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<br/>increased scrutiny because of its involvement in the global carbon cycle and<br/>consequently climate change. Dissolved organic nitrogen (DON) and phosphorus<br/>(DOP), which have historically been ignored because of their suggested “biological<br/>unavailability”, have now received greater attention due to their importance in nutrient<br/>cycling, particularly in oligotrophic ecosystems. DOM, a byproduct of photosynthetic<br/>production, has important ecological significance as a substrate that supports<br/>heterotrophic bacterial growth, thereby causing oxygen consumption and regenerating<br/>inorganic nutrients. In the open ocean the net production of DOC is ultimately due to<br/>the decoupling of biological production and consumption processes. Concentrations of<br/>DOM in the surface oceans, therefore, are controlled by both physical and biological<br/>processes. This research investigates the biological factors that control the distributions<br/>of DOC, DON and DOP in surface waters, the importance of DOC degradation to<br/>oxygen consumption, the importance of DON and DOP degradation to remineralised<br/>dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP), and the<br/>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<br/>temperate region, the southern subtropical gyre, the equatorial region, the northern<br/>subtropical gyre, and the northern temperate region. This work described here was<br/>performed as a component of the AMT programme.<br/>Concentrations of DOC and TDN were determined using a high-temperature catalytic<br/>combustion technique, and TDP concentrations were determined using a UV oxidation<br/>method. Concentrations of DON and DOP were estimated as the difference between<br/>the independent measurements of TDN and TDP. The results showed that the highest<br/>DOM concentrations were found in surface (0-30 m) waters, ranging from 70-80 µM<br/>DOC, 4.8-6.5 µM DON and 0.2-0.3 µM DOP, and decreased with increasing water<br/>depth to 45-55 µM DOC, 2.6-4.0 µM DON and 0.04-0.05 µM DOP at 300 m. The<br/>lowest DOM concentrations were observed in the deep (&gt;1000 m) ocean, averaging 44<br/>µM DOC, 2.3 µM DON and 0.02 µM DOP. In the upper 300 m, the concentrations of<br/>semilabile (and labile) DOC decreased by 45-95% from the surface values. DON and<br/>DOP were the dominant components of the total dissolved nutrient pools in the upper<br/>50 m, accounting for up to 99% and 80% of the TDN and TDP pools, respectively. In<br/>the upper 300 m, semilabile (and labile) DON and DOP decreased by 50-65% and<br/>90-95% from the surface values, respectively.<br/>The decoupled correlations between DOC/DON/DOP and chlorophyll-a and rates of<br/>carbon fixation suggested that phytoplankton biomass and rates of primary production<br/>were not the important controls of the cumulative DOC, DON and DOP. Zooplankton<br/>grazing was hypothesised to be an important factor in regulating the distributions of<br/>DOC, DON and DOP in surface waters. Poor correlations between DOC/DON/DOP<br/>and DIN/DIP suggested that inorganic nutrients were not the significant controls in<br/>DOC, DON and DOP distributions. N and P were probably retained mainly in the<br/>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<br/>bringing new nutrients into the surface waters, strong correlations between dissolved<br/>organic and inorganic nutrients should not be anticipated. Prochlorococcus spp.<br/>abundance was statistically linked with the concentrations of DOC, DON and DOP.<br/>The significant correlations may reflect the ability of Prochlorococcus to assimilate the<br/>labile forms of dissolved organic nutrients (including DOC), which may be<br/>quantitatively significant in surface waters of the Atlantic Ocean.<br/>The C:N, N:P and C:P stoichiometry of the bulk DOM pool deviated from the Redfield<br/>ratio of 6:1, 16:1 and 106:1, ranging from 12-18, 20-100 and 300-1400, respectively, in<br/>the upper 300 m, suggesting that the cumulative DOM was rich in C relative to N and P,<br/>and N relative to P compared to the Redfield trajectories. The offsets of the C:N:P<br/>stoichiometry relatively to the Redfield ratio were due to nutrient limitations that<br/>imposed on prokaryotic and eukaryotic microbial populations. The C:N:P<br/>stoichiometry of the bulk DOM pool showed an increased trend, with C:N = 12-16,<br/>N:P = 20-25, and C:P = 300-350 in the upper 30 m, C:N = 12-18, N:P = 50-100, and<br/>C:P = 700-1400 at 300 m, and C:N = 17-24, N:P = 79-132; C:P = 1791-2442 at 1000 m.<br/>The differences in the C:N:P stoichiometry of the bulk DOM pool between the upper<br/>and deep waters suggested preferential remineralisation of P relative to C and N, and N<br/>relative to C. A greater remineralisation length scale for DOC relative to DON and<br/>DOP produced a long-term, steady flux of C from the surface to the deep ocean.<br/>Therefore, CO2 fixed in the upper ocean during planktonic photosynthesis was<br/>continuously “pumped” into the ocean interior, and stored in the deep ocean up to<br/>thousands of years. The C:N, N:P and C:P stoichiometry of the semilabile (and labile)<br/>DOM pool generally agreed with the Redfield ratio (C:N = 6; N:P = 16; C:P = 106) in<br/>the upper 30 m. At 100 m C:N ratio was 5-12, C:P ratio was 20-30, and C:P ratio was<br/>100-150. At 300 m, C:N ratio was 5-12, N:P ratio was 25-100, and C:P ratio was<br/>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<br/>semilabile (and labile) DOP seemed to be preferentially remineralised relative to the<br/>semilabile (and labile) DOC and DON.<br/>In the upper thermocline (i.e. above 300 m), DOC degradation was important with<br/>respect to oxygen consumption, contributing to as much as 25% of the apparent<br/>oxygen utilization (AOU). The remaining of 75% was attributable to POC<br/>decomposition. However, the AOU contributable to DOC showed a function of latitude,<br/>with 15-55% found in the central subtropical Atlantic gyres and 15-25% in the<br/>equatorial region. The most likely explanation for the variation of DOC relative to<br/>POC degradation with respect to AOU was the regional variability in the export of<br/>POC, which was suggested to be highest in the high nutrient regions of the equator and<br/>at the poleward margins of the subtropical gyres. As a result, DOC formed an<br/>important contribution to AOU in oligotrophic regions, while POC was the dominant<br/>control of AOU in upwelling regions. <br/>Some freshly-produced fractions of DON and DOP with turnover times of months to<br/>years were capable of escaping rapid microbial degradation in surface waters and<br/>became entrained into deep waters via diffusive mixing. Subsequent microbial<br/>degradation of these DON and DOP took place in the thermocline, regenerating<br/>inorganic nutrients. Statistically significant correlations were observed between the<br/>DON-to-DIN and DOP-to-DIP relationships. Calculations of the fluxes of dissolved<br/>organic nutrients relative to inorganic nutrients suggested that in the upper thermocline<br/>(i.e. above 300 m), the downward fluxes of DON and DOP contributed to a total of 4%<br/>and 5% of the upward fluxes of DIN and DIP, respectively, into the euphotic zone. The<br/>remaining of 95% of the upward dissolved inorganic nutrients fell out of the euphotic<br/>zone as particles in order to prevent nutrient accumulation and to maintain nutrient<br/>integrity of the pelagic ecosystem.

Name dc:type.qualificationname

Ph.D.

Level dc:type.qualificationlevel

doctoral

Grantor dc:publisher.institution

University of Southampton

Year dc:date.issued

2007

Author dc:creator

• Pan, Xi