Xosé Antón Álvarez-Salgado

Hydrographic variability off the Rías Baixas (NW Spain) during the upwelling season

During the Galicia X cruise, from May to October 1989 an intensive collection of hydrographic data was carried out at a single station on the shelf off the western coast of Galicia. It allows us to follow the response of the water column to the intermittent equatorward wind stress during the upwelling season. Upwelling events occur with biweekly periodicity, bringing Eastern North Atlantic Water (ENAW) to the subsurface layer at our station. A trend of the thermohaline properties of the upwelled water to increase in time was observed. This seems to be mostly due to the southwestward displacement of the origin of this water mass during the year. Although the saltier and warmer ENAW is less nutrient-rich, nutrient levels increase because of the rapid remineralization of organic matter from the Rias, which takes place in the bottom water on the shelf.

The Portugal coastal counter current off NW Spain: new insights on its biogeochemical variability

Abstract Time series of wind-stress data, AVHRR and SeaWiFS satellite images, and in situ data from seven cruises are used to assemble a coherent picture of the hydrographic variability of the seas off the Northwest Iberian Peninsula from the onset (September–October) to the cessation (February–May) of the Portugal coastal counter current (PCCC). During this period the chemistry and the biology of the shelf, slope and ocean waters between 40° and 43°N have previously been undersampled. Novel information extracted from these observations relate to: 1. The most frequent modes of variability of the alongshore coastal winds, covering event, seasonal and long-term scales; 2. The conspicuous cycling between stratification and homogenisation observed in PCCC waters, which has key implications for the chemistry and biology of these waters; 3. The seasonal evolution of nitrite profiles in PCCC waters in relation to the stratification cycle; 4. The Redfield stoichiometry of the remineralisation of organic matter in Eastern North Atlantic Central Water (ENACW)—the water mass being transported by the PCCC; 5. The separation of coastal (mesotrophic) from PCCC (oligotrophic) planktonic populations by a downwelling front along the shelf, which oscillates to and fro across the shelf as a function of coastal wind intensity and continental runoff; and 6. The photosynthetic responses of the PCCC and coastal plankton populations to the changing stratification and light conditions from the onset to the cessation of the PCCC.

Simultaneous determination of dissolved organic carbon and total dissolved nitrogen in seawater by high temperature catalytic oxidation: Conditions for precise shipboard measurements

Abstract Appropriate conditions have been achieved for the accurate, rapid, and highly precise shipboard simultaneous determination of dissolved organic carbon and total dissolved nitrogen in seawater by high temperature catalytic oxidation. A nitrogen-specific Antek 705D chemiluminescence detector and a CO 2 -specific LiCor Li6252 IRGA have been coupled in-series with a Shimadzu TOC-5000 organic carbon analyser. Precision of both simultaneous measurements is ≤1.5%, i.e. ±1 μ mol C l −1 and ±0.3 μ mol N l −1 , respectively. Quality of analysis is not compromised by vibrations associated with ocean going research vessels.

Net Production and Consumption of Fluorescent Colored Dissolved Organic Matter by Natural Bacterial Assemblages Growing on Marine Phytoplankton Exudates

ABSTRACT An understanding of the distribution of colored dissolved organic matter (CDOM) in the oceans and its role in the global carbon cycle requires a better knowledge of the colored materials produced and consumed by marine phytoplankton and bacteria. In this work, we examined the net uptake and release of CDOM by a natural bacterial community growing on DOM derived from four phytoplankton species cultured under axenic conditions. Fluorescent humic-like substances exuded by phytoplankton (excitation/emission [Ex/Em] wavelength, 310 nm/392 nm; Cobles peak M) were utilized by bacteria in different proportions depending on the phytoplankton species of origin. Furthermore, bacteria produced humic-like substances that fluoresce at an Ex/Em wavelength of 340 nm/440 nm (Cobles peak C). Differences were also observed in the Ex/Em wavelengths of the protein-like materials (Cobles peak T) produced by phytoplankton and bacteria. The induced fluorescent emission of CDOM produced by prokaryotes was an order of magnitude higher than that of CDOM produced by eukaryotes. We have also examined the final compositions of the bacterial communities growing on the exudates, which differed markedly depending on the phytoplankton species of origin. Alteromonas and Roseobacter were dominant during all the incubations on Chaetoceros sp. and Prorocentrum minimum exudates, respectively. Alteromonas was the dominant group growing on Skeletonema costatum exudates during the exponential growth phase, but it was replaced by Roseobacter afterwards. On Micromonas pusilla exudates, Roseobacter was replaced by Bacteroidetes after the exponential growth phase. Our work shows that fluorescence excitation-emission matrices of CDOM can be a helpful tool for the identification of microbial sources of DOM in the marine environment, but further studies are necessary to explore the association of particular bacterial groups with specific fluorophores.

The water masses along the western boundary of the south and equatorial Atlantic

A quasi-meridional hydrographic section located offshore from South America from 50°S to 10°N, and three shorter transverse lines to the continental slope, are used for a descriptive study of the water masses along the western boundary of the South and Equatorial Atlantic. At the upper and intermediate levels, the tracer analysis provides geographical limits of the wind-driven circulation regimes, and a comparison of the tracer values at the continental slope and along the meridional section shows where the boundary currents originate. At depths shallower than about 200 m, the subdivision of the subtropical gyre into two cells separated by the Subtropical Countercurrent near 28°S, that was pointed out in a previous study, is corroborated. South of this front, a warm variety (∼18°C) of Subtropical Mode Water in the inner recirculation of the Brazil Current appears, despite its limited extent, as a southern counterpart of the North Atlantic 18°C water. At the deep levels, the Upper Circumpolar Water and Upper North Atlantic Deep Water enter the South Atlantic in a significantly overlapping density range. The ensuing lateral encounter of both water masses occurs at 26°S near the western boundary, where most of the boundary flow of the latter water is stopped and deflected seaward by the base of the subtropical gyre. Other tracer anomalies signal significant eastward escapes of North Atlantic Deep Water: within two jets at about two degrees of latitude on either side of the equator, in another narrow current at 10°S, and at 34°S. The latter latitude marks the confluence, and eastward deflection, of the opposite boundary currents of Lower North Atlantic Deep Water and Lower Circumpolar Water. Near the bottom of the Argentine Basin, the Weddell Sea Deep Water that flows westward north of the Zapiola Ridge is more recently ventilated than the water carried by the boundary current near the Falkland Escarpment. While a part of it flows anticyclonically around the ridge, another part turns equatorward and enhances the southern property signatures of the water farther north.

Off-shelf fluxes of labile materials by an upwelling filament in the NW Iberian Upwelling System

Daily changes in the concentrations of carbon and nitrogen species were monitored during the course of a Lagrangian drifter experiment in a recurrent upwelling filament south of Cape Finisterre (NW Iberian Upwelling System). A drifting buoy released at the southern edge of the upwelling centre generated by the Cape moved 60km southwestwards from 3 to 7 August 1998. Organic matter in the 50m deep study water mass (average 77±2 µM C) consisted of: 57µ MC of dissolved organic matter (DOM) with a C/N molar ratio of 19±2; 6µM C of DOM with a C/N ratio of 9±2, and 14µM C of 50% DOM and 50% suspended organic matter (POM susp) with a C/N ratio of 6.0±0.4. Net conversion of consumed inorganic salts into accumulated TOM=POMsusp+DOM was 40% for nitrogen and 30% for carbon. Since the parcel of water crossed the shelf-edge, these conversion efficiencies are equivalent to net horizontal export-ratio of 0.4 and 0.3 respectively. A second drifter was deployed in the offshore-end of the filament, and was displaced 20km west between 14 and 17 August 1998. Nitrate was exhausted in the surface water and no significant changes were observed in the variables measured during the course of the second experiment. Low C/N ratios (6.5±0.4) and rapid POMsusp/DOM inter-conversion in the 20 µM C excess observed in the study volume points to the persistence of the labile materials formed on the shelf during transport to the ocean. Our data demonstrate

Nutrient mineralization patterns in shelf waters of the Western Iberian upwelling

Abstract A set of hydrographic data collected during several cruises to the NW Iberian upwelling system has been used to assess the patterns of nutrient mineralization over the shelf. Mineralized nutrient ratios (N/P and N/Si) in the lower water column are clearly below the ratios characteristic of freshly upwelled oceanic waters. Nitrogen mineralization, in comparison to phosphorus mineralization, is incomplete, in agreement with phosphates faster regeneration rate and the low residence time of water, which precludes complete mineralization. Silicate shows the highest enrichment as a consequence of the selective concentration of particulate biogenic silica in the near bottom waters. Diffusion of nutrients mineralized in the sediments seems to represent an important contribution. The lowest mineralization of the surveyed area is found to the north of Cape Finisterre, especially with regard to silicate. To the South, the Rias Baixas (four large embayments) return as reducing particulate organic matter (POM) a small fraction of the upwelled nutrients imported from the shelf. However, the out-flowing freshwater contributes to stratification of coastal waters, which favours POM production, accumulation and sedimentation. In addition, high levels of new dissolved organic matter (DOM) in the outflow can also enhance primary production in shelf surface waters. Finally, highest mineralization was found to the south of the River Mino, where there are no embayments and the shelf is wider. Mineralization tends to: (1) increase the potential primary production of this ecosystem by up to 50%, (nitrogen limitation) or up to 80% (silicon limitation); (2) favour the development of phytoplankton assemblages dominated by diatoms (Si enrichment double that of N and P); and (3) buffer the large spatial and temporal differences introduced by the advected nutrients.

New production of the NW Iberian shelf during the upwelling season over the period 1982–1999

New production (NP) is calculated for NW Iberian shelf waters from 421 to 431N (3500 km 2 ), at the fortnight, upwelling-season (March–October) and inter-annual time-scales. The time series used are (1) upwelling rates (daily values of offshore Ekman transport from 1982 to 1999), (2) bottom shelf temperatures (twice a week values from 1987 to 1999), and (3) the nutrient–temperature relationships ofupwelled Eastern North Atlantic Central Water (ENACW) obtained during 14 hydrographic cruises to the study area (between 1977 and 1998). Marked inter-annual variability is observed, both at the fortnight and the seasonal time-scales. Average NP over the upwelling-season ranged from 330 to 815 mg C m � 2 d � 1 (mean, 4907145 mg C m � 2 d � 1 ) in the 1982–1999 period. Large inter-annual changes ofupwelling rates are the reason behind the NP fluctuations: 83% ofthe variability ofNP can be explained by the offshore Ekman Transport ð� QX Þ: NP is compared with satellite-derived net microbial community production (NCP) during the 1998– 1999 upwelling seasons, when SeaWiFS images are available. An average upwelling-season NP/NCP ratio of0.33 was obtained, indicating that 67% of NCP is respired in situ and 33% is exported off-shelf to the surrounding oligotrophic ocean.

Hydrographic conditions associated with the relaxation of an upwelling event off the Galician coast (NW Spain)

During the GALICIA-XI cruise froIn May 10 to May 14, 1991, the relaxation of a previous strong tipwelling event took place along the Galician coast caused by a shift in the position of the Azores High. The coldest tenperatures and highest nitrate levels were forrod near the coast corresponding to the advection of subsurface waters, particularly of Eastern North Atlantic Water (ENAW). On the basis of thermohaline properties, ENAW of polar (ENAWp) and tropical (ENAWt) origin are distinguished. The highest salinity values found correspond to a saline wedge of ENAW t located off the west coast of Galicia. The relaxation of tipwelling produced a shoreward migration of surface water from oceanic stations which crone into contact with upwelled Eastern North Atlantic Water ti-mn the coastal stations to tbrm a convergence front 28 lan off the coast. Meanwhile, the saline wedge displaced towards both the north and the shore which is thvored by the compression of the tipwelling system to a narrow coastal strip outwardly limited by the convergence zone.

Evidence of in situ diel vertical migration of a red-tide microplankton species in Ria de Vigo (NW Spain)

The migration capacity of red-tide species in the natural environment was studied at a station in the Ría de Vigo (Rías Bajas, NW Spain) over a 24 h period in September 1991. The Ría de Vigo, where red tides are frequent, normally shows a positive estuarine circulation and is subjected to seasonal upwelling and downwelling phenomena. A marked diel pattern was observed for five species that are capable of causing red tides (Ceratium furca, Scrippsiella trochoidea, Dinophysis acuminata, Mesodinium rubrum, and Eutreptiella sp.). Such diel behaviour could be clearly advantageous in a stratified environment where light and nutrients are often in two separate layers. Active movement enables species such as dinoflagellates and some ciliates to exploit high levels of irradiance at the surface during the day and to take up nutrients in deeper layers at night. Patchy distribution of phytoplankton, shear-induced horizontal dispersion and density variations were considered, but none of them accounted for the vertical changes observed. Vertical migration is thought to be one of the mechanisms that could promote blooms in nutrient-depleted surface layers.