Anthropogenic N is Driving the East China and Yellow Seas to P Limitation: Implication of a change in bloom-forming phytoplankton from diatom to dinoflagellate

Abstract

Increasing emissions of anthropogenic reactive nitrogen from northeastern Asian countries (i.e. China, Japan, and Korea) and its addition to the East China and Yellow seas via riverine input and atmospheric deposition have led to an unparalleled increase in the nitrate (N) concentration in the upper ocean. However, this N increase has not been accompanied by an increase in phosphate (P) concentration. The disproportionate increase of N over the past four decades has progressively transformed extensive areas of the East China and Yellow seas from N limitation to P limitation. The area of P limitation was nearly absent in the 1980s but rapidly expanded in a northeast direction along the downstream of the Changjiang River to coastal waters of Korea, where the change was also confirmed at the Ieodo Ocean Research Station: Explicitly, a 3 fold increase from 6.32 × 104 km2 (1990s) to 18.55 × 104 km2 (2010s). The areas other than those downstream of the Changjiang River have remained in N deficit relative to P, but the extent of N deficiency has decreased over time. Our results provide direct observational evidence showing a nutrient regime shift from N limitation to P limitation. This shift has transformed the study area to conditions where the occurrence of dinoflagellate blooms exceeds that of diatom blooms; dinoflagellates are known to be better able to acquire P in both inorganic and organic forms.