55 research outputs found
SIZE-REACTIVITY OF DISSOLVED ORGANIC MATTER IN THE CAPE VERDE FRONTAL ZONE
Oral communicationDissolved organic matter (DOM) plays a major role in the recycling,
export and sequestration of biogenic organic carbon, being a key
component of ocean biogeochemical cycles and of the biological and
microbial carbon pumps. Microbial degradation of DOM not only
produces CO 2 but also generates dissolved molecules of decreasing
bioavailability that can accumulate in the oceans for hundreds to
thousands of years. The size-reactivity continuum (SRC) model is the
conceptual framework to explain the DOM reactivity on a size basis,
although field tests are still scarce and some of the pieces of this
puzzle remain unclear. Taking advantage of the FLUXES-I cruise in the
Cape Verde Frontal Zone (CVFZ), we have studied the size fractionated reactivity of the high (HMW; >1 KDa) and low (LMW; <1
KDa) molecular weight fractions of the DOM from surface down to
4000 m, using a high-efficiency and low-concentration-factor
ultrafiltration cell. The wide ageing range covered by the water
masses of the CVFZ makes it an excellent site to test the SRC model.
Regarding the bulk C and N pools, the water masses with higher
oxygen utilization were more depleted in HMW molecules, with a
significant preference for the degradation of large N-containing
compounds. Accordingly, preferential degradation of HMW
fluorescent protein-like compounds was observed. In parallel,
fluorescent humic-like compounds of both HMW and LMW were
generated as by-product of the degradation of HMW organic
compounds, and the remineralization of the DOM increases the
aromaticy of both fractions, but especially the LMW one.ASL
DISSOLVED ORGANIC MATTER MOLECULAR FINGERPRINT OF THE WATER MASSES IN THE CAPE VERT FRONTAL ZONE
Oral communicationOcean water masses have been traditionally characterized by the
thermohaline and conservative chemical properties (e.g. preformed
nutrients) at their respective source regions. However, water masses
also can exhibit characteristic levels of other individual compounds or
emerging properties associated to compound classes. In this regard,
the objective of this contribution is to characterize the dissolved
organic matter (DOM) molecular fingerprint of the water masses
present in the Cape Vert Frontal Zone (CVFZ). For this purpose, a set
of 133 samples was collected from the surface to 4000 m depth in
the CVFZ during the FLUXES I cruise (12 July - 11 August 2017) and
isolated by solid-phase extraction (SPE), using styrene divinyl benzene
polymer cartridges (PPL). The molecular analysis of these SPE-PPL
extracts was performed using Fourier Transform Ion Cyclotron
Resonance Mass Spectrometry (FT-ICR MS), a method capable of
identifying thousands of molecular formulae in DOM. These analyses
have been combined with an optimum multiparameter (OMP) water
mass analysis to obtain characteristic molecular indices for the
eleven water masses present in the CVFZ, stemming from the
subtropical and subpolar North and South Atlantic as well as from the
Arctic and Antarctic Oceans. In particular, emerging properties such
as the molecular diversity (D), mean molecular mass (MW), mean C:N
ratio, aromaticity index (AI), double bond equivalent (DBE), and main
molecular groups, as well as different compounds (e.g. Lignin) and
individual heteroatoms were quantified.ASL
Mesopelagic respiration near the ESTOC (European Station for Time-Series in the Ocean, 15.5°W, 29.1°N) site inferred from a tracer conservation model
© The Author(s), 2016. This is the author's version of the work and is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Deep Sea Research Part I: Oceanographic Research Papers 115 (2016): 63–73, doi:10.1016/j.dsr.2016.05.010.Remineralization of organic matter in the mesopelagic zone (ca. 150–700 m) is a
key controlling factor of carbon export to the deep ocean. By using a tracer conservation model applied to climatological data of oxygen, dissolved inorganic carbon (DIC) and
nitrate, we computed mesopelagic respiration at the ESTOC (European Station for Time-
Series in the Ocean, Canary Islands) site, located in the Eastern boundary region of the
North Atlantic subtropical gyre. The tracer conservation model included vertical Ekman
advection, geostrophic horizontal transport and vertical diffusion, and the biological remineralization terms were diagnosed by assuming steady state. Three different approaches
were used to compute reference velocities used for the calculation of geostrophic velocities and flux divergences: a no-motion level at 3000 m, surface geostrophic velocities
computed from the averaged absolute dynamic topography field, and surface velocities
optimized from the temperature model. Mesopelagic respiration rates computed from the
model were 2.8–8.9molO2 m2 y=1, 2.0–3.1mol Cm2 y=1 and 0.6–1.0molNm2 y=1, consistent with remineralization processes occurring close to Redfield stoichiometry. Model
estimates were in close agreement with respiratory activity, derived from electron transport
system (ETS) measurements collected in the same region at the end of the winter bloom
period (3.61 ± 0.48molO2 m=2 y=1). According to ETS estimates, 50% of the respiration
in the upper 1000 m took place below 150 m. Model results showed that oxygen, DIC and
nitrate budgets were dominated by lateral advection, pointing to horizontal transport as the
main source of organic carbon fuelling the heterotrophic respiration activity in this region.Funding for this study was provided by the Xunta de Galicia under the research project VARITROP (09MDS001312PR, PI B. Mouriño-Carballido) and by the Ministerio de Educación y
Cultura under the research project MESOPELAGIC (MAR97-1036, PI S. Hernández-León).
B. Fernández-Castro acknowledges the receipt of FPU grant from the Spanish government
(AP2010-5594).2017-05-2
Nitrogen inputs influence on biomass and trophic structure of ocean plankton: a study using biomass and stable isotope size-spectra.
Large scale patterns in planktonic food web structure were studied by applying continuous size-scaled models of biomass and δ15N to plankton samples, collected at 145 stations during the Malaspina-2010 Expedition across three ocean basins and including major biomes. Carbon biomass and δ15N were determined in size-fractionated samples (40 to 5000 μm) collected by vertical hauls (0-200 m). Biomass-normalized size-spectra were constructed to summarize food web structure and spatial patterns in spectral parameters were analyzed using geographically-weighted regression analysis. Except in the northwestern Atlantic, size-spectra showed low variability, reflecting a large homogeneity in nitrogen sources and food web structure for the central oceans. Estimated predator-to-prey mass ratios 20% (Trades and Westerlies biomes) suggested that oceanic plankton food webs could support a larger number of trophic levels than current estimates based on high efficiency values. The largest changes in spectral parameters and nitrogen sources were related to inputs of atmospheric nitrogen, either from diazotrophic organisms or dust deposition. These results suggest geographic homogeneity in the net transfer of nitrogen up the food web.CONSOLIDER-INGENIO 2010 (CSD2008-00077) ; EURO-BASIN (FP7-ENV-2010 264933)Preprint1,749
Breaking of internal waves and turbulent dissipation in an anticyclonic mode Water Eddy
A four-month glider mission was analyzed to assess turbulent dissipation in an anticyclonic eddy at the western boundary of the subtropical North Atlantic. The eddy (radius ≈ 60 km) had a core of low potential vorticity between 100–450 m, with maximum radial velocities of 0.5 m s−1 and Rossby number ≈ −0.1. Turbulent dissipation was inferred from vertical water velocities derived from the glider flight model. Dissipation was suppressed in the eddy core (ε ≈ 5×10−10 W kg−1) and enhanced below it (> 10−9 W kg−1). Elevated dissipation was coincident with quasi-periodic structures in the vertical velocity and pressure perturbations, suggesting internal waves as the drivers of dissipation. A heuristic ray-tracing approximation was used to investigate the wave-eddy interactions leading to turbulent dissipation. Ray-tracing simulations were consistent with two types of wave-eddy interactions that may induce dissipation: the trapping of near-inertial wave energy by the eddy’s relative vorticity, or the entry of an internal tide (generated at the nearby continental slope) to a critical layer in the eddy shear. The latter scenario suggests that the intense mesoscale field characterizing the western boundaries of ocean basins might act as a ‘leaky wall’ controlling the propagation of internal tides into the basins’ interior
Seasonality modulates wind-driven mixing pathways in a large lake
Turbulent mixing controls the vertical transfer of heat, gases and nutrients in stratified water bodies, shaping their response to environmental forcing. Nevertheless, due to technical limitations, the redistribution of wind-derived energy fuelling turbulence within stratified lakes has only been mapped over short (sub-annual) timescales. Here we present a year-round observational record of energy fluxes in the large Lake Geneva. Contrary to the standing view, we show that the benthic layers are the main locus for turbulent mixing only during winter. Instead, most turbulent mixing occurs in the water-column interior during the stratified summer season, when the co-occurrence of thermal stability and lighter winds weakens near-sediment currents. Since stratified conditions are becoming more prevalent –possibly reducing turbulent fluxes in deep benthic environments–, these results contribute to the ongoing efforts to anticipate the effects of climate change on freshwater quality and ecosystem services in large lakes
Biogeochemistry of dissolved and suspended organic matter in the Cape Vert Frontal Zone (NW Africa)
Oral communicationThe Cape Verde Frontal Zone (CVFZ) in the southern boundary of the
Canary Current Upwelling Ecosystem, is a highly dynamic area,
featuring large vertical and horizontal export fluxes of organic matter
(OM) due to the interaction of the Cape Verde Front (CVF) with the
Mauritanian upwelling. To study the interplay between transport and
biogeochemical processes driving the distribution of OM in the CVFZ,
full-depth profiles of dissolved (DOM) and suspended particulate
(POM) OM were obtained during the FLUXES I cruise in August 2017.
Distributions of surface DOM and POM and their stoichiometry were
influenced by the mesoscale variability at the frontal region, showing
significant differences north and south of the CVF and between
stations close and distant to the Mauritanian coast. The C:N
molar ratio of DOM and POM showed average vertical gradients,
increasing from 12.1 and 8.0 in surface to 15.6 and 17.0 respectively
in deeps waters, deviating from the traditional Redfield ratio. In the
meso- and bathypelagic zones, meridional and cross-shore gradients
were detected within samples belonging to the same water mass,
indicating that their properties were re-shaped by biogeochemical
processes within the CVFZ. Correlations between apparent oxygen
utilization and OM indicate that DOM+POM contributed only to
8.1% of the carbon and 17.8% of the nitrogen
mineralisation in the water column, suggesting that the local carbon
demand is mainly supported by sinking POM and N containing
compounds are mineralised to a larger extend than C containing compoundsASL
Factors controlling the community structure of picoplankton in contrasting marine environments
The
effect of inorganic nutrients on planktonic assemblages has traditionally
relied on concentrations rather than estimates of nutrient supply. We
combined a novel dataset of hydrographic properties, turbulent mixing,
nutrient concentration, and picoplankton community composition with the aims
of (i) quantifying the role of temperature, light, and nitrate fluxes as
factors controlling the distribution of autotrophic and heterotrophic
picoplankton subgroups, as determined by flow cytometry, and (ii) describing
the ecological niches of the various components of the picoplankton
community. Data were collected at 97 stations in the Atlantic Ocean,
including tropical and subtropical open-ocean waters, the northwestern
Mediterranean Sea, and the Galician coastal upwelling system of the northwest
Iberian Peninsula. A generalized additive model (GAM) approach was used to
predict depth-integrated biomass of each picoplankton subgroup based on three
niche predictors: sea surface temperature, averaged daily surface irradiance,
and the transport of nitrate into the euphotic zone, through both diffusion
and advection. In addition, niche overlap among different picoplankton
subgroups was computed using nonparametric kernel density functions.
Temperature and nitrate supply were more relevant than light in predicting
the biomass of most picoplankton subgroups, except for
Prochlorococcus and low-nucleic-acid (LNA) prokaryotes, for which irradiance also played a
significant role. Nitrate supply was the only factor that allowed the
distinction among the ecological niches of all autotrophic and heterotrophic
picoplankton subgroups. Prochlorococcus and LNA prokaryotes were
more abundant in warmer waters (>20 ∘C) where the nitrate fluxes
were low, whereas Synechococcus and high-nucleic-acid (HNA)
prokaryotes prevailed mainly in cooler environments characterized by
intermediate or high levels of nitrate supply. Finally, the niche of
picoeukaryotes was defined by low temperatures and high nitrate supply. These
results support the key role of nitrate supply, as it not only promotes the
growth of large phytoplankton, but it also controls the structure of marine
picoplankton communities.Ministerio de Economía y Competitividad | Ref. CTM2012-30680Ministerio de Economía y Competitividad | Ref. CTM2008-0626I-C03-01Ministerio de Economía y Competitividad | Ref. REN2003-09532-C03-01Ministerio de Economía y Competitividad | Ref. CTM2004-05174 -C02Ministerio de Economía y Competitividad | Ref. CTM2011-25035Xunta de Galicia | Ref. 09MMA027604PRXunta de Galicia | Ref. EM2013/021European Commission | Ref. FP7, n. 261860Ministerio de Economía y Competitividad | Ref. FJCI-641 2015-2571
Nutrient supply does play a role on the structure of marine picophytoplankton communities
Conference communicationThe Margalef´s mandala (1978) is a simplified bottom-up control model that explains how mixing and nutrient concentration determine the composition of marine phytoplankton communities. Due to the difficulties of measuring turbulence in the field, previous attempts to verify this model have applied different proxies for nutrient supply, and very often used interchangeably the terms mixing and stratification. Moreover, because the mandala was conceived before the discovery of smaller phytoplankton groups (picoplankton <2 µm), it describes only the succession of vegetative phases of microplankton. In order to test the applicability of the classical mandala to picoplankton groups, we used a multidisciplinary approach including specifically designed field observations supported by remote sensing, database analyses, and modeling and laboratory chemostat experiments. Simultaneous estimates of nitrate diffusive fluxes, derived from microturbulence observations, and picoplankton abundance collected in more than 200 stations, spanning widely different hydrographic regimes, showed that the contribution of eukaryotes to picoautotrophic biomass increases with nutrient supply, whereas that of picocyanobacteria shows the opposite trend. These findings were supported by laboratory and modeling chemostat experiments that reproduced the competitive dynamics between picoeukaryote sand picocyanobacteria as a function of changing nutrient supply. Our results indicate that nutrient supply controls the distribution of picoplankton functional groups in the ocean, further supporting the model proposed by Margalef.Spanish Governmen
Small-scale turbulence and mixing: energy fluxes in stratified lakes
Aim: in this article, we describe the energetics and dynamics of small-scale turbulence and mixing in stratified water bodies. Turbulence in the stratified interior of a water body arises when wind-driven internal motions become unstable and transfer their energy towards smaller-scale motions. The resulting turbulent mixing drives weak but continuous vertical fluxes of heat, nutrients and gases, as well as any dissolved or particulate substances, playing a critical role in ecological processes. We outline the fundamental concepts describing the dynamics of stratified turbulence and provide practical guidelines for measuring turbulent fluxes in the field. Main concepts: we first take an energy balance approach and trace the energy pathways from wind-driven basin-scale motions to small-scale turbulent mixing and compare the energy available for mixing with that stored in stratification. We then describe how turbulent eddies operate at small-scales to generate net vertical downgradient fluxes, the intensity of which can be quantified in terms of a turbulent diffusivity parameter. Finally, we illustrate the mixing mechanisms operating in different compartments of stratified water-bodies. Main methods: the most commonly used methods to estimate vertical turbulent fluxes are described. These include direct flux measurements using the eddy-covariance technique, microstructure measurements, and basin-scale budget approaches. Conclusion: turbulent mixing is a highly episodic phenomenon, challenging to measure with sufficient spatio-temporal resolution. Notwithstanding, a consistent picture is emerging from decades of research showing that a small fraction (close to 0.3%) of the available wind energy is invested in mixing the stratified interior of the water body. This energy is too low to cause major changes in stratification but strong enough to drive ecologically-relevant fluxes. Despite this progress, many open questions remain regarding the spatio-temporal distribution of turbulent fluxes in different systems and their evolution under climate warming, which is altering the stratification dynamics in lakes globally
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