Transformation processes of nitrogen, phosphorus and iron in sub-euphotic waters and surface sediments

In the ocean, concentrations and ratios of nitrogen (N), phosphorus (P) and iron (Fe) determine primary production. In upwelling regions, nutrient-rich deep waters fuel this primary production. Knowledge of nutrient transformation processes in deep waters is still scarce. Therefore N-removal via denitrification and anaerobic ammonium oxidation (anammox) as well as P and Fe transformation processes were investigated. In the Arabian Sea up to 50% of N-loss was due to benthic N-removal, emphasizing the high potential of sediments to contribute to the N-deficit in the water column. In sandy permeable sediments off Mauritania, denitrification rates correlated with sediment grain size, indicating a strong relationship between benthic denitrification rates and advective porewater transport. On the shelf off Mauritania and Namibia, P-uptake was investigated in sub-euphotic waters and found to be largely biologically driven. In conclusion, transformation processes determine concentration and bioavailability of nutrients in the sub-euphotic water column and sediments and may eventually have an impact on the primary production in the surface ocean.