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Title: The marine biogeochemistry of dissolved organic carbon and dissolved organic nutrients in the Atlantic Ocean
Author: Pan, Xi
Awarding Body: University of Southampton
Current Institution: University of Southampton
Date of Award: 2007
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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.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QD Chemistry ; QH301 Biology ; GC Oceanography