Microbial and photochemical reactivity of fluorescent dissolved organic matter in a coastal upwelling system

Original research articleWe observed significant changes in the dissolved oxygen content and the fluorescence of humic substances anddissolved aromatic amino acids after 24 h light and dark incubations in the coastal upwelling system of the Rı´a deVigo under a wide variety of meteorologic and oceanographic conditions. Respiration rates were inverselycorrelated with the net production of humic fluorescence in the dark at a net rate of 20.027 6 0.003 mgequivalents of quinine sulphate per mmol of O2, suggesting that marine humics are a by-product of the bacterialrespiration of dissolved organic matter (DOM). On the contrary, humic fluorescence consumption in the lightminus dark incubations was positively correlated with the net production in the dark, indicating a rapidphotodegradation of recently produced marine humic substances. Parallel incubation experiments demonstratedthat daily photodegradation rates and residual humic fluorescence levels followed a seasonal pattern characterizedby a marked autumn maximum. Finally, a significant linear correlation between the gross primary production(Pg) and the net production of aromatic amino acids fluorescence in the light pointed to the rapid consumption ofdissolved protein-like materials at a net average rate of 21.4 6 0.2 ppb equivalents of tryptophan per day, whichaccumulates in the water column only when Pg exceeds 80 6 20 mmol kg21d21Financial support came from the Spanish Ministerio de Cienciay Tecnologı´a (MCyT) grant REN2000-0880-C02-01 and Xunta deGalicia grant PGIDT01MAR40201PN and a fellowship from theMCyT and the I3P-CSIC ProgramVersión del editor3,38

Phytoplankton diversity effect on ecosystem functioning in a coastal upwelling system.

This work is part of the time series project RADIALES conducted and funded by the Instituto Español de Oceanografía (http://www.seriestemporales-ieo.net) with additional support from project MarRisk (Interreg POCTEP Spain-Portugal) grant number 0262 MARRISK 1 E, from grants Contrato-Programa GAIN-IEO, and grant number IN607A2018/2 of the Axencia Galega de Innovación (GAIN, Xunta de Galicia, Spain). JO was supported by a Junta para la Ampliación de Estudios Fellowship (JAE-Doc programme 2011) from the CSIC and ESF.Species composition plays a key role in ecosystem functioning. Theoretical, experimental and field studies show positive effects of biological diversity (biodiversity) on ecosystem processes. However, this link can differs between taxonomic and functional diversity components and also across trophic levels. These relationships It haves been hardly studied in planktonic communities occurring inof coastal upwelling systems. Using a 28 yr time series of phytoplankton and zooplankton assemblages we examined the effects of phytoplankton diversity on resource use efficiency at the two trophic levels (RUE, ratio of biomass to limiting resource) in the Galician upwelling system (NW Iberian peninsula). Fitting Generalized Least Squares models we show that phytoplankton diversity was the best predictor for RUE across planktonic trophic levels. This link varied depending on the biodiversity component considered: while the effect of phytoplankton richness on RUE was positive for phytoplankton RUE and negative for zooplankton RUE, phytoplankton evenness effect was negative for phytoplankton RUE and positive for zooplankton RUE. Overall, taxonomic diversity had higher explanatory power than functional diversity, and variability in phytoplankton and zooplankton RUE decreased with increasing phytoplankton taxonomic diversity. Phytoplankton used resources more efficiently in warmer waters and at greater upwelling intensity although these effects were not as strong as those for biodiversity. These results suggest that phytoplankton species numbers in highly dynamic upwelling systems are important for maintaining the planktonic biomass production leading us to hypothesize the relevance of likely through complementarity effects. However, we further postulate that a selection effect may operate also because assemblages with low evenness were dominated by diatoms with specific functional traits increasing their ability to exploit resources more efficiently.This work is part of the time series project RADIALES conducted and funded by the Instituto Español de Oceanografía (http://www.seriestemporales-ieo.net) with additional support from project MarRisk (Interreg POCTEP Spain-Portugal) grant number 0262 MARRISK 1 E, from grants Contrato-Programa GAIN-IEO, and grant number IN607A2018/2 of the Axencia Galega de Innovación (GAIN, Xunta de Galicia, Spain). JO was supported by a Junta para la Ampliación de Estudios Fellowship (JAE-Doc programme 2011) from the CSIC and ESF.Versión del edito

Fresh and saline submarine groundwater discharge in a large coastal inlet affected by seasonal upwelling

Original research paperSubmarine groundwater discharge is recognized as a major source of chemicals to the global ocean, exerting large control over coastal water composition. Radon and 226Ra are used to evaluate, for the first time, the occur rence and magnitude of submarine groundwater discharge in the Ría de Vigo, a large, highly productive embay ment affected by seasonal, wind-driven upwelling. The system is naturally enriched in 222Rn due to the regional granitic basement geology: high 222Rn activities (up to 106 Bq m−3 ) are detected in wells and boreholes in the drainage basin of the embayment. High 222Rn activities (>400 Bq m−3 ) are also measured in certain areas of the embayment. Comparatively lower 226Ra activities (<4 103 Bq m−3 ) were measured in the freshwater sources to the bay. Mass balances obtained with a box model are used to perform a volumetric estimate of fresh and saline submarine groundwater discharge in the Ría de Vigo under contrasting circulation patterns. Fresh groundwater is shown to be a relevant hydrological component of the Ria de Vigo water balance, equivalent to 9% 4% and 23% 9% of the volume discharged by tributary rivers during winter and summer, respectively. On the other hand, recirculation of seawater through permeable sediments is capable of filtering the entire upper volume of the Ria de Vigo through its seafloor in <100 days and might thus be a previously overlooked major source of regenerated solutes to the system.Irish Research Council and European Union Commission Horizon 2020 research and innovation programmeVersión del editor3,38

Surface waters of the NW Iberian margin: upwelling on the shelf versus outwelling of upwelled waters from the Rías Baixas

A set of hydrographic surveys were carried out in the Ría of Vigo (NW Spain) at 2–4 d intervals during four 2–3 week periods in 1997, covering contrasting seasons. Residual exchange fluxes with the adjacent shelf were estimated with a 2-D, non-steady-state, salinity–temperature weighted box model. Exchange fluxes consist of a steady-state term (dependent on the variability of continental runoff) and a non-steady-state term (dependent on the time changes of density gradients in the embayment). More than 95% of the short-time-scale variability of the exchange fluxes in the middle and outer ría can be explained by the non-steady-state term that, in turns, is correlated (R2>75%) with the offshore Ekman transport. Conversely, 96% of the variability of exchange fluxes in the inner ría rely on the steady-state term. The outer and middle ría are under the direct influence of coastal upwelling, which enhances the positive residual circulation pattern by an order of magnitude: from 10 2 to 10 3 m3s−1. On the contrary, downwelling provokes a reversal of the circulation in the outer ría. The position of the downwelling front along the embayment depends on the relative importance of Ekman transport (Qx, m3s−1km−1) and continental runoff (R, m3s−1). When Qx/ R>7±2 the reversal of the circulation affects the middle ría. Our results are representative for the ‘Rías Baixas’, four large coastal indentations in NW Spain. During the upwelling season (spring and summer), 60% of shelf surface waters off the ‘Rías Baixas’ consist of fresh Eastern North Atlantic Central Water (ENACW) upwelled in situ. The remaining 40% consists of upwelled ENACW that previously enters the rías and it is subsequently outwelled after thermohaline modification. During the downwelling season (autumn and winter), 40% of the warm and salty oceanic subtropic surface water, which piled on the shelf by the predominant southerly winds, enters the rias

Effect of dissolved organic matter (DOM) of contrasting origins in Cu and Pb speciation and toxicity to Paracentrotus lividus larvae

Original research paperWater samples of contrasting origin, including natural seawater, two sediment elutriates and sewage-influenced seawater, were collected and obtained to examine the effect of the dissolved organic matter(DOM) present on metal bioavailability. The carbon content (DOC) and the optical properties (absorbanceand fluorescence) of the coloured DOM fraction (CDOM) of these materials were determined. Cu and Pbcomplexation properties were measured by anodic stripping voltammetry (ASV) and the effect of DOMon Cu and Pb bioavailability was studied by means of theParacentrotus lividusembryo-larval bioassay.Sediment elutriates and sewage-influenced water (1) were enriched 1.4–1.7 times in DOC; (2) absorbedand reemitted more light; and (3) presented higher Cu complexation capacities (LCu) than the naturalseawater used for their preparation.LCuvaried from 0.08 M in natural seawater to 0.3 and 0.5 Min sediment elutriates and sewage-influenced water, respectively. Differences in DOC, CDOM and Cucomplexation capacities were reflected in Cu toxicity. DOM enriched samples presented a Cu EC50of0.64 M, significantly higher than the Cu EC50of natural and artificial seawater, which was 0.38 M. Theprotecting effect of DOM on Cu toxicity greatly disappeared when the samples were irradiated with highintensity UV-light. Cu toxicity could be successfully predicted considering ASV-labile Cu concentrationsin the samples. Pb complexation by DOM was only detected in the DOM-enriched samples and causedlittle effect on Pb EC50. This effect was contrary for both elutriates: one elutriate reduced Pb toxicity incomparisonwiththecontrolartificialseawater,whiletheotherincreasedit.UVirradiationofthesamplescaused a marked increase in Pb toxicity, which correlated with the remaining DOC concentration. DOMparameterswererelatedtoCuspeciationandtoxicity:goodcorrelationswerefoundbetweenDOCandCuEC50, whileLCucorrelated better with the fluorescence of marine humic substances. The present resultsstress the importance of characterizing not only the amount but also the quality of seawater DOM tobetter predict ecological effects from total metal concentration dataSpanish Ministry of Education and Science (CTM2006-13880-C03-01/MAR), Xunta of Galicia (PGIDIT-05MA40201) and Basque Government (predoctoral grant)Versión del editor3,76

Dissolved organic nitrogen production and export by meridional overturning in the eastern subpolar North Atlantic

Research articleDissolved organic matter (DOM) is produced in the surface and exported towards the deep ocean, adding ∼ 2 PgC/year to the global carbon export. Due to its central role in the Meridional Overturning Circulation, the eastern subpolar North Atlantic (eSPNA) contributes largely to this export. Here we quantify the transport and budget of dissolved organic nitrogen (DON) in the eSPNA, in a box delimited by the OVIDE 2002 section and the Greenland-Iceland-Scotland sills. The Meridional Overturning Circulation exports >15.9 TgN/year of DON downward and, contrary to the extended view that these are materials of subtropical origin, up to 33% of the vertical flux derives from a net local DON production of 7.1 ± 2.6 TgN/year. The low C:N molar ratio of DOM production (7.4 ± 4.1) and the relatively short transit times in the eSPNA (3 ± 1 year) suggest that local biogeochemical transformations result in the injection of fresh bioavailable DOM to the deep ocean.Versión del editor3,79

Fluorescence: Absorption coefficient ratio — Tracing photochemical and microbial degradation processes affecting coloured dissolved organic matter in a coastal system

Original research paperThe optical properties of coloured dissolved organic matter (CDOM) – absorption coefficient, induced fluorescence, and fluorescence quantum yield – were determined in the coastal eutrophic system of the Ría de Vigo (NW Spain) under two contrasting situations: a downwelling event in September 2006 and an upwelling event in June 2007. Significantly different optical properties were recorded in the shelf surface (higher absorption coefficient and lower quantum yield) and bottom (lower absorption coefficient and higher quantum yield) waters that entered the embayment during downwelling and upwelling conditions, respectively. Continental waters presented distinctly high CDOM levels. The spatial and temporal variability of the induced fluorescence to absorption coefficient ratio during the mixing of shelf and continental waters was used to quantify the relative importance of photochemical and microbial processes under these contrasting hydrographic conditions. Photochemical processes were dominant during the downwelling episode: 86% of the variability of CDOM can be explained by photochemical degradation. On the contrary, microbial processes prevailed during the upwelling event: 77% of the total variability of CDOM was explained by microbial respiration.The Xunta de Galicia, grant number PGIDIT-05MA40201PR; the project SUMMER, grant number CTM2008-03309/MAR; a I3P-CSIC predoctoral fellowship and a Marie Curie I.O.F.Versión del editor2,75

Dissolved Organic Matter (DOM) in the open Mediterranean Sea. I. Basin-Wide distribution and drivers of chromophoric DOM

Original research articleChromophoric dissolved organic matter (CDOM) in the open Mediterranean Sea (MedSea) is barely documented, remaining the basin–wide patterns in intermediate and deep waters still enigmatic. Here, full–depth distributions of CDOM absorption coefficients and spectral slopes recorded during the HOTMIX 2014 cruise are presented and their respective environmental drivers resolved. General Additive Models (GAMs) in surface waters and Optimum MultiParameter (OMP) water mass analysis in deep waters were applied. In the surface, apparent oxygen utilisation (AOU), a proxy to cumulative net community respiration, explained most of the variability of dissolved organic carbon (DOC) and the absorption coefficient at 254 nm (a254), whereas the absorption coefficient at 325 nm (a325), and the spectral slopes were mostly explained by potential temperature, a proxy to stratification and solar radiation, indicating that both water column stability and photobleaching may drive the variability of the UV–A absorbing CDOM components. In deep waters, the effect of water mass mixing and basin–scale mineralization were discerned from local mineralization processes. Water mass mixing and basin–scale mineralization contributed more substantially to explain the variability of DOC, a254 and a325 (82–91%) than the variability of the spectral slopes (35–64%). Local mineralization processes indicate that DOC and CDOM play a more relevant role in the carbon cycle in the Eastern (EastMed) than in the Western (WestMed) Mediterranean: whereas DOC contributed to 66 ± 10% of the oxygen demand in the EastMed, it represented only 24 ± 4% in the WestMed. Independently of basins and layers, a254 revealed as an excellent proxy to the concentration of DOC in the MedSea. Also, the unexpected inverse relationship of a325 with AOU indicates that the consumption of the UV–A absorbing CDOM fraction prevails over their productionSpanish Ministry of Education and Culture, Spanish Ministry of Economy and Competitiviness, FEDER, CSIC, University of GranadaVersión del editor3,26

Dissolved organic matter (DOM) in the open Mediterranean Sea. II: Basin-wide distribution and drivers of fluorescent DOM

Research articleFluorescent dissolved organic matter (FDOM) in the Mediterranean Sea was analysed by excitation–emission matrix (EEM) spectroscopy and parallel factor (PARAFAC) analysis during the cruise HOTMIX 2014. A 4–component model, including 3 humic–like and 1 protein–like compounds, was obtained. To decipher the environmental factors that dictate the distributions of these components, we run generalized additive models (GAMs) in the epipelagic layer and an optimum multiparametric (OMP) water masses analysis in the meso– and bathypelagic layers. In the epipelagic layer, apparent oxygen utilization (AOU) and temperature presented the most significant effects on the variability of the marine humic-like peak M fluorescence, suggesting that its distribution was controlled by the net community respiration of organic matter and photobleaching. On the contrary, the variability of the soil humic-like peak E and the protein–like peak T fluorescence was explained mainly by the prokaryotic heterotrophic abundance, which decreased eastwards. In the meso– and bathypelagic layers, water mass mixing and basin–scale mineralization processes explained >72% and 63% of the humic–like and protein–like fluorescence variability, respectively. When analysing the two basins separately, the OMP model offered a better explanation of the distribution of fluorescence in the eastern Mediterranean Sea, as expected from the reduced biological activity in this ultra–oligotrophic basin. Furthermore, while western Mediterranean deep waters display the usual trend in the global ocean (increase of humic–like fluorescence and decrease of protein–like fluorescence with higher AOU values), the eastern Mediterranean deep waters presented an opposite trend. Different initial fluorescence intensities of the water masses that mix in the eastern basin, with Adriatic and Aegean origins, seem to be behind this contrasting pattern. The analysis of the transect–scale mineralization processes corroborate this hypothesis, suggesting a production of humic–like and a consumption of protein–like fluorescence in parallel with water mass ageing. Remarkably, the transect–scale variability of the chromophoric dissolved organic matter (CDOM) absorbing at the excitation wavelength of the humic–like peak M indicates an unexpected loss with increasing AOU, which suggests that the consumption of the non–fluorescent fraction of CDOM absorbing at that wavelength exceeded the production of the fluorescent fraction observed hereProject HOTMIX (reference CTM2011–30010–C02 01–MAR and 02–MAR), co–financed with FEDER funds (re ference BES–2012–056175) ; the project MODMED from CSIC (PIE, 201730E020) and the project FERMIO (MINECO, CTM2014–57334–JIN), co–financed with FEDER fundsVersión del editor3,26

Influence of the oceanographic conditions during spring 2003 on the transport of the Prestige tanker fuel oil to the Galician coast

Hydrographic data collected during the cruise HIDROPRESTIGE were combined with meteorological and dynamic data provided by remote sensors and drifting/moored buoys, to describe the surface circulation of the Northern Iberian basin in March–April 2003. Sea surface winds transported the floating Prestige oil slicks from the sinking area to the continental slope off the Rías Baixas in 1/2 month: the surface current intensity was 2% of the wind intensity and it was rotated clockwise 5 from the wind direction. Mesoscale cyclonic and anticyclonic structures west of 10º W increased the residence time of oil patches in the Northern Iberian basin, as compared with the expected southwards flow of the Iberian current (IC). On the other hand, the Iberian poleward current (IPC) formed a marked surface front with coastal waters, preventing the entry of fuel oil into the rías. PAHs in the surface layer during the cruise were <0.5 ug L- 1, except in the Galicia bank ( 1 ug L- 1; where the Prestige tanker was still leaking) and the vicinity of Cape Fisterra ( 1.5 ug L -1; where the convergence front between the IPC and coastal waters vanished)