Part of the global DOC versus AOU (dissolved organic carbon/apparent oxygen utilization) data compilation, Azores-I

Temperate, transitional and subtropical waters of the remote Azores Front region east of Azores (24-40°N, 22-32°W) were sampled during three cruises conducted under increasing stratification conditions (April 1999, May 1997 and August 1998). Despite the temporal increase of surface temperature (by 5 °C) and stratification (by 2.1 1/min**2), as well as the thermocline shoaling (by ~15 m), dissolved organic carbon (DOC) and nitrogen (DON) in the surface layer were not significantly different for the early spring, late spring and summer periods, with average concentrations of 69±2 µM-C and 5.2±0.4 µM-N, respectively. The surface excess of semi-labile DOC, compared with the baseline DOC concentration in the deep ocean (47±2 µM-C), represents 33% of the bulk DOC concentration and as much as 85% of the TOC (=POC+DOC) excess. When compared with the winter baseline (56±2 µM-C), the seasonal surface DOC excess is 20% of the bulk DOC concentration and 87% of the seasonal TOC excess. These results confirm the major role played by DOC in the carbon cycle of surface waters of the Azores Front region. The total amount of bioreactive DOC transported from the temperate to the subtropical North Atlantic by the Ekman flux between March and December represents only ~15% of the average annual primary production, and ~15% and ~30% of the measured sinking POC flux+vertical DOC eddy diffusion during early spring and summer, respectively. Vertical eddy diffusion is 35% and 2% of the spring and summer sinking POC flux, respectively. On the other hand, DOC only contributes 13% to the local oxidation of organic matter in subsurface waters (between the pycnocline and 500 m) of the study region.

Stable carbon isotope record of planktonic foraminifera and organic matter from sediment cores in the Gulf of Aqaba

The stable carbon isotopic composition of the planktonic foraminifera Globigerinoides sacculifer and G. ruber (white) and sedimentary organic matter from the northern Gulf of Aqaba have been investigated to estimate changes in delta13CDIC in surface waters during the last 1,000 years. The high sedimentation rates at the core sites (about 54 cm/Kyear) provide high temporal resolution (~10 years). Recent sediments at the top of the cores reflect conditions younger than 1950. The delta13C records of the planktonic foraminifera from three multicores display similar trends, showing a uniform and consistent pattern before the 1750s, and a gradual decrease of approximately 0.63? over the last two centuries. This decrease seems to track the decrease of delta13CDIC in surface waters, which is mainly caused by the increase of anthropogenic input of 13C-depleted CO2 into the atmosphere. Similarly, a trend towards lighter values of the carbon isotopic composition of sedimentary organic matter (delta13Corg) during the last 200 years supports the interpretation obtained from the planktonic foraminiferal delta13C. Furthermore, direct measurements of seawater show that delta13C of the dissolved inorganic carbon (DIC) in the northern Gulf of Aqaba has decreased by about 0.44 per mil during the period 1979-2000. The average annual decrease is 0.021 per mil, which is similar to that observed globally. The delta13C values of planktonic foraminifera combined with organic matter delta13C from marine sediments are good indicators for reconstructing past changes in atmospheric CO2 concentrations from the northern Gulf of Aqaba.

Experiment: Organic matter exudation by Emiliania huxleyi under simulated future ocean conditions

Emiliania huxleyi (strain B 92/11) was exposed to different nutrient supply, CO2 and temperature conditions in phosphorus controlled chemostats to investigate effects on organic carbon exudation and partitioning between the pools of particulate organic carbon (POC) and dissolved organic carbon (DOC). 14C incubation measurements for primary production (PP) and extracellular release (ER) were performed. Chemical analysis included the amount and composition of high molecular weight (>1 kDa) dissolved combined carbohydrates (HMW-dCCHO), particulate combined carbohydrates (pCCHO) and the carbon content of transparent exopolymer particles (TEP-C). Applied CO2 and temperature conditions were 300, 550 and 900 µatm pCO2 at 14 °C, and additionally 900 µatm pCO2 at 18 °C simulating a greenhouse ocean scenario. Enhanced nutrient stress by reducing the dilution rate (D) from D = 0.3 /d to D = 0.1 /d (D = µ) induced the strongest response in E. huxleyi. At µ = 0.3 /d, PP was significantly higher at elevated CO2 and temperature and DO14C production correlated to PO14C production in all treatments, resulting in similar percentages of extracellular release (PER; (DO14C production/PP) × 100) averaging 3.74 ± 0.94%. At µ = 0.1 /d, PO14C production decreased significantly, while exudation of DO14C increased. Thus, indicating a stronger partitioning from the particulate to the dissolved pool. Maximum PER of 16.3 ± 2.3% were observed at µ = 0.1 /d at elevated CO2 and temperature. While cell densities remained constant within each treatment and throughout the experiment, concentrations of HMW-dCCHO, pCCHO and TEP were generally higher under enhanced nutrient stress. At µ= 0.3 /d, pCCHO concentration increased significantly with elevated CO2 and temperature. At µ = 0.1 /d, the contribution (mol % C) of HMW-dCCHO to DOC was lower at elevated CO2 and temperature while pCCHO and TEP concentrations were higher. This was most pronounced under greenhouse conditions. Our findings suggest a stronger transformation of primary produced DOC into POC by coagulation of exudates under nutrient limitation. Our results further imply that elevated CO2 and temperature will increase exudation by E. huxleyi and may affect organic carbon partitioning in the ocean due to an enhanced transfer of HMW-dCCHO to TEP by aggregation processes.

Composition of organic matter in waters of the Barents Sea

Concentration of dissolved and suspended organic carbon in the Barents Sea decreases with increasing depth and in meridional direction from south to north. This fact is attributed to uneven supply of organic matter (OM) from different sources. Large amounts of OM are present in the water column of the Barents Sea, which is a result of high productivity of this water mass and slow decomposition of OM at low temperatures and lowered activity of microflora. Larger amount of OM reaches bottom sediments of the Barents Sea more than in seas, which have similar depth and productivity, but are located at low latitudes. Characteristic features of distribution of lipids and suspended matter in sea water are examined.