The influence of atmospheric organic carbon and organic nitrogen on biogeochemistry of the (sub-) tropical North Atlantic Ocean

Abstract

The (sub-) tropical North Atlantic Ocean is a region which is influenced by oceanic upwelling along the West African coast, an extensive oxygen minimum zone and also by atmospheric dust deposition from the Sahara and Sahel regions and the industrial continental regions of Europe and North America. In this study, samples from water column profiles were collected to investigate the distributions of dissolved organic carbon (DOC) and dissolved organic itrogen (DON), including dissolved free amino acids (DFAA) and dissolved hydrolysable amino acids (DHAA). In addition, bulk aerosol samples were collected on the island of São Vicente (Cape Verde Islands) to quantify the deposition fluxes into the surface waters of the (sub-) tropical North Atlantic Ocean of atmospheric organic matter; namely leachable organic carbon (LOC), leachable organic nitrogen (LON), leachable free amino acids (LFAA) and leachable hydrolysable amino acids (LHAA). DOC, LOC, total dissolved nitrogen (TDN) and leachable total nitrogen (LTN) were determined using a high temperature combustion technique (HTC). DON values were defined as the difference between TDN and dissolved inorganic nitrogen (DIN). LON values were derived as the difference between LTN and leachable inorganic nitrogen (LIN). DFAA, LFAA, DHAA and LHAA were analysed using the method optimized in this study. The determination was performed using reversed-phase high performance liquid chromatography (RP-HPLC) combined with AQC pre-column derivatisation. Hydrolysis was conducted using vapour phase hydrolysis for 24 hours at a temperature of 110-115 ºC.

In general, DOC concentrations were high in surface waters of the (sub-) tropical North Atlantic Ocean and declined toward the thermocline, whereas DON exhibited non-typical vertical profiles, with the concentrations increasing with depth, in the tropical North Atlantic Ocean. Mean surface water DOC and DON concentrations ranged from 67.2 ?M to 72.3 ?M and 4.6 ?M to 8.1 ?M, respectively. Enhanced DON values in sub-surface waters were observed in the tropical North Atlantic Ocean along the 12ºN latitude section. The North Atlantic subtropical gyre and the tropical North Atlantic Ocean showed small contributions of DOC oxidation to apparent oxygen utilization (AOU) (14.1% - 16.5%) indicating that DOC is primarily remineralised in the surface waters and the bulk of respiration in sub-surface waters was supported by particulate organic carbon. In the Mauritanian shelf region, high particle sinking rates are primarily responsible for oxygen consumption. The stoichiometric ratios (C:N:P) indicated that the DOM pool in surface waters was enriched in carbon relative to nitrogen and phosphorus, and nitrogen relative to phosphorus, as a result of preferential mineralization of nitrogen and phosphorus relative to carbon in organic matter. The correlation between DOC and DON with density, chlorophyll a, bacterial abundance and DIN revealed that distributions of DOC and DON in the North Atlantic subtropical gyre and in the tropical North Atlantic Ocean were controlled by a combination of physical and biogeochemical processes. However, physical processes, particularly water mixing, appeared to be the main influence. DFAA and DHAA concentrations were below the limits of detection of the optimized method which are 0.07 and 0.66 ?M, respectively.

The aerosols were classified into 6 types associated with air mass back trajectory analysis using the HYSPLIT model and according to the colour of the aerosols. The air masses were derived from different source regions; namely the Sahara region, the Sahel and Tropical Rainforest region, Europe and America. The aerosols in the air masses deposited organic carbon of 3.67 mmol/m2/year, total nitrogen of 11.9 mmol/m2/year and organic nitrogen 0.47 mmol/m2/year into the (sub-) tropical North Atlantic Ocean. Air masses transported from the Saharan region supplied the highest LOC and LTN deposition fluxes, while LON fluxes were derived pre-dominantly from air masses of multiple origins including the Saharan and Sahel regions, Europe and North America. Aerosol leaching solutions contained low concentrations of LFAA (< 0.054 nmol/m3) and LHAA (< 0.64 nmol/m3). The aerosols from all origins were enriched in organic nitrogen in relation to phosphorus whereas the aerosols originating from the Saharan region were enriched in carbon relative to nitrogen. Relatively short residence times of LOC and LON were observed in the Mauritania Shelf region indicating the presence of rapid biotic removal processes coinciding with an enhanced abundance of heterotrophic bacteria and marine phytoplankton.

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