Vertical distribution of phytoplankton and primary production in relation to nutricline depth in the open ocean

Global ocean primary production (PP) is a function of both light and nutrient availability. The vertical distribution of nutrients in the euphotic zone differs in both time and space. As a result, the vertical distribution of PP varies as well. Differences in the vertical distribution of PP have not, however, been systematically studied. Here, we focus on the open ocean and use nutricline depth, D-NO3 (defined as the depth where [NO3-] = 1 mu mol kg(-1)), as a proxy for nutrient availability in the euphotic zone. Using our own and archived (WOD, HOT, BATS, CARIACO) data, we show universal relationships between D-NO3 and (1) depth of the deep chlorophyll maximum (DCM), (2) total water column PP and (3) vertical distribution of PP. When D-NO3 is located between similar to 20 and 90 m, the DCM and D-NO3 are juxtaposed. However, the DCM is located above nutriclines found at > similar to 90 m. The observed relationships between DCM and D-NO3 depths can be explained with a simple model including light and nutrient limitation. The global PP estimates indicate that similar to 25% of ocean PP occurs in the upper 10 m. Estimating total global ocean PP from surface optical characteristics and the relationship between vertical PP distribution and D-NO3 indicates that oligotrophic regions of the ocean may be more productive than usually assumed. The relationship shown here between water column PP and D-NO3 suggests that considering stratification characteristics in a future ocean is critical for predicting climate change effects on global PP.