MEPS prepress abstract  -  DOI:

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

Katherine Richardson*Jørgen Bendtsen


ABSTRACT: 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, DNO3 (defined as [NO-3]=1μmol 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 DNO3 and (1) depth of the deep chlorophyll maximum (DCM), (2) total water column PP, (3) vertical distribution of PP. When DNO3 is located between ~20 and 90 m, the DCM and DNO3 are juxtaposed. However, the DCM is located above nutriclines found at > ~90 m. The observed relationships between DCM and DNO3 depths can be explained with a simple model including light and nutrient limitation. The global primary production estimates indicate that ~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 DNO3 indicates that oligotrophic regions of the ocean may be more productive than usually assumed. The relationship shown here between water column PP and DNO3 suggests that considering stratification characteristics in a future ocean is critical for predicting climate change effects on global PP.