The composition and photochemical transformations of dissolved organic matter (DOM) in the northern North Pacific Ocean were investigated at the molecular level using ultrahigh resolution mass ...spectrometry and geochemical tracers. Analyses included vertical profiles and experiments in which deep sea DOM was exposed to sunlight and incubated in the dark. The composition of the deep sea DOM was found to be approximately uniform and enriched with highly unsaturated compounds, with highly aromatic compounds, and with polycyclic aromatics. Surface DOM had a significantly different composition, being enriched with both highly unsaturated and with unsaturated aliphatic compounds potentially due to the addition of photodegradation products and phytoplankton inputs. Deep sea DOM composition is transformed by photoreactions, becoming more similar to surface DOM. The influence of photochemistry extends beyond the photic zone, presumably because of vertical export of DOM previously modified at the surface.
Key Points
Composition of deep North Pacific DOM is approximately uniform to lowest order
Photoreactions transform deep sea DOM, making it more similar to surface DOM
Influence of photochemical transformations extends beyond the photic zone
During theDeepwater Horizonoil well blowout in the Gulf of Mexico, the application of 7 million liters of chemical dispersants aimed to stimulate microbial crude oil degradation by increasing the ...bioavailability of oil compounds. However, the effects of dispersants on oil biodegradation rates are debated. In laboratory experiments, we simulated environmental conditions comparable to the hydrocarbon-rich, 1,100 m deep plume that formed during theDeepwater Horizondischarge. The presence of dispersant significantly altered the microbial community composition through selection for potential dispersant-degradingColwellia,which also bloomed in situ in Gulf deep waters during the discharge. In contrast, oil addition to deepwater samples in the absence of dispersant stimulated growth of natural hydrocarbon-degradingMarinobacter.In these deepwater microcosm experiments, dispersants did not enhance heterotrophic microbial activity or hydrocarbon oxidation rates. An experiment with surface seawater from an anthropogenically derived oil slick corroborated the deepwater microcosm results as inhibition of hydrocarbon turnover was observed in the presence of dispersants, suggesting that the microcosm findings are broadly applicable across marine habitats. Extrapolating this comprehensive dataset to real world scenarios questions whether dispersants stimulate microbial oil degradation in deep ocean waters and instead highlights that dispersants can exert a negative effect on microbial hydrocarbon degradation rates.
The California Current System is characterized by upwelling and rich mesoscale eddy activity. Cyclonic eddies generally pinch off from meanders in the California Current, potentially trapping ...upwelled water along the coast and transporting it offshore. Here, we use satellite-derived measurements of particulate organic carbon (POC) as a tracer of coastal water to show that cyclones located offshore that were generated near the coast contain higher carbon concentrations in their interior than cyclones of the same amplitude generated offshore. This indicates that eddies are in fact trapping and transporting coastal water offshore, resulting in an offshore POC enrichment of 20.9 ± 11 Gg year
. This POC enrichment due to the coastally-generated eddies extends for 1000 km from shore. This analysis provides large-scale observational-based evidence that eddies play a quantitatively important role in the offshore transport of coastal water, substantially widening the area influenced by highly productive upwelled waters in the California Current System.
An increasing number of studies have aimed to clarify the factors leading human groups to prioritize the use of some woody plant species compared to others. Some of these studies have tested the ...apparency hypothesis in aiming to understand this phenomenon. According to the apparency hypothesis, the most commonly available local plant species on a forest path are the most useful to that local human population. However, the sparse and diverse nature of the results from studies investigating the factors that influence human exploitation of plant resources motivated us to perform a meta-analysis on the apparency hypothesis. We searched in the main databases (Scopus, ScienceDirect, Google Scholar, and Scielo) for studies that correlated the environmental availability of woody species (estimated through vegetation parameters) with the degree of importance of such species to the local human population (estimated by means of the use value index). Overall, this meta-analysis supported the apparency hypothesis, although we also found high levels of heterogeneity in these studies. When the distinct uses of woody flora were considered separately, we found that local species availability is important for fuelwood (firewood and charcoal) and construction (houses, fences, etc.) purposes but does not explain medicinal and technological (object manufacture) plant use. We found no important differences in correlation values between the degree of species importance for people and the different vegetation parameters, although correlations are slightly higher for the dominance and importance value index. Our findings suggest that the exploitation of woody flora is influenced by local availability.
The Amazon River is a large source of terrigenous dissolved organic carbon (tDOC) to the Atlantic Ocean. The fate of this tDOC in the ocean remains unclear despite its importance to the global carbon ...cycle. Here, we used two decades of satellite ocean color to describe variability in tDOC in the Amazon River plume. Our analyses showed that tDOC distribution has a distinct seasonal pattern, reaching northwest toward the Caribbean during high discharge periods, and moving eastward entrained in the North Brazil Current retroflection during low discharge periods. Elevated tDOC content extended beyond the shelfbreak in all months of the year, suggesting that cross‐shelf carbon transport occurs year‐round. Maximum variability was found at the plume core, where seasonality accounted for 40% of the total variance, while interannual variability accounted for 15% of the variance. Our results revealed a seasonal pattern in tDOC removal over the shelf with increased consumption in May when river discharge is high. Anomalies in tDOC removal over the shelf with respect to the seasonal cycle were significantly correlated with anomalies in tDOC concentration offshore of the shelfbreak with a lag of 30–40 days, so that anomalously high inshore tDOC removal was associated with anomalously low tDOC content offshore. This suggests that variability in the offshore transport of tDOC in the Amazon River plume is modulated by interannual changes in tDOC removal over the shelf.
Plain Language Summary
The Amazon River is an important source of dissolved organic carbon (DOC) of terrigenous origin to the Atlantic Ocean. It is often difficult to characterize the distribution and variability of this material in the ocean due to the lack of high resolution in situ observations spanning long periods of time. Here, we used satellite observations of terrigenous DOC (tDOC) to describe the behavior of the plume at seasonal and interannual scales. Our analyses showed that enhanced tDOC content is observed offshore of the continental margin during all months of the year, extending toward the Caribbean during high river discharge conditions and eastward during low discharge conditions associated with variability in the large‐scale circulation. Our results also revealed that tDOC removal over the shelf varies seasonally, being enhanced earlier in the year when river discharge is high. Anomalies in tDOC removal over the shelf are significantly correlated with anomalies in tDOC content offshore with a lag of 30–40 days, suggesting that the offshore transport of carbon is modulated by tDOC consumption over the shelf.
Key Points
Satellite ocean color was used to describe variability in terrigenous dissolved organic carbon (tDOC) in the Amazon River plume
tDOC degradation and/or flocculation over the shelf increase early in the year when river discharge also increases
Interannual variability in tDOC content off the continental margin is related to variability in tDOC removal over the shelf
Coastal waters off west Greenland are strongly influenced by the input of low salinity water from the Arctic and from meltwater from the Greenland Ice Sheet. Changes in freshwater content in the ...region can play an important role in stratification, circulation, and primary production; however, investigating salinity variability in the region is challenging because in situ observations are sparse. Here, we used satellite observations of sea surface salinity (SSS) from the Soil Moisture and Ocean Salinity mission produced by LOCEAN and by the Barcelona Expert Center (SMOS LOCEAN and SMOS BEC) and from the Soil Moisture Active Passive mission produced by the Jet Propulsion Laboratory (SMAP JPL) as well as by Remote Sensing Systems (SMAP RSS) to investigate how variability in a narrow coastal band off west Greenland is captured by these different products. Our analyses revealed that the various satellite SSS products capture the seasonal freshening off west Greenland from late spring to early fall. The magnitudes of the freshening and of coastal salinity gradients vary between the products however, being attenuated compared to historical in situ observations in most cases. The seasonal freshening off southwest Greenland is intensified in SMAP JPL and SMOS LOCEAN near the mouth of fjords characterized by large inputs of meltwater near the surface, which suggests an influence of meltwater from the Greenland Ice Sheet. Synoptic observations from 2012 following large ice sheet melting revealed good agreement with the spatial scale of freshening observed with in situ and SMOS LOCEAN data. Our analyses indicate that satellite SSS can capture the influence of meltwater input and associated freshwater plumes off coastal west Greenland, but those representations differ between products.
The Amazon River is a major source of terrestrially-derived dissolved organic matter (DOM) to the Atlantic Ocean. Measurements of dissolved organic carbon (DOC) and its optical properties (absorbance ...and fluorescence) were made in the water within and adjacent to the Amazon River plume during two cruises conducted at periods of low (September–October/2011) and high (July/2012) river discharges. Four fluorescent components were resolved by excitation emission matrix fluoresces combined with parallel factor analysis (EEM-PARAFAC) as two terrestrial humic-like components, one marine humic-like component, and one autochthonous protein-like component. The distribution and dynamics of optically active fraction of DOM were largely controlled by physical mixing along the Amazon River-ocean continuum, with the exception of the protein-like component whose distribution was driven by factors other than dilution. Principal component analysis (PCA) enabled differentiation of water samples following gradient of terrestrial signatures and distinction of source materials associated with different fluorescent components. On board water incubation experiments with photochemical and microbial alterations revealed that photochemistry was primarily responsible for the removal of optically active fraction of DOM whereas the contribution of microbial transformation to this fraction was minor. Results from this work will provide a better understanding of DOM compositions and transformations in a globally significant river plume-ocean continuum.
•Four fluorescent components are revealed in the Amazon River plume and ocean waters.•Physical mixing was the dominant driver for CDOM and humic-like fluorescent DOM.•Protein-like fluorescent component was controlled by factors other than dilution.•Photochemistry is more effective and efficient in degrading CDOM than microbes.
Many environmental samples contain complex mixtures of organic compounds with different sources, polarities and reactivities. This study reports a method for the analysis of both polar/water-soluble ...and apolar organic compounds in several kinds of environmental samples. The analytical method consists of extraction with a mixture of dichloromethane:methanol (2:1, v/v), silylation using BSTFA (
N,O-bis-(trimethylsilyl)trifluoroacetamide) and analysis by gas chromatography–mass spectrometry (GC–MS), a common device in chemical and environmental laboratories. Fifty individual sugar standards, including monosaccharides, sugar alcohols, anhydrosugars, disaccharides and trisaccharides, were analyzed for the determination of their fragmentation patterns and retention times. Recoveries (at three concentrations) and limits of detection (LOD) were determined for a standard mixture containing glucose (monosaccharide), sorbitol (sugar alcohol), levoglucosan (anhydrosugar) and sucrose (disaccharide), and they varied from 68 to 119% and 130 to 360
ng
mL
−1, respectively. The method was used for the analysis of aerosol particle, soil and sediment samples, and demonstrated its feasibility in detecting not only several important environmental sugars (e.g., glucose, fructose, inositol, mannitol, sorbitol, levoglucosan, sucrose, mycose), but also a large range of organic compound classes from other polar components (e.g., dicarboxylic acids) to apolar compounds such as
n-alkanes. Therefore, the analytical method presented here demonstrated its usefulness for a better understanding of sources and transport of various organic compounds in different environmental compartments.
Coastal oceans link terrestrial and marine carbon cycles. Yet, carbon sources and sinks in these biomes remain poorly understood. Here, we explore the dynamics of dissolved organic matter (DOM) along ...the Amazon River-to-ocean continuum from the lower mainstem at Óbidos to the open ocean of the western tropical North Atlantic. We molecularly characterized DOM via ultrahigh-resolution Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS), determined DOM stable carbon isotopes, and interpreted the data in the context of bacterial abundance and production, phytoplankton biomass and composition. Multivariate analysis revealed that the DOM molecular variability in the plume was mainly influenced by the input of terrigenous DOM. Incubation experiments with water from close to the river mouth showed that photo- and bio-degradation preferentially removed 13C-depleted and 13C-enriched terrigenous DOM, respectively. However, there was no significant quantitative change in the total amount of dissolved organic carbon (DOC) over five days. This result suggests that most of the reactive DOM had already been bio-degraded upstream within the river and that photo-degradation was diminished in the turbid plume close to the river mouth. Terrigenous DOM therefore appeared to be relatively non-reactive nearshore. In the less turbid offshore plume, enhanced light penetration stimulated growth of phytoplankton and increased bacterial production. Although marine DOM compounds became relatively enriched, bulk DOC concentrations were 9 to 30% below levels expected from conservative mixing of river and ocean endmembers suggesting that quantitative removal of terrigenous DOM was not compensated by marine DOM production. We propose that removal of terrigenous DOM in the outer plume may be enhanced by (i) bio-degradation primed by reactive algal DOM, (ii) photo-degradation, which may further break down DOM into more bio-available forms, and possibly (iii) sorption of DOM to sinking particles.
•DOM was characterized using ultrahigh-resolution mass spectrometry from the lower Amazon River to the Caribbean Sea•DOM molecular variability in the plume was primarily influenced by terrigenous river DOM input•In the intermediate plume, phytoplankton biomass and bacterial activity were significantly correlated to DOM composition•9 - 30% of initial DOC was lost along the plume: removal of terrigenous DOM was not compensated by in situ new production•Molecular DOM patterns suggest bio- and photo-degradation as DOM sinks
The offshore transport of Greenland coastal waters influenced by freshwater input from ice sheet melting during summer plays an important role in ocean circulation and biological processes in the ...Labrador Sea. Many previous studies over the last decade have investigated shelfbreak transport processes in the region, primarily using ocean model simulations. Here, we use 27 years of surface geostrophic velocity observations from satellite altimetry, modified to include Ekman dynamics based on atmospheric reanalysis, and virtual particle releases to investigate seasonal and interannual variability in transport of coastal water in the Labrador Sea. Two sets of tracking experiments were pursued, one using geostrophic velocities only, and another using total velocities including the wind effect. Our analysis revealed substantial seasonal variability, even when only geostrophic velocities were considered. Water from coastal southwest Greenland is generally transported northward into Baffin Bay, although westward transport off the west Greenland shelf increases in fall and winter due to winds. Westward offshore transport is increased for water from southeast Greenland so that, in some years, water originating near the east Greenland coast during summer can be transported into the central Labrador Sea and the convection region. When wind forcing is considered, long-term trends suggest decreasing transport of Greenland coastal water during the melting season toward Baffin Bay, and increasing transport into the interior of the Labrador Sea for water originating from southeast Greenland during summer, where it could potentially influence water column stability. Future studies using higher-resolution velocity observations are needed to capture the role of submesoscale variability in transport pathways in the Labrador Sea.
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