Nutricline

The nutricline is the depth zone of steep vertical gradient in dissolved nutrients, where concentrations of nitrate, phosphate, and silicate rise sharply from the depleted euphotic zone into the nutrient-rich interior. It usually lies near the base of the thermocline because the same stratification that blocks vertical mixing keeps surface nutrients low and deep nutrients high. The nutricline depth sets new production: where it shoals, as in upwelling zones and equatorial divergences, nutrient supply to the sunlit layer rises and productivity climbs. A deep nutricline marks oligotrophic subtropical gyres.