Role of the circulation on the anthropogenic CO 2 inventory in the North-East Atlantic: A climatological analysis

Climatology-based storage rate of anthropogenic CO2 (Cant, referred to year 2000) in the North-East Atlantic (53 ± 9 kmol s−1, 0.020 ± 0.003 Pg-C yr−1) is described on annual mean terms. Cant advection (32 ± 14 kmol s−1) occurs mostly in the upper 1800 m and contributes to 60% of the Cant storage rate. The Azores and Portugal Currents act as ‘Cant streams’ importing 389 ± 90 kmol s−1, most of which recirculates southwards with the Canary Current (−214 ± 34 kmol s−1). The Azores Counter Current (−79 ± 36 kmol s−1) and the northward-flowing Mediterranean Water advective branch (−31 ± 12 kmol s−1) comprise secondary Cant export routes. By means of Cant transport decomposition, we find horizontal circulation to represent 11% of the Cant storage rate, while overturning circulation is the main driver (48% of the Cant storage rate). Within the domain of this study, overturning circulation is a key mechanism by which Cant in the upper layer (0–500 dbar) is drawdown (74 ± 14 kmol s−1) to intermediate levels (500–2000 dbar), and entrained (37 ± 7 kmol s−1) into the Mediterranean Outflow Water to form Mediterranean Water. This newly formed water mass partly exports Cant to the North Atlantic at a rate of −39 ± 9 kmol s−1 and partly contributes to the Cant storage in the North-East Atlantic (with up to 0.015 ± 0.006 Pg-C yr−1). Closing the Cant budget, 40% of the Cant storage in the North-East Atlantic is attributable to anthropogenic CO2 uptake from the atmosphere (21 ± 10 kmol s−1)

Evidencias do cambio climático na hidrografía e a dinámica das rías e da plataforma galega

16 páginas, 9 figuras, 3 tablas[EN] This study is focused on multidecadal-scale variations of the hydrographic and hydrodynamic features of Rías and shelf waters off Galicia. Water masses take, through their thermohaline characteristics, a footprint of the climatic variability, mainly caused by changes either in the hydrological budget (precipitation, evaporation, runoff), in heat exchange and wind stress. In this study we focus in the evidences of change in the surface and sub-surface water mass characteristics. Regarding surface water masses, a significant warming during the last 45 years has been noticed, at a rate of 0.17 ºC/ decade. This warming was especially intense from the 1970’s, 0.27 ºC/decade. The tendency is opposite in subsurface waters, but it only takes into account data from the last 15 years. Regarding hydrodynamics, because of its strong influence in the renewal time of the Rías and consequently in their living resources, a study of decadal changes in the estuarine circulation of the Rías de Vigo and Arousa has been made. Results showed a progressive decay in the estuarine circulation, which causes an increase of renewal times. On the other hand, indirect evidence of a possible deceleration of the winter Iberian Poleward Current (IPC) are reported. This slow down is partially caused by a decrease in the meridional (N-S) temperature gradient off the Iberian coast. Nevertheless, the fall of the meridional gradient is not clear because it has been inferred from surface waters. On the contrary, IPC seems to be intensified by the increasing southerly autumn winds, although it appears weakened by the decreasing southerly winter winds. Thus, the overall effect of the CIP is still uncertain. Finally, a study of sea level and wave changes on the Galician coast has been made. An increase of (2-2.5 cm/ decade) in the sea level as well as a decrease in the wave height (3 cm/decade) has been reported, but in this last topic results are not significant[GA] Este estudo centrarase en determinar as variacións que ocorren a escalas multidecadais nas características hidrográficas e hidrodinámicas das augas das rías e da plataforma fronte a Galicia. As masas de auga reciben a través das súas características termohalinas unha pegada das alteracións climáticas, principalmente a causa de cambios no balance hidrolóxico (precipitación, evaporación, achegas continentais), do intercambio de calor e da acción do vento. Neste estudo analizaremos as evidencias de cambio nas características das masas de auga superficiais e subsuperficiais. Nas primeiras detectouse quentamento de xeito significativo nos últimos 45 anos a unha taxa media de 0,17 ºC por década, especialmente intenso desde hai 30 anos, 0,27 ºC por década. Nas augas subsuperficiais a tendencia é a inversa, aínda que só se conta con datos dos últimos 15 anos. En canto á hidrodinámica, e pola súa forte implicación nos tempos de renovación da ría e, polo tanto, nos recursos mariños que viven nela, fíxose un estudo sobre os cambios na circulación estuárica das rías de Vigo e Arousa nas últimas décadas. Os resultados amosan un descenso paulatino a longa escala temporal da circulación que causa un aumento dos tempos de renovación. Por outra banda, achéganse evidencias indirectas dunha posible ralentización da corrente invernal cara ao Polo (CIP) causada por un descenso no gradiente meridional (N-S) de temperatura fronte ás costas ibéricas. Non obstante, o descenso do gradiente meridional non está claro, pois induciuse só a partir de datos superficiais. Ademais, a propia CIP amosa estar intensificada pola maior intensidade dos ventos sur no outono e debilitada polo descenso dos ventos do sur no inverno, polo que o efecto global na CIP é aínda incerto. Por último, faise un estudo dos cambios do nivel do mar e da ondada nas costas galegas que amosa un aumento daquel (2-2,5 cm/década) e un descenso desta (3 cm/década), aínda que no caso da ondada os resultados non son concluíntesPeer reviewe

Origin and fate of a bloom of Skeletonema costatum during a winter upwelling/downwelling sequence in the Ría de Vigo (NW Spain)

Original research paperThe onset, development and decay of a winter bloom of the marine diatom Skeletonema costatum was monitored during a 10 d period in the coastal upwelling system of the Rı´a de Vigo (NW Spain). The succession of upwelling, relaxation and downwelling-favorable coastal winds with a frequency of 10 –20 d is a common feature of the NW Iberian shelf. The onset of the bloom occurred during an upwelling-favorable 1⁄2 wk period under winter thermal inversion conditions. The subsequent 1⁄2 wk coastal wind relaxation period allowed development of the bloom (gross primary production reached 8gCm–2 d–1) utilizing nutrients upwelled during the previous period. Finally, downwelling during the following 1⁄2 wk period forced the decay of the bloom through a combination of cell sinking and downward advection.Financial support came from the Spanish Ministerio de Ciencia y Tecnologı´a (MCyT) grant REN2000-0880-C02-01 and Xunta de Galicia grant PGIDT01MAR40201PN; a fellowship from the MCyT and the I3P-CSIC Program.Versión del editor0,98

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

Geochemical vs. microbial approach to the new production of the coastal upwelling system of the Ría de Vigo (NW Spain)

Symposium GLOBEC–IMBER España, Valencia, 28-30 marzo 2007The fate of the inorganic and organic N trapped in the coastal upwelling system of Ría de Vigo (NW Spain), accumulation/export versus production/ consumption, was studied at the short time–scale (2–4 d) during July 2002. A transient geochemical box model was applied to the measured residual currents and concentrations of inorganic (NT), dissolved (DON) and particulate (PON) organic N to obtain the i) net balance of inputs minus outputs (i – o); ii) the net accumulation (V·dN/dt); and iii) the net ecosystem production (NEP) of NT, DON and PON. The average NEP during July (107 mg N m-2 d-1) indicates an autotrophic metabolism of the ría. About 25% of this material was exported to the shelf and the remaining 75% was transferred to the sediments or promoted to higher trophic levels. Measurements of oxygen production (Pg) and respiration (R) were performed in a single site twice a week at five depths. In addition, microzooplankton grazing and sedimentation rates were measured for first time in the Ría de Vigo. The high grazing rates observed reduces the efficiency of the ría to transfer organic matter directly from phytoplankton to the metazoans Comparison of the metabolic state of the Ría de Vigo derived from these in vitro measurements (Pg, R, grazing and sedimentation) and the in situ geochemical budget shows that they agree in 2 of the 3 study cases. Both methods are complementary and their simultaneous application allows obtaining a better knowledge of coastal upwelling ecosystems functioningN

Oceanographic processes and morphosedimentary products along the Iberian margins: A new multidisciplinary approach

Postprin

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

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

Nitrogen cycling in an estuarine upwelling system, the Ría de Arousa (NW Spain). I. Short-time-scale patterns of hydrodynamic and biogeochemical circulation

13 pages, 9 figures, 1 table.Nitrogen species distributions in the Ría de Arousa, NW Spain, during the upwelling season are strongly affected by the influence of shelf wind stress on water circulation. Intermittent northerly winds forced large volumes of nitrate-rich Eastern North Atlantic Central Water into the ría with a fortnightly periodicity and led to net community production maxima supported by external nitrate. Net community production in the central segment of the estuary was high (average 251 mg m-2 d-1 of nitrogen). However, only 20% of the total nutrient input was net taken up due to rapid water exchange. An intensive culture of hanging mussels favoured dissolved organic nitrogen formation (58% of net community production) and particulate organic nitrogen sedimentation (21% of net community production, or 50% of net particulate organic nitrogen formation). Consequently, more than 75% of the net dissolved+particulate organic nitrogen production was exported from the central segment, with a dissolved/particulate ratio of ~3. In addition, 40% of phytoplankton gross primary production was internally recycled by pelagic+benthic ammonification in the central segment, mainly during prolonged upwelling relaxations. Low rates of particulate organic nitrogen sedimentation and low levels of conversion to dissolved organic nitrogen were observed during the development of a red tide assemblage. This appears to be field evidence of limited filtration by mussels in the presence of these organisms. A strong reversal in the residual circulation occurred in mid-October, during the transition from northerly to southerly winds. Ammonium released from the estuarine sediments, at a high average net rate of 162 mg N m-2 d-1, was exported towards the shelf.Support for this work came from C.I.C.Y.T Grant No. MAR88-0245 and funds from the Consellería de Pesca da Xunta de Galicia. Fellowships from the Spanish Ministerio de Educación y Cienca allowed X.A.A.-S., G.R. and Y.P. to carry out their Ph.D. theses as part of this project.Peer reviewe