and resuspension the deep-sea floor determine strongly the nutritional cycle of the benthic community.

The elevated turbulence levels in the BBL close to the seafloor stimulates microbial activity which may result in the rapid transfer of dissolved organic carbon (DOC) to bio-available particulate organic carbon (POC).

Adaptation of the benthic community to seasonal pulses of fresh detritus reveals different types of metabolism corresponding to a "food input" and a "resting" mode.

Understanding the benthic degradation, recycling and diagenetic processes at the sediment-water interface and during burial is also essential for the interpretation of global environmental change, especially the change in net CO₂ flux at a typical western European margin and of paleo-oceanographic sequences in the sedimentary record. Previous studies on early diagenesis have established the role of oxidants such as molecular oxygen in the oxic zone, manganese-oxides and nitrate in the hypoxic zone and iron and sulphide in the anoxic zone of the sediment. Modelling of pore-water oxidant or by-product profiles allows the estimation of carbon mineralisation rates and diagenetic processes and fluxes across the sediment-water interface.

The sediment record may provide evidence on depositional fluxes, their fluctuations through past climates and hence local sensitivity to global climate changes. With particular regard to upwelling areas, the mean wind strength in mid-latitudes appears stronger during glacial than interglacial periods and leads to stronger upwelling, enhanced productivity and delivery of organic matter to the deep-sea bed.

Satellite remote sensing provides large-scale, synoptic and high-resolution, high-frequency, physical and biogeochemical observations not feasible from research vessels.

In the north-west Iberian context, Advanced Very High Resolution Radiometer (AVHRR; operated 1979-present) images of Sea Surface Temperature (SST) exhibit many physical processes that modify the surface water masses distribution (upwelling, filaments, fronts, meso-scale eddies, slope currents). Images of "ocean colour" derived from Coastal Zone Colour Scanner (CZCS; operated 1978-1986) and Sea-viewing Wide Field of view Sensor (SeaWiFS; operated 1997-present) reveal variations in phytoplankton abundance and some physical processes via their influence on nutrient input or retention of phytoplankton cells in the photic layer. The latter sensor provides improved estimates of phytoplankton pigment concentration and coloured dissolved organic matter. Near-real-time transmission to research cruises helps to improve the strategy of ship-borne samplings and deployments. The Iberian margin region not only offers a high frequency of cloud-free satellite observations, but also exhibits a wide range in SST and chlorophyll concentrations, in an environment with meso-scale patterns of upwelling, filaments, eddies and river-generated plumes. Thus it is an ideal location to improve remotely sensed biogeochemical algorithms.

The "f-ratio", i.e., the ratio of new (NO₃-fuelled) production to total production could be mapped using regional correlation between ship-derived NO₃ concentration and remotely sensed SST, and between NO₃ and new production obtained from ¹⁵N isotopic nitrogen onboard incubations. This approach shows the enormous potential for combining remote sensing, sea-truthing with surface biogeochemical measurements and proxies to spatially and temporally map a wide range of processes including primary production, sedimentation, CO₂ exchange fluxes and nutrient inputs at the ocean margin.

Ultimately, proxies for sedimentation rates developed from relations with surface total production, runoff and new production could form the basis of using remotely sensed data for extrapolating site-specific sedimentation rates to the whole of the Iberian margin.

The overall objective of OMEX II-II is to measure and understand the fluxes contributing to ocean margin exchange for e.g., water, carbon, nutrients, lithogenic and biogenic particulate matter. Ultimately, understanding is tested by the ability to form balanced budgets, to formulate models and to improve the measurement strategy in both hindcast and forecast modes.

To simulate the physical and ecological processes in a highly dynamic and spatially diverse coupled dynamic system, models require hydrodynamical input which can resolve the main spatial (upwelling regions and filaments) and temporal (upwelling and relaxation / downwelling processes) scales.

The ultimate aim of OMEX II-II is to design a complete model:

- by synthesising the measurements obtained on the complementary cruises,

- by forming consistent flux and budget estimates from these data,

- by integrating specific models to form these budgets (hindcast mode) and

- by studying the sensitivity to global climate change and anthropogenic inputs (forecast mode).

The 3-dimensional (3D) hydrodynamical model used in OMEX II-II has a horizontal resolution of a few kilometres, is equipped with a nesting facility in order to speed up the computations and will be further developed to include moving boundaries. This model defined a sequence of fixed horizontal layers, so-called baroclinic levels. Each level has a fixed thickness, with the exception of the level at the surface and the one that is close to the bottom. A typical application includes 20 levels with thickness of a few meters at the surface and thus provides a precise representation of the upper part of the water column that is important for uses in biological models. The external driving forces are wind, air pressure, density field and flow through the open boundaries, fresh water supply from land and heat flux, calculated from humidity, cloud cover and wind velocity. In addition to the carbonate system, this model has eight state variables representing phytoplankton, micro- and mesozooplankton, nutrients and detritus.

Modelling of water circulation at the Iberian margin represents a major numerical and conceptual challenge arising from the wide range of scales controlling circulation, mixing, formation of filaments, vertical exchange and sedimentation. The major circulation features (slope currents, upwelling) of the Iberian margin can be modelled using climatologically driven 3-D model for the NE Atlantic (25N to 65N; 05E to 25W) operating on a 20-km grid. Initial conditions will be set by temperature and salinity fields from the NOAA World Ocean Atlas. Turbulence closure will be from direct measurements of turbulent kinetic energy, and validation will be against historical flow fields and actual flows recorded during OMEX II-II period from current meter moorings and from shipborne acoustic current profilers for the upper 500 metres. Real meteorological forcing during the OMEX periods will be carried out using meteorological-data streams from ECMWF. The model will then be used to track water masses, the large-scale evolution of nutrients and carbon and of particle residence times for biogeochemical budgeting across the OMEX Box.

Coupled pelagic and benthic ecological models specifically developed to simulate the short-term evolution and rate experiments and incorporating nutrient flows, light, biomass and growth data will be nested within a validated 3-D hydrodynamic ensemble.

Regulation of vertical flux through aggregation will be incorporated into mathematical models in the near future without risking that the predictability of the model decreases due to the increase in complexity. This is based on the assumption that aggregation can be predicted as a function of physical forcing and diatom abundance and physiological state. Except for the physiological determination of aggregate stickiness, models can simulate the observed processes reasonably well.

Pelagic biological models that simulate the carbon flow at ocean margins have highlighted differences between rates of pelagic production and the sedimentation and benthic consumption of organic matter at the slopes. Using the rigors of mass balance and turbulence closure, modelling provides a unique tool for integrating and reconciling inventories and rates of biological processes.

Updated : 11 September 2003,  Feedback :

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