ALBERT

Description: The Mauritanian coastal area is one of the most biologically productive upwelling regions in the world ocean. Shipboard observations carried out during maximum upwelling season and short-term moored observations are used to investigate diapycnal mixing processes and to quantify diapycnal fluxes of nutrients. The observations indicate strong tide-topography interactions that are favored by near-critical angles occurring on large parts of the continental slope. Moored velocity observations reveal the existence of highly nonlinear internal waves and bores and levels of internal wave spectra are strongly elevated near the buoyancy frequency. Dissipation rates of turbulent kinetic energy at the slope and shelf determined from microstructure measurements in the upper 200 m averages to ɛ = 5 × 10−8 W kg−1. Particularly elevated dissipation rates were found at the continental slope close to the shelf break, being enhanced by a factor of 100 to 1000 compared to dissipation rates farther offshore. Vertically integrated dissipation rates per unit volume are strongest at the upper continental slope reaching values of up to 30 mW m−2. A comparison of fine-scale parameterizations of turbulent dissipation rates for shelf regions and the open ocean to the measured dissipation rates indicates deficiencies in reproducing the observations. Diapycnal nitrate fluxes above the continental slope at the base of the mixed layer yielding a mean value of 12 × 10−2 μmol m−2 s−1 are amongst the largest published to date. However, they seem to only represent a minor contribution (10% to 25%) to the net community production in the upwelling region.

Description: Nitrous oxide (N2O) flux densities across the ocean/atmosphere interface from the Mauritanian upwelling (16°–18.5°W, 16°–21°N) were computed with a simple upwelling model using N2O measurements from four cruises between 2006 and 2008 as well as wind data from the QuikSCAT satellite. The resulting N2O flux densities show a strong seasonality reflecting the wind-driven seasonality of the upwelling: N2O flux densities are highest in the northern part (19.5°–21°N) and show a decreasing trend towards the south. The summer periods with no upwelling (and thus associated with no or negligible N2O flux densities) are most pronounced in the southern part (16°–17°N). The mean seasonally and regionally weighted annual N2O emissions from the Mauritanian upwelling were estimated to 1.0 Gg N. This is low compared to other major upwelling areas (Arabian Sea, off Chile) indicating that N2O emissions from the Mauritanian upwelling are a minor source of atmospheric N2O.

Description: Anthropogenically induced increases in nitrogen deposition to the ocean can stimulate marine productivity and oceanic emission of nitrous oxide. We present the first global ocean model assessment of the impact on marine N2O of increases in nitrogen deposition from the preindustrial era to the present. We find significant regional increases in marine N2O production downwind of continental outflow, in coastal and inland seas (15–30%),and nitrogen limited regions of the North Atlantic and North Pacific (5–20%). The largest changes occur in the northern Indian Ocean (up to 50%) resulting from a combination of high deposition fluxes and enhanced N2O production pathways in local hypoxic zones. Oceanic regions relatively unaffected by anthropogenic nitrogen deposition indicate much smaller changes (〈2%). The estimated change in oceanic N2O source on a global scale is modest (0.08–0.34 Tg N yr-1, ~3–4% of the total ocean source), and consistent with the estimated impact on global export production (~4%).

Description: Continuous high-resolution underway measurements of dimethyl sulfide (DMS) and isoprene in the ocean surface were conducted from Germany to South Africa in November 2008. DMS, total dimethylsulfoniopropionate (DMSPt), isoprene and 19'-hexanoyloxyfucoxanthin (19'-hex) correlated in nitrogen-depleted regions when they were clustered by nitrogen to phosphorous ratio (N:P). The 19'-hex-containing algae groups might be a common source of DMS, DMSPt, and isoprene in the low N: P regions. Additionally, DMS and isoprene correlated in nitrate-depleted regions when they were clustered against nitrate concentrations. Correlations between DMS and isoprene were also found within nitrate-depleted eddies encountered along the cruise track. Eddies with N: P of similar to 2.8 showed the highest positive correlations between DMS and isoprene. We conclude that the DMS/isoprene relationships in the eastern Atlantic Ocean were influenced by nutrient availability, with implications for using nutrients to predict the DMS and isoprene concentrations over a range of oceanographic areas depleted in nitrogen