The distribution of nitrate throughout the global ocean has been measured in a number of global sampling campaigns, from WOCE to GEOTRACEs, and nitrogen has long been identified as the primary limiting nutrient in the low latitude ocean. Using the stable isotopic signatures in nitrate, the processes which influence Atlantic nitrogen cycling are discussed - the supply, uptake and regeneration of nitrate, and its relationship with phosphate and silicate.
Mode and intermediate waters which form in the Southern Ocean are the principal supply of macronutrients to the surface low latitude ocean. The mechanisms which promote the retention of nitrate and phosphate in mode water formation regions in contrast to silicate are explored using nitrogen isotope data (δ15NNO3, δ18ONO3 and δ15NPN) between 40-60°S in the Atlantic. Through this approach we identify cross frontal mixing of subtropical and Antarctic Surface Water and seasonal changes in the subantarctic thermocline. Nitrification is most prominent at the base of the summer mixed layer (~50m), the shallow depth of remineralisation helps to retain nitrate within the seasonal mixed layer of ~200m and within mode water formation regions. We thus identify nitrification and the physical processes controlling subantarctic mixed layer depths as key in enhancing nitrate export in mode waters to support primary production in the subtropical Atlantic. Uptake and remineralisation processes within the low latitude Atlantic can determine the extent at which fixed N is utilised and/or produced by N-fixers, per unit of phosphorous. Water mass modification through the Atlantic basin is investigated to determine the extent at which nitrate in the upper intermediate waters is recycled as it transits through the low latitudes. Decreases in δ18ONO3 in mode waters indicate the importance of N-fixers in producing high N:P particle ratios (18-21:1) for remineralisation, this is further confirmed by a decrease in δ15NNO3 through the subtropical Atlantic. This reveals the dominance of recycling processes and diazotrophy in producing high N:P ratios in the North Atlantic thermocline. The talk will explore the importance of these processes in understanding the stability of the marine N cycle.
