Carmen G. Castro

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.

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.

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.

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.

Water masses and distribution of physico-chemical properties in the Western Bransfield Strait and Gerlache Strait during Austral summer 1995/96

In the framework of the FRUELA project, two oceanographic surveys were conducted by R/V Hesperides in the eastern Bellingshausen Sea, western basin of the Bransfield Strait and Gerlache Strait area during December 1995 and January 1996. The main hydrographic structures of the study domain were the Southern Boundary of the ACC and the Bransfield Front. The characteristics and zonation of local water masses are discussed in terms of temperature, salinity, dissolved oxygen, nutrient and inorganic carbon concentrations. Concentration intervals for water mass labelling, on the basis of chemical parameters in addition to the common θ/S-based classification, are defined. Silicate seems to be a very good discriminator for local water masses.

Transient hydrographic and chemical conditions affecting microplankton populations in the coastal transition zone of the Iberian upwelling system (NW Spain) in September 1986

The coastal transition zone and adjacent continental shelf of the Iberian upwelling system was studied in September 1986, during the seasonal transition from upwelling-favored to downwellingfavored winds. The most striking features in the coastal transition zone were: (1) a poleward flow of high salinity Eastern North Atlantic Central Water (subtropical) off the Rias Baixas (Galician western coast); and (2) an anticyclonic eddy off Cape Ortega1 (Galician northern coast). Chemical and biological similarities between both structures, clearly different from the surrounding oceanic waters, suggest that the eddy was an isolated and aged parcel of water originating from the poleward flow. The continental shelf was characterized by: (1) outwelling of chlorophyll-rich waters from the four large coastal embayments (the “Rias Baixas”) in the western coast and; (2) an upwelling front off the northern coast. The coexistence of opposite hydrographic structures, as the poleward flow and the upwelling front, was the consequence of transitional wind conditions in September-October, and we hypothesized transitional conditions to be crucial for the development of the eddy. Both the poleward flow and the eddy precluded the shelf-edge exchange of microplankton populations developed over the shelf, leading to massive in situ sedimentation and subsequent nutrient mineralization over the shelf.

Mixing analysis of nutrients, oxygen and inorganic carbon in the Canary Islands region

Abstract We show the distribution of nutrients, oxygen, total dissolved inorganic carbon ( C T ) and total alkalinity ( A T ) along three sections close to the Canary Islands, between 18°W and the African coast during Meteor 37/2 cruise (January 1997). From the thermohaline properties of Eastern North Atlantic Central Water (ENACW), Mediterranean Water (MW), Antarctic Intermediate Water (AAIW) and North Atlantic Deep Water (NADW), a mixing model has been established based on the water mass description. It can explain most of the variabilities found in the distribution of the chemical variables, including the carbon system, and it is validated through the use of conservative chemical variables like ‘NO.’ From nutrients, oxygen, A T and C T , the chemical characterisation of the water masses was performed by calculating the concentration of these variables in the previously defined thermohaline end-members. The relative variation of nutrient concentrations, resulting from the regeneration of organic matter, was estimated. Close to the African shelf-break, a poleward undercurrent conveying as much as a 11% of AAIW was observed only in the southern section (28.5°N). From the chemical and thermohaline properties of the end-members, a comparison with data from other oceanic regions was made in respect to conservative chemical variables (‘NO’). In addition, a north–south gradient in the ventilation pattern of water masses is observed from the residuals of the model.

Nutrient utilisation and chlorophyll distribution in the Atlantic sector of the Southern Ocean during Austral summer 1995-96

The chemical fields in the Eastern Bellingshausen Sea, Gerlache and Bransfield Straits were studied for nutrients, apparent oxygen utilisation (AOU) and total inorganic carbon (TIC) in the upper mixed layer of the water column during the FRUELA 95 (December 1995-January 1996) and FRUELA 96 (January-February 1996) cruises. In Bellingshausen Sea area, a strong silicic acid gradient clearly traced the Southern Boundary of the ACC front during the two cruises. In the Bransfield Strait, the lowest concentrations of nutrients, AOU and TIC (NO 3 <28 μmol kg -1 , PO 4 < 2 μmol kg -1 , Si(OH) 4 < 80 μmol kg -1 , AOU < 10 μmol kg -1 , TIC<2170 μmol kg -1 ) corresponded to transition zonal water with Bellingshausen Sea influence. The southeastern part, which was dominated by transition zonal water with Weddell Sea influence, had the highest levels of these properties. In the Gerlache Strait, the chemical field was marked by low-salinity water in the southwestern extreme, which had low nutrient levels. Though the chemical field strongly reflected the thermohaline field, significant nutrient consumption was observed where there was high chlorophyll a and particulate organic matter. During the FRUELA 95 cruise, the highest nutrient consumption in the upper mixed layer was localised in the Gerlache Strait (ΔNO 3 = 7±2 μmol kg -1 , ΔPO 4 = 0.5±0.1 μmol kg -1 , ΔSi(OH) 4 = 6±3 μmol kg -1 , ΔAOU = 65±15 μmol kg -1 , ΔTIC = 40±8 μmol kg -1 ) and in the Southern Boundary of the ACC front (ΔNO 3 = 10±3 μmol kg -1 , ΔPO 4 = 0.6±0.2 μmol kg -1 , ΔSi(OH) 4 = 1.9±1.6 μmol kg -1 , ΔAOU = 78±10 μmol kg -1 , ΔTIC = 69±17 μmol kg -1 ), which was associated with high biomass. During FRUELA 96 cruise, the highest nutrient consumption also was observed in the Gerlache Strait (ΔNO 3 = 14±3 μmol kg -1 , ΔPO 4 = 1.1±0.2 μmol kg -1 , ΔSi(OH) 4 = 12±5 μmol kg -1 , ΔAOU = 94±22 μmol kg -1 , ΔTIC = 70±16 μmol kg -1 ), with average chlorophyll a of 7±2 mg m -3 Using a simple approach, we can ascertain whether the estimated nutrient consumption can account for the observed biomass accumulation during the two cruises.

Stoichiometry of the degradation of dissolved and particulate biogenic organic matter in the NW Iberian upwelling

[1] The average composition of the dissolved and particulate products of early degradation of marine phytoplankton has been established for the first time in a coastal upwelling system using a mixing analysis along isopycnal surfaces combined with a stoichiometric model. About 17–18% of the mineralized organic matter is derived from the decomposition of organic particulates, and 16–35% is from the dissolved organic matter. The remaining 50–70% is derived probably from large fast sinking particles. On average, the mineralized material on large particles has the closest composition to the Redfield formula. The ratio of dissolved saccharides to dissolved organic matter respiration is >40% higher than expected from a material of Redfield composition. Finally, the ratio of lipid to particulate organic matter respiration is >80% larger than expected from a material of Redfield composition. Regarding the decomposition of hard structures, biogenic silica dissolves predominantly in the inner shelf, where organic carbon oxidation is more intense, and diatom deposition occurs preferentially.

Generation of null alleles for the functional analysis of six genes from the right arm of Saccharomyces cerevisiae chromosome II

Using PCR‐ligated long flanking homology cassettes, null alleles of six open reading frames (ORFs) from chromosome II have been created in Saccharomyces cerevisiae. Deletants were constructed in three genetic backgrounds: FY1679, W303 and CEN.PK2. Tetrad analysis of heterozygous deletants revealed that none of the ORFs is essential for vegetative growth. Basic phenotypic analysis of haploid deletants showed that deletion of the YBR283c ORF causes a slight growth defect at 30°C and 37°C on glycerol‐complete, glucose‐complete, and glucose‐minimal media only in the FY1679 and W303 backgrounds. Transformation of these deletants with the corresponding cognate gene in a centromeric plasmid complements the defects. Deletion of the YBR287w ORF leads to poor growth on glucose‐minimal medium at 15°C in the FY1679 background. None of the six ORFs seems to be involved in mating or sporulation. Copyright