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The marine biogeochemistry of dissolved organic carbon and dissolved organic nutrients in the Atlantic Ocean

The marine biogeochemistry of dissolved organic carbon and dissolved organic nutrients in the Atlantic Ocean
The marine biogeochemistry of dissolved organic carbon and dissolved organic nutrients in the Atlantic Ocean
The marine biogeochemistry of dissolved organic carbon (DOC) has come under
increased scrutiny because of its involvement in the global carbon cycle and
consequently climate change. Dissolved organic nitrogen (DON) and phosphorus
(DOP), which have historically been ignored because of their suggested “biological
unavailability”, have now received greater attention due to their importance in nutrient
cycling, particularly in oligotrophic ecosystems. DOM, a byproduct of photosynthetic
production, has important ecological significance as a substrate that supports
heterotrophic bacterial growth, thereby causing oxygen consumption and regenerating
inorganic nutrients. In the open ocean the net production of DOC is ultimately due to
the decoupling of biological production and consumption processes. Concentrations of
DOM in the surface oceans, therefore, are controlled by both physical and biological
processes. This research investigates the biological factors that control the distributions
of DOC, DON and DOP in surface waters, the importance of DOC degradation to
oxygen consumption, the importance of DON and DOP degradation to remineralised
dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP), and the
C:N:P stoichiometry of DOM pool in the Atlantic Ocean. Samples were collected on Atlantic Meridional Transects (AMT) cruise 16 and 17, which crossed the southern
temperate region, the southern subtropical gyre, the equatorial region, the northern
subtropical gyre, and the northern temperate region. This work described here was
performed as a component of the AMT programme.
Concentrations of DOC and TDN were determined using a high-temperature catalytic
combustion technique, and TDP concentrations were determined using a UV oxidation
method. Concentrations of DON and DOP were estimated as the difference between
the independent measurements of TDN and TDP. The results showed that the highest
DOM concentrations were found in surface (0-30 m) waters, ranging from 70-80 µM
DOC, 4.8-6.5 µM DON and 0.2-0.3 µM DOP, and decreased with increasing water
depth to 45-55 µM DOC, 2.6-4.0 µM DON and 0.04-0.05 µM DOP at 300 m. The
lowest DOM concentrations were observed in the deep (>1000 m) ocean, averaging 44
µM DOC, 2.3 µM DON and 0.02 µM DOP. In the upper 300 m, the concentrations of
semilabile (and labile) DOC decreased by 45-95% from the surface values. DON and
DOP were the dominant components of the total dissolved nutrient pools in the upper
50 m, accounting for up to 99% and 80% of the TDN and TDP pools, respectively. In
the upper 300 m, semilabile (and labile) DON and DOP decreased by 50-65% and
90-95% from the surface values, respectively.
The decoupled correlations between DOC/DON/DOP and chlorophyll-a and rates of
carbon fixation suggested that phytoplankton biomass and rates of primary production
were not the important controls of the cumulative DOC, DON and DOP. Zooplankton
grazing was hypothesised to be an important factor in regulating the distributions of
DOC, DON and DOP in surface waters. Poor correlations between DOC/DON/DOP
and DIN/DIP suggested that inorganic nutrients were not the significant controls in
DOC, DON and DOP distributions. N and P were probably retained mainly in the
organic pool in the surface waters due to a hypothesised insufficient functioning of the microbial degradation. If the vertical migration of zooplankton was significant in
bringing new nutrients into the surface waters, strong correlations between dissolved
organic and inorganic nutrients should not be anticipated. Prochlorococcus spp.
abundance was statistically linked with the concentrations of DOC, DON and DOP.
The significant correlations may reflect the ability of Prochlorococcus to assimilate the
labile forms of dissolved organic nutrients (including DOC), which may be
quantitatively significant in surface waters of the Atlantic Ocean.
The C:N, N:P and C:P stoichiometry of the bulk DOM pool deviated from the Redfield
ratio of 6:1, 16:1 and 106:1, ranging from 12-18, 20-100 and 300-1400, respectively, in
the upper 300 m, suggesting that the cumulative DOM was rich in C relative to N and P,
and N relative to P compared to the Redfield trajectories. The offsets of the C:N:P
stoichiometry relatively to the Redfield ratio were due to nutrient limitations that
imposed on prokaryotic and eukaryotic microbial populations. The C:N:P
stoichiometry of the bulk DOM pool showed an increased trend, with C:N = 12-16,
N:P = 20-25, and C:P = 300-350 in the upper 30 m, C:N = 12-18, N:P = 50-100, and
C:P = 700-1400 at 300 m, and C:N = 17-24, N:P = 79-132; C:P = 1791-2442 at 1000 m.
The differences in the C:N:P stoichiometry of the bulk DOM pool between the upper
and deep waters suggested preferential remineralisation of P relative to C and N, and N
relative to C. A greater remineralisation length scale for DOC relative to DON and
DOP produced a long-term, steady flux of C from the surface to the deep ocean.
Therefore, CO2 fixed in the upper ocean during planktonic photosynthesis was
continuously “pumped” into the ocean interior, and stored in the deep ocean up to
thousands of years. The C:N, N:P and C:P stoichiometry of the semilabile (and labile)
DOM pool generally agreed with the Redfield ratio (C:N = 6; N:P = 16; C:P = 106) in
the upper 30 m. At 100 m C:N ratio was 5-12, C:P ratio was 20-30, and C:P ratio was
100-150. At 300 m, C:N ratio was 5-12, N:P ratio was 25-100, and C:P ratio was
150-500. The findings suggested that in the upper 300 m, there was no preferential remineralisation between the semilabile (and labile) DOC and DON, however, the
semilabile (and labile) DOP seemed to be preferentially remineralised relative to the
semilabile (and labile) DOC and DON.
In the upper thermocline (i.e. above 300 m), DOC degradation was important with
respect to oxygen consumption, contributing to as much as 25% of the apparent
oxygen utilization (AOU). The remaining of 75% was attributable to POC
decomposition. However, the AOU contributable to DOC showed a function of latitude,
with 15-55% found in the central subtropical Atlantic gyres and 15-25% in the
equatorial region. The most likely explanation for the variation of DOC relative to
POC degradation with respect to AOU was the regional variability in the export of
POC, which was suggested to be highest in the high nutrient regions of the equator and
at the poleward margins of the subtropical gyres. As a result, DOC formed an
important contribution to AOU in oligotrophic regions, while POC was the dominant
control of AOU in upwelling regions.
Some freshly-produced fractions of DON and DOP with turnover times of months to
years were capable of escaping rapid microbial degradation in surface waters and
became entrained into deep waters via diffusive mixing. Subsequent microbial
degradation of these DON and DOP took place in the thermocline, regenerating
inorganic nutrients. Statistically significant correlations were observed between the
DON-to-DIN and DOP-to-DIP relationships. Calculations of the fluxes of dissolved
organic nutrients relative to inorganic nutrients suggested that in the upper thermocline
(i.e. above 300 m), the downward fluxes of DON and DOP contributed to a total of 4%
and 5% of the upward fluxes of DIN and DIP, respectively, into the euphotic zone. The
remaining of 95% of the upward dissolved inorganic nutrients fell out of the euphotic
zone as particles in order to prevent nutrient accumulation and to maintain nutrient
integrity of the pelagic ecosystem.
Pan, Xi
ff91c9f9-28c6-465b-a4aa-51b879dbd611
Pan, Xi
ff91c9f9-28c6-465b-a4aa-51b879dbd611

Pan, Xi (2007) The marine biogeochemistry of dissolved organic carbon and dissolved organic nutrients in the Atlantic Ocean. University of Southampton, School of Ocean and Earth Science, Doctoral Thesis, 133pp.

Record type: Thesis (Doctoral)

Abstract

The marine biogeochemistry of dissolved organic carbon (DOC) has come under
increased scrutiny because of its involvement in the global carbon cycle and
consequently climate change. Dissolved organic nitrogen (DON) and phosphorus
(DOP), which have historically been ignored because of their suggested “biological
unavailability”, have now received greater attention due to their importance in nutrient
cycling, particularly in oligotrophic ecosystems. DOM, a byproduct of photosynthetic
production, has important ecological significance as a substrate that supports
heterotrophic bacterial growth, thereby causing oxygen consumption and regenerating
inorganic nutrients. In the open ocean the net production of DOC is ultimately due to
the decoupling of biological production and consumption processes. Concentrations of
DOM in the surface oceans, therefore, are controlled by both physical and biological
processes. This research investigates the biological factors that control the distributions
of DOC, DON and DOP in surface waters, the importance of DOC degradation to
oxygen consumption, the importance of DON and DOP degradation to remineralised
dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP), and the
C:N:P stoichiometry of DOM pool in the Atlantic Ocean. Samples were collected on Atlantic Meridional Transects (AMT) cruise 16 and 17, which crossed the southern
temperate region, the southern subtropical gyre, the equatorial region, the northern
subtropical gyre, and the northern temperate region. This work described here was
performed as a component of the AMT programme.
Concentrations of DOC and TDN were determined using a high-temperature catalytic
combustion technique, and TDP concentrations were determined using a UV oxidation
method. Concentrations of DON and DOP were estimated as the difference between
the independent measurements of TDN and TDP. The results showed that the highest
DOM concentrations were found in surface (0-30 m) waters, ranging from 70-80 µM
DOC, 4.8-6.5 µM DON and 0.2-0.3 µM DOP, and decreased with increasing water
depth to 45-55 µM DOC, 2.6-4.0 µM DON and 0.04-0.05 µM DOP at 300 m. The
lowest DOM concentrations were observed in the deep (>1000 m) ocean, averaging 44
µM DOC, 2.3 µM DON and 0.02 µM DOP. In the upper 300 m, the concentrations of
semilabile (and labile) DOC decreased by 45-95% from the surface values. DON and
DOP were the dominant components of the total dissolved nutrient pools in the upper
50 m, accounting for up to 99% and 80% of the TDN and TDP pools, respectively. In
the upper 300 m, semilabile (and labile) DON and DOP decreased by 50-65% and
90-95% from the surface values, respectively.
The decoupled correlations between DOC/DON/DOP and chlorophyll-a and rates of
carbon fixation suggested that phytoplankton biomass and rates of primary production
were not the important controls of the cumulative DOC, DON and DOP. Zooplankton
grazing was hypothesised to be an important factor in regulating the distributions of
DOC, DON and DOP in surface waters. Poor correlations between DOC/DON/DOP
and DIN/DIP suggested that inorganic nutrients were not the significant controls in
DOC, DON and DOP distributions. N and P were probably retained mainly in the
organic pool in the surface waters due to a hypothesised insufficient functioning of the microbial degradation. If the vertical migration of zooplankton was significant in
bringing new nutrients into the surface waters, strong correlations between dissolved
organic and inorganic nutrients should not be anticipated. Prochlorococcus spp.
abundance was statistically linked with the concentrations of DOC, DON and DOP.
The significant correlations may reflect the ability of Prochlorococcus to assimilate the
labile forms of dissolved organic nutrients (including DOC), which may be
quantitatively significant in surface waters of the Atlantic Ocean.
The C:N, N:P and C:P stoichiometry of the bulk DOM pool deviated from the Redfield
ratio of 6:1, 16:1 and 106:1, ranging from 12-18, 20-100 and 300-1400, respectively, in
the upper 300 m, suggesting that the cumulative DOM was rich in C relative to N and P,
and N relative to P compared to the Redfield trajectories. The offsets of the C:N:P
stoichiometry relatively to the Redfield ratio were due to nutrient limitations that
imposed on prokaryotic and eukaryotic microbial populations. The C:N:P
stoichiometry of the bulk DOM pool showed an increased trend, with C:N = 12-16,
N:P = 20-25, and C:P = 300-350 in the upper 30 m, C:N = 12-18, N:P = 50-100, and
C:P = 700-1400 at 300 m, and C:N = 17-24, N:P = 79-132; C:P = 1791-2442 at 1000 m.
The differences in the C:N:P stoichiometry of the bulk DOM pool between the upper
and deep waters suggested preferential remineralisation of P relative to C and N, and N
relative to C. A greater remineralisation length scale for DOC relative to DON and
DOP produced a long-term, steady flux of C from the surface to the deep ocean.
Therefore, CO2 fixed in the upper ocean during planktonic photosynthesis was
continuously “pumped” into the ocean interior, and stored in the deep ocean up to
thousands of years. The C:N, N:P and C:P stoichiometry of the semilabile (and labile)
DOM pool generally agreed with the Redfield ratio (C:N = 6; N:P = 16; C:P = 106) in
the upper 30 m. At 100 m C:N ratio was 5-12, C:P ratio was 20-30, and C:P ratio was
100-150. At 300 m, C:N ratio was 5-12, N:P ratio was 25-100, and C:P ratio was
150-500. The findings suggested that in the upper 300 m, there was no preferential remineralisation between the semilabile (and labile) DOC and DON, however, the
semilabile (and labile) DOP seemed to be preferentially remineralised relative to the
semilabile (and labile) DOC and DON.
In the upper thermocline (i.e. above 300 m), DOC degradation was important with
respect to oxygen consumption, contributing to as much as 25% of the apparent
oxygen utilization (AOU). The remaining of 75% was attributable to POC
decomposition. However, the AOU contributable to DOC showed a function of latitude,
with 15-55% found in the central subtropical Atlantic gyres and 15-25% in the
equatorial region. The most likely explanation for the variation of DOC relative to
POC degradation with respect to AOU was the regional variability in the export of
POC, which was suggested to be highest in the high nutrient regions of the equator and
at the poleward margins of the subtropical gyres. As a result, DOC formed an
important contribution to AOU in oligotrophic regions, while POC was the dominant
control of AOU in upwelling regions.
Some freshly-produced fractions of DON and DOP with turnover times of months to
years were capable of escaping rapid microbial degradation in surface waters and
became entrained into deep waters via diffusive mixing. Subsequent microbial
degradation of these DON and DOP took place in the thermocline, regenerating
inorganic nutrients. Statistically significant correlations were observed between the
DON-to-DIN and DOP-to-DIP relationships. Calculations of the fluxes of dissolved
organic nutrients relative to inorganic nutrients suggested that in the upper thermocline
(i.e. above 300 m), the downward fluxes of DON and DOP contributed to a total of 4%
and 5% of the upward fluxes of DIN and DIP, respectively, into the euphotic zone. The
remaining of 95% of the upward dissolved inorganic nutrients fell out of the euphotic
zone as particles in order to prevent nutrient accumulation and to maintain nutrient
integrity of the pelagic ecosystem.

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Published date: June 2007
Additional Information: 133pp & appendices.
Organisations: University of Southampton

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Local EPrints ID: 63139
URI: http://eprints.soton.ac.uk/id/eprint/63139
PURE UUID: 8daac3a1-832c-44f7-afb3-e445951be30d

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Date deposited: 12 Sep 2008
Last modified: 13 Mar 2019 20:26

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Author: Xi Pan

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