『Abstract
　This paper aims at a synthesis of our conceptual views on the marine nutrient cycling. Based on selected biological, physical and geochemical aspects of the system, we present a framework for the description of the nitrogen cycling in oligotrophic regions of the ocean. Recent findings about the importance of the mesopelagic for remineralization are combined with ideas on the role of primary production, external input and fixation in nitrogen cycling. Instead of the usual distinction between euphotic and aphotic zones (i.e., with and without net primary production), our concept suggests a three-layer model featuring the subsurface production remineralization layer (SPRL) between the base of the surface mixed layer (SML) and the nutrient maximum at about 1000-1200 m depth, which can be used as a biogeochemical definition for the lower boundary of the mesopelagic. The approach leads to revised definitions of new and regenerated production, as well as export into the deep ocean, which are more adequate to characterize the biological pump, at least in the oligotrophic ocean. One of the main conclusions is that the steady state nitrogen export from the SPRL is fully determined by the sum of atmospheric deposition of nitrogen and N2-fixation and can be best quantified at the depth of the nitrogen maximum. While the magnitude of primary production by non-diazotrophic phytoplankton is irrelevant for export into the deep ocean, biological processes are crucial in determining size, shape and density of sinking particles and the remineralization rate of detritus.

Keywords: Stratified ocean; Nutricline; Subsurface biomass maximum layer; Primary production; Biological pump; Export; Remineralization length scale Mesopelagic; Twilight zone; Conceptual model』

Introduction
Some characteristics of the surface nutrient depleted water column
　Vertical mixing
　Optical properties and the euphotic zone
　Macronutrient profiles
An updated concept for nutrient cycling in the oligotrophic ocean
Discussion
Conclusions
Acknowledgements
References