Application of multi-regression machine learning algorithms to solve ocean water mass mixing in the Atlantic Ocean

The distribution of any non-conservative variable in the deep open ocean results from the circulation and mixing of water masses (WMs) of contrasting origin and from the initial preformed composition, modified during ongoing simultaneous biological and/or geochemical processes. Estimating the contribution of the WMs composing a sample is useful to trace the distribution of each water mass and to quantitatively separate the physical (mixing) and biogeochemical components of the variability of any, non- conservative variable (e.g., dissolved organic carbon, prokaryote biomass) in the ocean. Other than potential temperature and salinity, additional semi-conservative and non-conservative variables have been used to solve the mixing of more than three water masses using Optimum Multi-Parameter (OMP) approaches. Successful application of an OMP analysis requires knowledge of the characteristics of the water masses in their source regions as well as their circulation and mixing patterns. Here, we propose the application of multi-regression machine learning models to solve ocean water mass mixing. The models tested were trained using the solutions from OMP analyses previously applied to samples from cruises in the Atlantic Ocean. Extremely Randomized Trees algorithm yielded the highest score (R2 = 0.9931; mse = 0.000227). Our model allows solving the mixing of water masses in the Atlantic Ocean using potential temperature, salinity, latitude, longitude and depth. Therefore, basic hydrographic data collected during typical research cruises or autonomous systems can be used as input variables and provide results in real time. The model can be fed with new solutions from compatible OMP analyses as well as with new water masses not previously considered in it. Our tool will provide knowledge on water mass composition and distribution to a broader community of marine scientists not specialized in OMP analysis and/or in the oceanography of the studied area. This will allow a quantitative analysis of the effect of water mass mixing on the variables or processes under study

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

Penetration of ultraviolet‐B radiation in oligotrophic regions of the oceans during the Malaspina 2010 expedition

Few studies have investigated ultraviolet (UV) radiation in the open ocean besides its harmful effects on organisms and influence on biogeochemical processes. Here, we assessed UV attenuation, with particular focus on UV‐B, across the (sub)tropical ocean during the Malaspina 2010 Circumnavigation. Vertical UV radiometer profiles together with Chl‐ a concentration, and UV absorption by CDOM ( a CDOM ( λ )) and by suspended particulate matter ( a p ( λ )) were measured at 117 stations. At photosynthetically active radiation (PAR) and across UV‐A and UV‐B wavelengths, the lowest downwelling attenuation coefficients ( K d ) during the expedition were recorded in ultra‐oligotrophic regions at 5°–15°S (mean K d (305 nm): 0.129 m −1 , mean K d (313 nm): 0.107 m −1 ) in the Indian and South Pacific Oceans. The waters here were comparatively more transparent than at 5°–15°N (mean K d (305 nm): 0.239 m −1 , mean K d (313 nm): 0.181 m −1 ) where equatorial upwelling occurs. K d was highest near the Costa Rica Dome ( K d (313 nm): 0.226 m −1 ) and at the confluence of the Benguela and Agulhas currents ( K d (313 nm): 0.251 m −1 ). The contribution of a p ( λ ) toward nonwater absorption ( a nw ( λ )) was significantly lower at 305 nm than at 313 and 320 nm, suggesting the contribution of absorption by detritus and phytoplankton particles decreases compared with that of CDOM absorption as UV‐B wavelength decreases. Both a CDOM ( λ ) and a p ( λ ) at UV‐B wavelengths were lowest in the Indian Ocean whereas K d was lowest in the South Pacific. This finding emphasizes that other factors besides absorption, such as scattering by reflective phytoplankton or inorganic particles, strongly influence UV‐B attenuation in open ocean waters.Plain Language Summary: We assessed water transparency to UV‐B radiation across the tropical and subtropical ocean as part of the Malaspina 2010 Expedition. UV‐B radiometer profiles, Chlorophyll‐a, and UV‐B absorption by organic matter and suspended particles were measured at 117 stations. The most UV‐B transparent waters were found in regions where nutrients are extremely low, particularly at 5°–15°S in the Indian and South Pacific Oceans. Here, ocean waters were considerably more transparent than 5°–15°N, which suggests that at a given depth southern hemisphere marine organisms experience higher UV‐B exposure than their northern counterparts. The least UV‐B transparent waters were near the Costa Rica Dome and at the confluence of the Benguela and Agulhas currents. UV‐B absorption by organic matter and suspended particles, and Chl‐ a concentration were lowest in the Indian Ocean. The contribution of suspended particles toward nonwater absorption was significantly lower at 305 nm than at 313 and 320 nm, and higher in the clearest waters of the Indian Ocean than in the Subtropical South Pacific. Absorptions by organic matter and suspended particles were lowest in the Indian Ocean whereas UV‐B attenuation was lowest in the South Pacific, which highlights the complex relationship between optical properties in the UV‐B spectrum.Ministerio de Ciencia e Innovación | Ref. CSD2008-00077King Abdullah University of Science and Technology | Ref. BAS/1/1072-01-0

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

Prokaryotic capability to use organic substrates across the global tropical and subtropical ocean

Prokaryotes play a fundamental role in decomposing organic matter in the ocean, but little is known about how microbial metabolic capabilities vary at the global ocean scale and what are the drivers causing this variation. We aimed at obtaining the first global exploration of the functional capabilities of prokaryotes in the ocean, with emphasis on the under-sampled meso- and bathypelagic layers. We explored the potential utilization of 95 carbon sources with Biolog GN2 plates® in 441 prokaryotic communities sampled from surface to bathypelagic waters (down to 4,000 m) at 111 stations distributed across the tropical and subtropical Atlantic, Indian, and Pacific oceans. The resulting metabolic profiles were compared with biological and physico-chemical properties such as fluorescent dissolved organic matter (DOM) or temperature. The relative use of the individual substrates was remarkably consistent across oceanic regions and layers, and only the Equatorial Pacific Ocean showed a different metabolic structure. When grouping substrates by categories, we observed some vertical variations, such as an increased relative utilization of polymers in bathypelagic layers or a higher relative use of P-compounds or amino acids in the surface ocean. The increased relative use of polymers with depth, together with the increases in humic DOM, suggest that deep ocean communities have the capability to process complex DOM. Overall, the main identified driver of the metabolic structure of ocean prokaryotic communities was temperature. Our results represent the first global depiction of the potential use of a variety of carbon sources by prokaryotic communities across the tropical and the subtropical ocean and show that acetic acid clearly emerges as one of the most widely potentially used carbon sources in the ocean

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

Water mass age and ageing driving chromophoric dissolved organic matter in the dark global ocean

Research articleThe omnipresence of chromophoric dissolved organic matter (CDOM) in the open ocean enables its use as a tracer for biochemical processes throughout the global overturning circulation. We made an inventory of CDOM optical properties, ideal water age (τ), and apparent oxygen utilization (AOU) along the Atlantic, Indian, and Pacific Ocean waters sampled during the Malaspina 2010 expedition. A water mass analysis was applied to obtain intrinsic, hereinafter archetypal, values of τ, AOU, oxygen utilization rate (OUR), and CDOM absorption coefficients, spectral slopes and quantum yield for each one of the 22 water types intercepted during this circumnavigation. Archetypal values of AOU and OUR have been used to trace the differential influence of water mass aging and aging rates, respectively, on CDOM variables. Whereas the absorption coefficient at 325nm (a325) and the fluorescence quantum yield at 340nm (Φ340) increased, the spectral slope over the wavelength range 275–295nm (S275–295) and the ratio of spectral slopes over the ranges 275–295nm and 350–400nm (SR) decreased significantly with water mass aging (AOU). Combination of the slope of the linear regression between archetypal AOU and a325 with the estimated global OUR allowed us to obtain a CDOM turnover time of 634 ± 120 years, which exceeds the flushing time of the dark ocean (>200 m) by 46%. This positive relationship supports the assumption of in situ production and accumulation of CDOM as a by-product of microbial metabolism as water masses turn older. Furthermore, our data evidence that global-scale CDOM quantity (a325) is more dependent on aging (AOU), whereas CDOM quality (S275–295, SR, Φ340) is more dependent on aging rate (OUR).Versión del editor4,785

Solid phase extraction of ocean dissolved organic matter with PPL cartridges: efficiency and selectivity

Our current knowledge of the chemical composition of ocean dissolved organic matter (DOM) is limited, mainly because of its extreme molecular diversity, low concentration of individual compounds and the elevated ionic strength of ocean waters. As a result, many analytical methods require a previous extraction step. The efficiency and selectivity of the extraction method defines the representativeness of the extracted DOM fraction. Nowadays, the most widespread procedure for concentrating DOM is solid phase extraction (SPE) using styrene divinyl benzene polymer cartridges (PPL). Here, we investigate the effect of SPE-PPL on DOM elemental and optical properties to assess the efficiency and selectivity of this extraction method on water samples from the main intermediate and deep water masses of Arctic, Mediterranean and Antarctic origin present in the Cape Vert Frontal Zone (CVFZ, NW Africa). Furthermore, North and South Atlantic Central waters converge in this area and coastal DOM is injected by the giant upwelling filament of Cape Blanc. On one side, the colored fraction of DOM (CDOM) presented extraction efficiencies comparable to that of the bulk dissolved organic carbon (DOC), but decreased significantly with increasing wavelength, suggesting an affinity of PPL cartridges for low molecular weight organic compounds. While the protein-like fluorescent fraction of DOM (FDOM) was also extracted with the same efficiency than DOC, the extraction efficiency of the humic-like fraction was comparatively much higher. On the other side, dissolved organic nitrogen (DON) extraction efficiencies were about half that of DOC. These contrasting extraction efficiencies of the different DOM pools indicated that the extracts were enriched in N-poor, low molecular weight and recalcitrant DOM, therefore showing less variability than the corresponding bulk DOM. Furthermore, DOC, DON, CDOM and FDOM extracted were not homogeneous through the water column but displayed certain significant differences among water masses in both efficiency and selectivity

A multidisciplinary approach to identify priority areas for the monitoring of a vulnerable family of fishes in Spanish Marine National Parks

Background Syngnathid fishes (Actinopterygii, Syngnathidae) are flagship species strongly associated with seaweed and seagrass habitats. Seahorses and pipefishes are highly vulnerable to anthropogenic and environmental disturbances, but most species are currently Data Deficient according to the IUCN (2019), requiring more biological and ecological research. This study provides the first insights into syngnathid populations in the two marine Spanish National Parks (PNIA—Atlantic- and PNAC—Mediterranean). Fishes were collected periodically, marked, morphologically identified, analysed for size, weight, sex and sexual maturity, and sampled for stable isotope and genetic identification. Due the scarcity of previous information, habitat characteristics were also assessed in PNIA. Results Syngnathid diversity and abundance were low, with two species identified in PNIA (Hippocampus guttulatus and Syngnathus acus) and four in PNAC (S. abaster, S. acus, S. typhle and Nerophis maculatus). Syngnathids from both National Parks (NP) differed isotopically, with much lower δ15N in PNAC than in PNIA. The dominant species were S. abaster in PNAC and S. acus in PNIA. Syngnathids preferred less exposed sites in macroalgal assemblages in PNIA and Cymodocea meadows in PNAC. The occurrence of very large specimens, the absence of small-medium sizes and the isotopic comparison with a nearby population suggest that the population of Syngnathus acus (the dominant syngnathid in PNIA) mainly comprised breeders that migrate seasonally. Mitochondrial cytochrome b sequence variants were detected for H. guttulatus, S. acus, and S. abaster, and a novel 16S rDNA haplotype was obtained in N. maculatus. Our data suggest the presence of a cryptic divergent mitochondrial lineage of Syngnathus abaster species in PNAC. Conclusions This is the first multidisciplinary approach to the study of syngnathids in Spanish marine NPs. Habitat preferences and population characteristics in both NPs differed. Further studies are needed to assess the occurrence of a species complex for S. abaster, discarding potential misidentifications of genus Syngnathus in PNAC, and evaluate migratory events in PNIA. We propose several preferential sites in both NPs for future monitoring of syngnathid populations and some recommendations for their conservation.Postprin