The composition and distribution of semi-labile dissolved  organic matter across the southwest Pacific

Abstract. The distribution and dynamics of dissolved organic carbon (DOC) and dissolved combined neutral sugars (DCNS) were studied across an increasing oligotrophic gradient (18 to 22∘ S latitude) in the tropical South Pacific Ocean, spanning from the Melanesian Archipelago (MA) area to the western part of the South Pacific gyre (WGY), in austral summer as a part of the OUTPACE project. Our results show that DOC and DCNS concentrations exhibited no statistical differences between the MA and WGY areas (0–200 m: 47–81 µM C for DOC and 0.2-4.2 µM C for DCNS). However, due to a deepening of the euphotic zone, a deeper penetration of DOC was noticeable at 150 m of depth at the WGY area. Excess DOC (DOCEX) was determined as the difference between surface and deep-sea DOC values, and euphotic zone integrated stocks of both DOC and DOCEX were higher in the WGY than the MA area. Considering DOCEX as representative of semi-labile DOC (DOCSL), its residence time was calculated as the ratio of DOCSL to bacterial carbon demand (BCD). This residence time was 176±43 days (n=3) in the WGY area, about 3 times longer than in the MA area (Tr=51±13 days, n=8), suggesting an accumulation of semi-labile dissolved organic matter (DOM) in the surface waters of WGY. Average epipelagic (0–200 m) DCNS yields (DCNS × DOC−1) based on volumetric data were roughly similar in both areas, accounting for ∼2.8 % of DOC. DCNS exhibited a longer residence time in WGY (Tr=91±41 days, n=3) than in MA (Tr=31±10 days, n=8), further suggesting that this DCNS pool persists longer in the surface waters of the WGY. The accumulation of DOCEX in the surface waters of WGY is probably due to very slow bacterial degradation due to nutrient and/or energy limitation of heterotrophic prokaryotes, indicating that biologically produced DOC can be stored in the euphotic layer of the South Pacific gyre for a long period.

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The composition and distribution of labile dissolved organic matter across the south west Pacific

10.5194/bg-2018-158 ◽

2018 ◽

Cited By ~ 1

Author(s):

Christos Panagiotopoulos ◽

Mireille Pujo Pay ◽

Mar Benavides ◽

France Van Wambeke ◽

Richard Sempéré

Keyword(s):

Organic Matter ◽

Residence Time ◽

Surface Waters ◽

Austral Summer ◽

Bacterial Degradation ◽

The South ◽

Long Period ◽

South West ◽

Neutral Sugars ◽

South Pacific Gyre

Abstract. The distribution and dynamics of dissolved organic carbon (DOC) and dissolved combined neutral sugars (DCNS) were studied across an increasing oligotrophic gradient (−18 to −22° N latitude) spanning from the Melanesian Archipelago (MA) area to the western part of the south Pacific gyre (WGY), in austral summer, as a part of the OUTPACE project. Our results showed DOC and DCNS concentrations exhibited little differences among the MA and WGY areas (0–200 m: 55–78 µMC for DOC and 1.5–2 µMC for DCNS), however, a deeper penetration of DOC was noticeable at 150 m depth at the WGY area. This finding was also reflected to the DOC and semi-labile DOC (DOCSL) stocks values (integration 0–200 m) for which we found higher values in the WGY than the MA area. The high excess DOCSL measured in WGY was characterized by a high residence time (130 ± 31 days (n = 3)) about three times higher than the MA region (Tr = 40 ± 7 days (n = 8)) suggesting an accumulation of the semi-labile DOM in the surface waters of WGY. DCNS yields (DCNS-C x DOC−1 %) also followed this pattern with higher values recorded in the WGY (3.2 ± 1.3 %) than MA (2.8 ± 0.8 %) highlighting the presence of semi-labile dissolved organic material (DOM) in the form of polysaccharides. These polysaccharides also exhibited a higher residence time in WGY (Tr = 8 ± 4 days, n = 3) than in MA (Tr = 3 ± 1days, n = 8) suggesting that this DCNS pool persists longer in the surface waters of the WGY. The accumulation of DOCSL in the surface waters of WGY is probably due to the very slow bacterial degradation due to nutrient limitation indicating that biologically produced DOC can be stored in the euphotic layer for a very long period.

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Distribution of inorganic and organic nutrients in the South Pacific Ocean − evidence for long-term accumulation of organic matter in nitrogen-depleted waters

Biogeosciences ◽

10.5194/bg-5-281-2008 ◽

2008 ◽

Vol 5 (2) ◽

pp. 281-298 ◽

Cited By ~ 83

Author(s):

P. Raimbault ◽

N. Garcia ◽

F. Cerutti

Keyword(s):

Organic Matter ◽

Organic Carbon ◽

Dissolved Organic Matter ◽

Dissolved Organic Carbon ◽

Chlorophyll A ◽

Organic Phosphorus ◽

South Pacific ◽

Nutrient Concentrations ◽

The South ◽

Photic Layer

Abstract. During the BIOSOPE cruise the RV Atalante was dedicated to study the biogeochemical properties in the South Pacific between the Marquesas Islands (141° W–8° S) and the Chilean upwelling (73° W–34° S). Over the 8000 km covered by the cruise, several different trophic situations were encountered, in particular strong oligotrophic conditions in the South Pacific Gyre (SPG, between 123° W and 101° W). In this isolated region, nitrate was undetectable between the surface and 160–180 m and only trace quantities (<20 nmoles l−1) of regenerated nitrogen (nitrite and ammonium) were detected, even in the subsurface maximum. Integrated nitrate over the photic layer, which reached 165 m, was close to zero. Despite this severe nitrogen-depletion, phosphate was always present in significant concentrations (≈0.1 μmoles l−1), while silicic acid was maintained at low but classical oceanic levels (≈1 μmoles l−1). In contrast, the Marquesas region (MAR) to the west and Chilean upwelling (UPW) to the east were characterized by high nutrient concentrations, one hundred to one thousand fold higher than in the SPG. The distribution of surface chlorophyll reflected the nitrate gradient, the lowest concentrations (0.023 nmoles l−1) being measured at the centre of the SPG, where integrated value throughout the photic layer was very low (≈ 10 mg m−2). However, due to the relatively high concentrations of chlorophyll-a encountered in the DCM (0.2 μg l−1), chlorophyll-a concentrations throughout the photic layer were less variable than nitrate concentrations (by a factor 2 to 5). In contrast to chlorophyll-a, integrated particulate organic matter (POM) remained more or less constant along the study area (500 mmoles m−2, 60 mmoles m−2 and 3.5 mmoles m−2 for particulate organic carbon, particulate organic nitrogen and particulate organic phosphorus, respectively), with the exception of the upwelling, where values were two fold higher. The residence time of particulate carbon in the surface water was only 4–5 days in the upwelling, but up to 30 days in the SPG, where light isotopic δ15N signal noted in the suspended POM suggests that N2-fixation provides a dominant supply of nitrogen to phytoplankton. The most striking feature was the large accumulation of dissolved organic matter (DOM) in the SPG compared to the surrounding waters, in particular dissolved organic carbon (DOC) where concentrations were at levels rarely measured in oceanic waters (>100 μmoles l−1). Due to this large pool of DOM in the SPG photic layer, integrated values followed a converse geographical pattern to that of inorganic nutrients with a large accumulation in the centre of the SPG. Whereas suspended particulate matter in the mixed layer had a C/N ratio largely conforming to the Redfield stochiometry (C/N≈6.6), marked deviations were observed in this excess DOM (C/N≈16 to 23). The marked geographical trend suggests that a net in situ source exists, mainly due to biological processes. Thus, in spite of strong nitrate-depletion leading to low chlorophyll biomass, the closed ecosystem of the SPG can accumulate large amounts of C-rich dissolved organic matter. The implications of this finding are examined, the conclusion being that, due to weak lateral advection, the biologically produced dissolved organic carbon can be accumulated and stored in the photic layer for very long periods. In spite of the lack of seasonal vertical mixing, a significant part of new production (up to 34%), which was mainly supported by dinitrogen fixation, can be exported to deep waters by turbulent diffusion in terms of DOC. The diffusive rate estimated in the SPG (134 μmolesC m−2 d−1), was quite equivalent to the particles flux measured by sediments traps.

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Origin and fate of particulate and dissolved organic matter in a naturally iron-fertilized region of the Southern Ocean

Biogeosciences Discussions ◽

10.5194/bgd-11-14097-2014 ◽

2014 ◽

Vol 11 (10) ◽

pp. 14097-14132 ◽

Cited By ~ 1

Author(s):

L. Tremblay ◽

J. Caparros ◽

K. Leblanc ◽

I. Obernosterer

Keyword(s):

Organic Matter ◽

Organic Carbon ◽

Dissolved Organic Matter ◽

Southern Ocean ◽

Surface Waters ◽

Bacterial Degradation ◽

Natural Iron ◽

Average Proportion ◽

Carbon Pump ◽

Microbial Carbon

Abstract. Natural iron fertilization of high-nutrient low-chlorophyll (HNLC) waters induces annually occurring spring phytoplankton blooms off Kerguelen Islands (Southern Ocean). To examine the origin and fate of particulate and dissolved organic matter (POM and DOM), D- and L-amino acids (AA) were quantified at bloom and HNLC stations. Total hydrolysable AA accounted for 21–25% of surface particulate organic carbon (%POCAA) at the bloom sites, but for 10% at the HNLC site. A marked decrease in %POCAA with depth was observed at the most productive stations leading to values between 3 and 5% below 300 m depth. AA contributed to only 0.9–4.4% of dissolved organic carbon (%DOCAA) at all stations. The only consistent vertical trend was observed at the most productive station (A3-2) where %DOCAA decreased from ∼2% in the surface waters to 0.9% near 300 m. These AA yields and other markers revealed that POM and DOM were more rapidly altered or mineralized at the bloom sites compared to the HNLC site. Different molecular markers indicated that POM mostly originated from diatoms and bacteria. The estimated average proportion of POM from intact phytoplankton cells in surface waters was 45% at the bloom station A3-2, but 14% at the HNLC site. Estimates based on D-AA yields indicated that ∼15% of POM and ∼30% of DOM was of bacterial origin (cells and cell fragments) at all stations. Surprisingly, the DOM in HNLC waters appeared less altered than the DOM from the bloom, had slightly higher dissolved AA concentrations, and showed no sign of alteration within the water column. Unfavorable conditions for bacterial degradation in HNLC regions can explain these findings. In contrast, large inputs of labile organic molecules and iron, likely stimulate the degradation of organic matter (priming effect) and the production of more recalcitrant DOM (microbial carbon pump) during iron-fertilized blooms.

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Distribution of inorganic and organic nutrients in the South Pacific Ocean – evidence for long-term accumulation of organic matter in nitrogen-depleted waters

Biogeosciences Discussions ◽

10.5194/bgd-4-3041-2007 ◽

2007 ◽

Vol 4 (4) ◽

pp. 3041-3087 ◽

Cited By ~ 19

Author(s):

P. Raimbault ◽

N. Garcia ◽

F. Cerutti

Keyword(s):

Organic Matter ◽

Organic Carbon ◽

Dissolved Organic Matter ◽

Dissolved Organic Carbon ◽

Chlorophyll A ◽

Chlorophyll Concentration ◽

South Pacific ◽

Nitrate Content ◽

The South ◽

Photic Layer

Abstract. The BIOSOPE cruise of the RV Atalante was devoted to study the biogeochemical properties in the South Pacific between the Marquesas Islands (141° W–8° S) and the Chilean upwelling (73° W–34° S). The 8000 km cruise had the opportunity to encounter different trophic situations, and especially strong oligotrophic conditions in the Central South Pacific Gyre (SPG, between 123° W and 101° W). In this isolated region, nitrate was undetectable between surface and 160–180 m, while regenerated nitrogen (nitrite and ammonium) only revealed some traces (<20 nmoles l−1), even in the subsurface maximum. Integrated nitrate over the photic layer, which reached 165 m, was close to zero. In spite of this severe nitrogen-depletion, phosphate was always present at significant concentrations (≈0.1 μmoles l−1), while silicate maintained at low but classical oceanic levels (≈1 μmoles l−1). In contrast, the Marquesas region (MAR) at west and Chilean upwelling (UPW) at east were characterized by large nutrient contents one hundred to one thousand fold higher than in the SPG. Distribution of surface chlorophyll concentration reflected this gradient of nitrate availability. The lowest value (0.023 nmoles l−1) was measured in the centre of the SPG, where integrated chlorophyll over the photic layer was very weak (≈10 mg m−2), since a great part (up to 50%) of the deep chlorophyll maximum (DCM) was located below the 1% light. But, because of the relative high concentration encountered in the DCM (0.2 μg l−1), chlorophyll a content over the photic layer varied much less (by a factor 2 to 5) than the nitrate content. In contrast to chlorophyll a, integrated content of particulate organic matter (POM) remained more or less constant along the investigated area (500 mmoles m−2, 60 mmoles m−2 and 3.5 mmoles m−2 for particulate organic carbon, particulate organic nitrogen and particulate organic phosphorus, respectively), except in the upwelling where values were two fold higher. Extensive comparison has shown that glass fiber GF/F filters efficiency collected particulate chlorophyll, while a significant fraction of POM (up to 50%) passed trough this filter and was retained by 0.2 μm Teflon membrane. The most striking feature was the large accumulation of dissolved organic matter (DOM) in the SPG relative to surrounding waters, especially dissolved organic carbon (DOC) where concentrations were at levels rarely measured in oceanic waters (>100 μmoles l−1). Due to this large pool of DOM over the whole photic layer of the SPG, integrated values followed an opposite geographical pattern than this of inorganic nutrients with a large accumulation within the centre of the SPG. While suspended particulate matter in the mixed layer had C/N ratio largely conform to Redfield stoichiometry (C/N≈6.6), marked deviations were observed in this excess DOM (C/N≈16 to 23). The existence of C-rich dissolved organic matter is recognized as a feature typical of oligotrophic waters, requiring the over consumption of carbon. Thus, in spite of strong nitrate-depletion leading to low chlorophyll biomass, the closed ecosystem of the SPG can produce a large amount of carbon. The implications of this finding are discussed, the conclusion being that, due to the lack of seasonal vertical mixing and weak lateral advection, the dissolved organic carbon biologically produced can be accumulated and stored in the photic layer for a very long period.

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Origin and fate of particulate and dissolved organic matter in a naturally iron-fertilized region of the Southern Ocean

Biogeosciences ◽

10.5194/bg-12-607-2015 ◽

2015 ◽

Vol 12 (2) ◽

pp. 607-621 ◽

Cited By ~ 17

Author(s):

L. Tremblay ◽

J. Caparros ◽

K. Leblanc ◽

I. Obernosterer

Keyword(s):

Organic Matter ◽

Organic Carbon ◽

Dissolved Organic Matter ◽

Southern Ocean ◽

Surface Waters ◽

Bacterial Degradation ◽

Natural Iron ◽

Average Proportion ◽

Carbon Pump ◽

Microbial Carbon

Abstract. Natural iron fertilization of high-nutrient low-chlorophyll (HNLC) waters induces annually occurring spring phytoplankton blooms off the Kerguelen Islands (Southern Ocean). To examine the origin and fate of particulate and dissolved organic matter (POM and DOM), D- and L-amino acids (AA) were quantified at bloom and HNLC stations. Total hydrolyzable AA accounted for 21–25% of surface particulate organic carbon (%POCAA) at the bloom sites, but for 10% at the HNLC site. A marked decrease in %POCAA with depth was observed at the most productive stations leading to values between 3 and 5% below 300 m depth. AA contributed to only 0.9–4.4% of dissolved organic carbon (%DOCAA) at all stations. The only consistent vertical trend was observed at the most productive station (A3-2) where %DOCAA decreased from ~ 2% in the surface waters to 0.9% near 300 m. These AA yields revealed that POM and DOM were more rapidly altered or mineralized at the bloom sites compared to the HNLC site. Alteration state was also assessed by trends in C / N ratio, %D-AA and degradation index. Different molecular markers indicated that POM mostly originated from diatoms and bacteria. The estimated average proportion of POM from intact phytoplankton cells in surface waters was 45% at the bloom station A3-2, but 14% at the HNLC site. Estimates based on D-AA yields indicated that ~ 15% of POM and ~ 30% of DOM was of bacterial origin (cells and cell fragments) at all stations. Surprisingly, the DOM in HNLC waters appeared less altered than the DOM from the bloom, had slightly higher dissolved AA concentrations, and showed no sign of alteration within the water column. Unfavorable conditions for bacterial degradation in HNLC regions can explain these findings. In contrast, large inputs of labile organic molecules and iron likely stimulate the degradation of organic matter (priming effect) and the production of more recalcitrant DOM (microbial carbon pump) during iron-fertilized blooms.

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Evidence for dissolved organic matter as the primary source and sink of photochemically produced hydroxyl radical in arctic surface waters

Environmental Science Processes & Impacts ◽

10.1039/c3em00596h ◽

2014 ◽

Vol 16 (4) ◽

pp. 807-822 ◽

Cited By ~ 39

Author(s):

Sarah E. Page ◽

J. Robert Logan ◽

Rose M. Cory ◽

Kristopher McNeill

Keyword(s):

Organic Matter ◽

Dissolved Organic Matter ◽

Hydroxyl Radical ◽

Residence Time ◽

Surface Waters ◽

Primary Source ◽

The Arctic ◽

Water Residence Time ◽

Arctic Surface ◽

Source And Sink

Photochemical hydroxyl radical formation decreases with increasing water residence time in a system of lakes connected by streams in the Arctic.

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Strong linkages between surface and deep-water dissolved organic matter in the East/Japan Sea

Biogeosciences ◽

10.5194/bg-14-2561-2017 ◽

2017 ◽

Vol 14 (9) ◽

pp. 2561-2570 ◽

Cited By ~ 3

Author(s):

Tae-Hoon Kim ◽

Guebuem Kim ◽

Yuan Shen ◽

Ronald Benner

Keyword(s):

Amino Acids ◽

Organic Matter ◽

Dissolved Organic Matter ◽

Deep Water ◽

Surface Waters ◽

North Pacific Ocean ◽

Deep Convection ◽

Japan Sea ◽

Euphotic Zone ◽

Deep Waters

Abstract. Vertical and horizontal distributions of total dissolved amino acids (TDAAs), dissolved organic carbon (DOC), and dissolved organic nitrogen (DON) were measured in the East/Japan Sea (EJS). The euphotic zone of this sea is N-limited, and the N : P ratio is ∼ 13 below 200 m depth. Elevated TDAA concentrations (137 ± 34 nM) and DOC-normalized yields (0.8 ± 0.2 % of DOC) were observed in deep waters ( ≥  1000 m) of the EJS and compared with those in the deep North Pacific Ocean. Significantly high TDAA concentrations and yields were observed in a region of deep-water formation, indicating the convection of margin-derived bioavailable dissolved organic matter (DOM) to deep waters. Declining TDAA concentrations (36 ± 12 %) and yields (33 ± 13 %) were observed between 1000 and 3000 m throughout the EJS, indicating the utilization of bioavailable DOM in deep waters. Concentrations of the D-enantiomers of amino acids (Ala, Glx, Asx, and Ser) were relatively high in deep waters of the EJS, indicating substantial bacterial contributions to DOM from surface and upper mesopelagic waters. Climate warming during the past few decades in the EJS is weakening deep convection during the winter, and one consequence of this reduction in deep convection is a decline in the supply of bioavailable DOM from surface waters.

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Carbon and nitrogen uptake in the South Pacific Ocean: evidence for efficient dinitrogen fixation and regenerated production leading to large accumulation of dissolved organic matter in nitrogen-depleted waters

Biogeosciences Discussions ◽

10.5194/bgd-4-3531-2007 ◽

2007 ◽

Vol 4 (5) ◽

pp. 3531-3579 ◽

Cited By ~ 8

Author(s):

P. Raimbault ◽

N. Garcia

Keyword(s):

Organic Matter ◽

Nitrogen Uptake ◽

Inorganic Nitrogen ◽

South Pacific ◽

Dinitrogen Fixation ◽

The South ◽

New Production ◽

Marquesas Islands ◽

South Pacific Ocean ◽

South Pacific Gyre

Abstract. A major goal of the BIOSOPE cruise on the R/V Atalante to the South Pacific Ocean (conducted in October–November 2004) was to establish rate of productivity along a longitudinal section across the oligotrophic South Pacific Gyre (SPG), and compared these measurements with those obtained in nutrient–repleted waters from Chilean upwelling and around Marquesas Islands. A dual 13C/15N isotopic technique was used to estimate rates of carbon fixation, inorganic nitrogen uptake (including dinitrogen fixation), ammonium (NH4) and nitrate (NO3) regeneration, and dissolved organic nitrogen (DON) release resulting from both NH4 and NO3 uptake. The SPG had revealed the lowest rates of primary production (0.1 gC.m−2.d−1), while rates were 7 to 20 fold higher around the Marquesas Islands and in the Chilean upwelling, respectively. In this very low productive area, most of primary production was sustained by active regeneration processes which fuelled up to 95% of the biological nitrogen demand. Since nitrification was very active in the surface layer and often balanced the biological demand of nitrate, dinitrogen fixation, although acting at low daily rate (≈1–2 nmoles l−1d−1), sustained the main part of new production. Then, new production in the SPG (0.008±0.007 gC m−2.d−1) was two orders of magnitude lower than this measured in the upwelling where it essentially sustained by nitrate (0.69±0.49 gC.m−2.d−1). In the whole investigated region, the percentage of nitrogen release as DON represented a large part of the inorganic nitrogen uptake (13–15% in average), and reaching 26–41% in the SPG where the production of DON appeared to be a major part of the nitrogen cycle. Due to the lack of annual vertical mixing and very low lateral advection, the high release rates could explain the large accumulation of dissolved organic matter observed in the nitrogen-depleted and low productive waters of the South Pacific Gyre.

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Niche partitioning by photosynthetic plankton as a driver of CO2-fixation across the oligotrophic South Pacific Subtropical Ocean

The ISME Journal ◽

10.1038/s41396-021-01072-z ◽

2021 ◽

Author(s):

Julia Duerschlag ◽

Wiebke Mohr ◽

Timothy G. Ferdelman ◽

Julie LaRoche ◽

Dhwani Desai ◽

...

Keyword(s):

Mixed Layer ◽

Niche Partitioning ◽

Euphotic Zone ◽

South Pacific ◽

Niche Differentiation ◽

Surface Mixed Layer ◽

The South ◽

The Core ◽

Photosynthetic Eukaryotes ◽

Phytoplankton Communities

AbstractOligotrophic ocean gyre ecosystems may be expanding due to rising global temperatures [1–5]. Models predicting carbon flow through these changing ecosystems require accurate descriptions of phytoplankton communities and their metabolic activities [6]. We therefore measured distributions and activities of cyanobacteria and small photosynthetic eukaryotes throughout the euphotic zone on a zonal transect through the South Pacific Ocean, focusing on the ultraoligotrophic waters of the South Pacific Gyre (SPG). Bulk rates of CO2 fixation were low (0.1 µmol C l−1 d−1) but pervasive throughout both the surface mixed-layer (upper 150 m), as well as the deep chlorophyll a maximum of the core SPG. Chloroplast 16S rRNA metabarcoding, and single-cell 13CO2 uptake experiments demonstrated niche differentiation among the small eukaryotes and picocyanobacteria. Prochlorococcus abundances, activity, and growth were more closely associated with the rims of the gyre. Small, fast-growing, photosynthetic eukaryotes, likely related to the Pelagophyceae, characterized the deep chlorophyll a maximum. In contrast, a slower growing population of photosynthetic eukaryotes, likely comprised of Dictyochophyceae and Chrysophyceae, dominated the mixed layer that contributed 65–88% of the areal CO2 fixation within the core SPG. Small photosynthetic eukaryotes may thus play an underappreciated role in CO2 fixation in the surface mixed-layer waters of ultraoligotrophic ecosystems.

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