Mar Nieto-Cid

A 29-year-long time series (1990–2018) of phyto- and zooplankton abundance and composition is analyzed to uncover regime shifts related to climate and local oceanography variability. At least two major shifts were identified: one between 1997 and 1998, affecting zooplankton group abundance, phytoplankton species assemblages and climatic series, and a second one between 2001 and 2002, affecting microzooplankton group abundance, mesozooplankton species assemblages and local hydrographic series. Upwelling variability was relatively less important than other climatic or local oceanographic variables for the definition of the regimes. Climate-related regimes were influenced by the dominance of cold and dry (1990–1997) vs. warm and wet (1998–2018) periods, and characterized by shifts from low to high life trait diversity in phytoplankton assemblages, and from low to high meroplankton dominance for mesozooplankton. Regimes related to local oceanography were defined by the shift from relatively low (1990–2001) to high (2002–2018) concentrations of nutrients provided by remineralization (or continental inputs) and biological production, and shifts from a low to high abundance of microzooplankton, and from a low to high trait diversity of mesozooplankton species assemblages. These results align with similar shifts described around the same time for most regions of the NE Atlantic. This study points out the different effects of large-scale vs. local environmental variations in shaping plankton assemblages at multiannual time scales.

Dissolved organic matter (DOM) is produced in the surface and exported towards the deep ocean, adding ∼2 PgC yr⁻¹ to the global carbon export. Due to its central role in the Meridional Overturning Circulation (MOC), 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 MOC exports >15.9 TgN yr⁻¹ of DON downwards 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 yr⁻¹. The low C:N molar ratio of DOM production (7.4 ± 4.1) and the relatively short‐transit times in the eSPNA (3 ± 1 yr) suggest that local biogeochemical transformations result in the injection of fresh bioavailable DOM to the deep ocean.

Despite of the major role ascribed to marine dissolved organic matter (DOM) in the global carbon cycle, the reactivity of this pool in the dark ocean is still poorly understood. Present hypotheses, posed within the size-reactivity continuum (SRC) and the microbial carbon pump (MCP) conceptual frameworks, need further empirical support. Here, we provide field evidence of the soundness of the SRC model. We sampled the high salinity core-of-flow of the Levantine Intermediate Water along its westward route through the entire Mediterranean Sea. At selected sites, DOM was size-fractionated in apparent high (aHMW) and low (aLMW) molecular weight fractions using an efficient ultrafiltration cell. A percentage decline of the aHMW DOM from 68–76% to 40–55% was observed from the Levantine Sea to the Strait of Gibraltar in parallel with increasing apparent oxygen utilization (AOU). DOM mineralization accounted for 30 ± 3% of the AOU, being the aHMW fraction solely responsible for this consumption, verifying the SRC model in the field. We also demonstrate that, in parallel to this aHMW DOM consumption, fluorescent humic-like substances accumulate in both fractions and protein-like substances decline in the aLMW fraction, thus indicating that not only size matters and providing field support to the MCP model.

The molecular composition of marine dissolved organic matter (DOM) is still poorly understood, particularly in the Mediterranean Sea. In this work, DOM from the open Mediterranean Sea and the adjacent Northeast Atlantic Ocean was isolated by solid‐phase extraction (SPE‐DOM) and molecularly characterized using Fourier‐transform ion cyclotron resonance mass spectrometry. We assessed the gradual reworking of the SPE‐DOM transported by the shallow overturning circulation of the Mediterranean Sea by following the increase in molecular weight (+20 Da), oxygenation (+5%), degradation index (Ideg +22%), and the proportional decrease of unsaturated aliphatic compounds (+34%) along the Levantine Intermediate Water. This reworked SPE‐DOM that leaves the Mediterranean Sea through the Strait of Gibraltar strongly contrasts with the fresh material transported by the inflow of Atlantic water (Ideg −25%). In the deep eastern and western overturning cells, the molecular composition of the deep waters varied according to their area and/or time of formation. SPE‐DOM of the waters formed in the Aegean Sea during the Eastern Mediterranean Transient (EMT) was more processed than the DOM in pre‐EMT waters formed in the Adriatic Sea (molecular weight and the proportion of unsaturated aliphatic compounds were increased by 5 Da and 9%, respectively). Furthermore, pre‐EMT waters contain more reworked SPE‐DOM (Ideg +7%) than post‐EMT waters formed also in the Adriatic Sea. In summary, our study shows that the Mediterranean Sea constitutes a laboratory basin where degradation processes and diagenetic transformations of DOM can be observed on close spatial and temporal scales.

Taurine (Tau), an amino acid‐like compound, is present in almost all marine metazoans including crustacean zooplankton. It plays an important physiological role in these organisms and is released into the ambient water throughout their life cycle. However, limited information is available on the release rates by marine organisms, the concentrations and turnover of Tau in the ocean. We determined dissolved free Tau concentrations throughout the water column and its release by abundant crustacean mesozooplankton at two open ocean sites (Gulf of Alaska and North Atlantic). At both locations, the concentrations of dissolved free Tau were in the low nM range (up to 15.7 nM) in epipelagic waters, declining sharply in the mesopelagic to about 0.2 nM and remaining fairly stable throughout the bathypelagic waters. Pacific amphipod–copepod assemblages exhibited lower dissolved free Tau release rates per unit biomass (0.8 ± 0.4 μmol g⁻¹ C‐biomass h⁻¹) than Atlantic copepods (ranging between 1.3 ± 0.4 μmol g⁻¹ C‐biomass h⁻¹ and 9.5 ± 2.1 μmol g⁻¹ C‐biomass h⁻¹), in agreement with the well‐documented inverse relationship between biomass‐normalized excretion rates and body size. Our results indicate that crustacean zooplankton might contribute significantly to the dissolved organic matter flux in marine ecosystems via dissolved free Tau release. Based on the release rates and assuming steady state dissolved free Tau concentrations, turnover times of dissolved free Tau range from 0.05 d to 2.3 d in the upper water column and are therefore similar to those of dissolved free amino acids. This rapid turnover indicates that dissolved free Tau is efficiently consumed in oceanic waters, most likely by heterotrophic bacteria.

Dissolved organic matter (DOM) plays a key role in global biogeochemical cycles and experiences changes in molecular composition as it undergoes processing. In the semi-closed basins of the oligotrophic Mediterranean Sea, these gradual molecular modifications can be observed in close proximity. In order to extend the spatial resolution of information on DOM molecular composition available from ultrahigh resolution mass spectrometry in this area, we relate this data to optical (fluorescence and absorption spectroscopy) measurements. Covariance between molecular formulae signal intensities and carbon-specific fluorescence intensities was examined by means of Spearman’s rank correlations. Fifty two per cent of the assigned molecular formulae were associated with at least one optical parameter, accounting for 70% of the total mass spectrum signal intensity. Furthermore, we obtained significant multiple linear regressions between optical and intensity-weighted molecular indices. The resulting regression equations were used to estimate molecular parameters such as the double bond equivalent, degradation state and occurrence of unsaturated aliphatic compounds from optical measurements. The statistical linkages between DOM molecular and optical properties illustrate that the simple, rapid and cost-efficient optical spectroscopic measurements provide valuable proxy information on the molecular composition of open ocean marine DOM.