Benthic oxygen fluxes in a coastal upwelling system (Ria de Vigo, NW Iberia) measured by aquatic eddy covariance

Organic carbon mineralization and nutrient cycling in benthic environments are critically important for their biogeochemical functioning, but are poorly understood in coastal up - welling systems. The main objective of this study was to determine benthic oxygen fluxes in a muddy sediment in the Ria de Vigo (NW Iberian coastal upwelling), by applying the aquatic eddy covariance (AEC) technique during 2 campaigns in different seasons (June and October 2017). The main drivers of benthic fluxes were studied and compared among days in each season and between seasons. The 2 campaigns were characterized by an upwelling-relaxation period followed by a downwelling event, the last of which was due to the extratropical cyclone Ophelia in October. The mean (±SD) seasonal benthic oxygen fluxes were not significantly different for the 2 campaigns despite differences in hydrodynamic and biogeochemical conditions (June: -20.9 ± 7.1 mmol m-2d-1vs. October: -26.5 ± 3.1 mmol m-2d-1). Benthic fluxes were controlled by different drivers depending on the season. June was characterized by sinking labile organic material, which enhanced benthic fluxes in the downwelling event, whereas October had a significantly higher bottom velocity that stimulated the benthic fluxes. Finally, a comparison with a large benthic chamber (0.50 m2) was made during October. Despite methodological differences between AEC and chamber measurements, concurrent fluxes agreed within an acceptable margin (AEC:benthic chamber ratio = 0.78 ± 0.13; mean ± SD). Bottle incubations of water sampled from the chamber interior indicated that mineralization could explain this difference. These results show the importance of using non-invasive techniques such as AEC to resolve benthic flux dynamicsPostprin

Tide and wind coupling in a semienclosed bay driven by coastal upwelling

The Ría de Vigo is a semi-enclosed bay in which tidal residual currents are associated with coastal upwelling events. Both upwelling and downwelling favourable winds generate a bidirectional exchange flow with the shelf – a two-layer circulation with surface waters leaving (entering) the ria and a compensating inflow (outflow) through the bottom layer under upwelling (downwelling) conditions. This vertical circulation changes the vertical density structure inside the ria. In the ria, the tide is mainly semidiurnal (M2, S2 and K2), with some energy in the diurnal band (K1). Our velocity observations show that the vertical structure of the tidal currents in the ria do not exhibit a classic barotropic profile with a bottom boundary layer beneath uniform “free-stream” flow as the tidal bottom boundary layer is affected by stratification. This links tidal circulation to the wind-driven residual circulation, since the latter also greatly helps to control the stratification. We quantify this effect by fitting tidal ellipses to observed velocities through the water column. In addition to this indirect coupling through stratification, there is a direct interaction in which velocities in the upper and bottom layers are best correlated with winds while the mid-water velocities are best correlated with tides. These wind-tide interactions are expected to play a key role in the resuspension and transport of nutrients and phytoplankton in the Ria.CTM2012-3515

Tidal and wind influences on circulation in the southern mouth of the Ría de Vigo (NW Iberian Peninsula)

Tidal and wind influences on the velocity field in the Ría de Vigo were assessed using atmospheric data from two meteorological stations located at Bouzas port and on an oceanic buoy off Silleiro Cape along with oceanic data from an ADCP moored in the Ría for a 72-day period. A two-layer circulation pattern was observed. Near-surface and near-bottom currents are primarily influenced by wind (especially remote winds), separated by an intermediate layer dominated by tidal variability. At subtidal frequencies, residual currents are well correlated with wind variability. Remote wind forcing exhibited a markedly high correlation with surface layer currents, indicating the major role played by wind in the long-term upwelling-modulated circulation of the Ría

Temperature calibration along the NW Iberian margin : multi-proxy approach

1 poster presented at the European Geosciences Union General Assembly 2012, Vienna, Austria, 22 – 27 April 2012Marine sediments, mainly the ones from coastal upwelling areas, are valuable archives for past reconstructions, including climate changes, global and regional oceanography, and the cycles of biochemical components in the ocean. However, the regional calibrations of the sediment properties (“proxy” data) to the present-day oceanographic and biological conditions are necessary for interpretation of high-resolution sediment-derived information. Many works have contributed directly or indirectly to improving proxy calibrations related to upwelling and nutrient cycling parameters, but there are few studies with a multi-proxy approach for the same region. Along NW Iberian margin, little work has been done regarding with calibration of proxy data. Thus, we lack understanding on (1) how the seasonally variable hydrography affects the various plankton groups and (2) how much of the water column signal is preserved in the sediments and available for paleo-reconstructions. In order to minimize this lack of information from the upper water column temperature point of view, we investigate trace element and stable isotopes from planktonic foraminifera, and alkenone Uk’37 index from a large set of core-top sediment samples from the NW Iberian margin (41.5-42.5ºN; 9-10ºW). Sediment data are compared with present-day water column data (CTD, stable isotopes, satellite measurements) and to “global” calibration data sets of the proxies with sea surface temperature. We explore reconstructing upper water column structure using three species (G. bulloides, N. pachyderma dextral, and G. inflata) with different depth habitats, different seasonal abundances, and linked to specific hydrographic conditions in this region. Preliminary results show that Mg/Ca and 18O temperatures of all species are consistent with seasonal temperatures observed in present-day water column data. In addition G. bulloides and N. pachyderma dextral appears with temperatures related with subtropical and subpolar Eastern North Atlantic Central Waters, respectively.N

Spatial and temporal variability in coccolithophore abundance and distribution in the NW Iberian coastal upwelling system

A systematic investigation of the spatial and temporal variability in coccolithophore abundance and distribution through the water column of the NW Iberian coastal up-welling system was performed. From July 2011 to June 2012, monthly sampling at various water depths was conducted at two parallel stations located at 42 degrees N. Total coccosphere abundance was higher at the outer-shelf station, where warmer, nutrient-depleted waters favoured coccolithophore rather than phytoplanktonic diatom blooms, which are known to dominate the inner-shelf location. In seasonal terms, higher coccosphere and coccolith abundances were registered at both stations during upwelling seasons, coinciding with high irradiance levels. This was typically in conjunction with stratified, nutrient-poor conditions (i.e. relaxing upwelling conditions). However, it also occurred during some upwelling events of colder, nutrient-rich subsurface waters onto the continental shelf. Minimum abundances were generally found during downwelling periods, with unexpectedly high coccolith abundance registered in subsurface waters at the inner-shelf station. This finding can only be explained if strong storms during these downwelling periods favoured resuspension processes, thus remobilizing deposited coccoliths from surface sediments, and hence hampering the identification of autochthonous coccolithophore community structure. At both locations, the major coccolithophore assemblages were dominated by Emiliania huxleyi, small Gephyrocapsa group, Gephyrocapsa oceanica, Florisphaera profunda, Syracosphaera spp., Coronosphaera mediterranea, and Calcidiscus leptoporus. Ecological preferences of the different taxa were assessed by exploring the relationships between environmental conditions and temporal and vertical variability in coccosphere abundance. These findings provide relevant information for the use of fossil coccolith assemblages in marine sediment records, in order to infer past environmental conditions, of particular importance for Paleoceanography. Both E. huxleyi and the small Gephyrocapsa group are proposed as proxies for the upwelling regime with a distinct affinity for different stages of the upwelling event: E. huxleyi was associated with warmer, nutrient-poor and more stable water column (i.e. upwelling relaxation stage) while the small Gephyrocapsa group was linked to colder waters and higher nutrient availability (i.e. early stages of the upwelling event), similarly to G. oceanica. Conversely, F. profunda is suggested as a proxy for the downwelling regime and low-productivity conditions. The assemblage composed by Syracosphaera pulchra, Coronosphaera mediterranea, and Rhabdosphaera clavigera may be a useful indicator of the presence of subtropical waters conveyed northward by the Iberian Poleward Current. Finally, C. leptoporus is proposed as an indicator of warmer, saltier, and oligotrophic waters during the downwelling/winter regime.EXCAPA project - Xunta de Galicia [10MDS402013PR]; CALIBERIA project (Fundacao para a Ciencia e a Tecnologia - Portugal) [PTDC/MAR/102045/2008]; CALIBERIA project [COMPETE/FEDER-FCOMP-01-0124-FEDER-010599, BI/PTDC/MAR/102045/2008/2010-016, BI/PTDC/MAR/102045/2008/2010-022, BI/PTDC/MAR/102045/2008/2011-027]; Ministerio de Economia y Competitividad [CGL2015-68459-P]; Ministry of Education of Spain [AP2010-2559]; ETH Zurich Postdoctoral Fellowship from the Swiss Federal Institute of Technology in Zurich (ETHZ); Xunta de Galicia (Spain); FCT [SFRH/BPD/111433/2015]; Plurianual/Estrategico project [UID/Multi/04326/2013]info:eu-repo/semantics/publishedVersio

Temporal changes in the water mass distribution and transports along the 20ºW CAIBOX section (NE Atlantic)

The CAIBOX cruise was conducted from 25 July to 14 August 2009. Three consecutive transects (zonal, meridional, and transverse) formed a closed box to the west of the Strait of Gibraltar. This study aimed to analyze the thermohaline properties, volume transports, and water mass distributions (percentages) along the meridional section (30–41.5º N, 20º W). We identified the main geostrophic current (Azores Current) and its associated volume transport and interannual changes. Data from previous cruises (AZORES I, A16N, CLIVAR, OACES, and CHAOS) with similar tracks were employed to compare with the CAIBOX meridional section. All but one (CHAOS) were summer cruises. We estimated a mean transport for the Azores Current at 20º W of 9.3 ± 2.6 Sv. There appears to be an inverse relation between the position of this current and its associated transport, with relatively high (low) transports when the current is located roughly south (north) of 35º N. Regarding water masses, an increase of 14.4% was found for Mediterranean Water compared with the 1993, 1998, and 2003 cruises; however, Labrador Sea Water decreased its contribution and southward spreading between 1998 and 2009. Key words: Northeast Atlantic, Azores Current, water masses, multiparametric mixing analysis.

Temporal changes in the water mass distribution and transports along the 20 degrees W CAIBOX section (NE Atlantic)

The CAIBOX cruise was conducted from 25 July to 14 August 2009. Three consecutive transects (zonal, meridional, and transverse) formed a closed box to the west of the Strait of Gibraltar. This study aimed to analyze the thermohaline properties, volume transports, and water mass distributions (percentages) along the meridional section (30-41.5 degrees N, 20 degrees W). We identified the main geostrophic current (Azores Current) and its associated volume transport and interannual changes. Data from previous cruises (AZORES 1, A16N, CLIVAR, OACES, and CHAOS) with similar tracks were employed to compare with the CAIBOX meridional section. All but one (CHAOS) were summer cruises. We estimated a mean transport for the Azores Current at 20 degrees W of 9.3 +/- 2.6 Sv. There appears to be an inverse relation between the position of this current and its associated transport, with relatively high (low) transports when the current is located roughly south (north) of 35 degrees N. Regarding water masses, an increase of 14.4% was found for Mediterranean Water compared with the 1993, 1998, and 2003 cruises; however, Labrador Sea Water decreased its contribution and southward spreading between 1998 and 2009.Entre el 25 de julio y el 14 de agosto de 2009 se llevó a cabo la campaña CAIBOX, que constó de tres transectos consecutivos (zonal, occidental y transversal) conformando una caja cerrada al oeste del estrecho de Gibraltar. El objetivo principal del presente trabajo fue el estudio de las propiedades termohalinas, los transportes (en términos de volumen) y la distribución de las masas de agua (en porcentajes) a lo largo de la sección occidental (30–41.5º N, 20º W). Se identificó la corriente geostrófica principal (Corriente de las Azores), el transporte asociado a ella y sus cambios interanuales. Adicionalmente se consideraron los datos de cinco campañas anteriores (AZORES I, A16N, CLIVAR, OACES y CHAOS) con secciones similares a las de la campaña CAIBOX. Todas excepto una (CHAOS) fueron campañas estivales. El transporte medio para la Corriente de las Azores a 20º W se estimó en 9.3 ± 2.6 Sv. Parece existir cierta relación inversa entre la posición de dicha corriente y su transporte asociado, con transportes relativamente altos (bajos) cuando la corriente se sitúa al sur (norte) de 35º N. En cuanto a las masas de agua, se observa un incremento de 14.4% del Agua Mediterránea en comparación con las campañas de 1993, 1998 y 2003; sin embargo, el Agua de Labrador ha disminuido su contribución y su propagación hacia el sur entre 1998 y 2009

Particle fluxes in the NW Iberian coastal upwelling system: Hydrodynamical and biological control

To better understand sources and transport of particulate material in the NW Iberian coastal upwelling system, a mooring line dotted with an automated PPS 4/3 sediment trap was deployed off Cape Silleiro at the base of the photic zone. The samples were collected from November 2008 through June 2012 over sampling periods of 4-12 days.Our study represents the first automated sediment trap database for the NW Iberian margin. The magnitude and composition of the settling material showed strong seasonal variability with the highest fluxes during the poleward and winter mixing periods (averages of 12.9 +/- 9.6 g m(-2) d(-1) and 5.6 +/- 5.6 g m(-2) d(-1) respectively), and comparatively lower fluxes (3.6 +/- 4.1 gm(-2) d(-1)) for the upwelling season. Intensive deposition events registered during poleward and winter mixing periods were dominated by the lithogenic fraction (80 +/- 3%). They were associated to high energy wave-driven re suspension processes, due to the occurrence of south-westerly storms, and intense riverine inputs of terrestrial material from Minho and Douro rivers.On the other hand, during the spring- summer upwelling season, the share of biogenic compounds (organic matter, calcium carbonate (CaCO3), biogenic silica (bSiO2)) to downward fluxes was higher, reflecting an increase in pelagic sedimentation due to the seasonal intensification of primary production and negligible river inputs and wave-driven resuspended material. Otherwise, the large variations of biogenic settling particles were mainly modulated by upwelling intensity, which by means of upwelling filaments ultimately controlled the offshore transport of the organic carbon fixed by primary producers towards the adjacent ocean. Based on the average downward flux of organic carbon (212 mg C m(-2) d(-1)) and considering an average primary production of 1013 mg C m(-2) d(-1) from literature, we estimated that about 21% of the fixed carbon is vertically exported during the upwelling season. (C) 2016 Elsevier Ltd. All rights reserved