Responses of phytoplankton functional types to changing winter vertical mixing: a modeling study

Abstract

Using the zero-dimensional European Regional Seas Ecosystem Model (ERSEM) for 2001-2012, we investigated the impacts of varying winter vertical mixing and atmospheric deposition on the lower trophic level ecosystem in the Ulleung Basin, East Sea. The model results show that as the winter maximum mixed layer depth (MMLD) changes, the growth and grazing loss of phytoplankton functional types (PFTs) are affected differently resulting in an altered succession process of PFTs in the upper mixed layer. Diatoms preempt the early spring growth by better utilization of light and nitrate. Diatoms’ advantages lessen as MMLD decreases. Flagellates and picophytoplankton show mixed responses to decreased MMLD. Their net primary productivity (NPP) and peak biomass decrease but their annual biomass increases due to decreased grazing. Dinoflagellates are always doing better when MMLD decreases. Taken together, if warming continues and winter vertical mixing decreases, the total NPP and peak biomass will decrease although the yearly biomass of flagellate, pico-, and dinoflagellate will increase.t Sea. The model results show that as the winter maximum mixed layer depth (MMLD) changes, the growth and grazing loss of phytoplankton functional types (PFTs) are affected differently resulting in an altered succession process of PFTs in the upper mixed layer. Diatoms preempt the early spring growth by better utilization of light and nitrate. Diatoms’ advantages lessen as MMLD decreases. Flagellates and picophytoplankton show mixed responses to decreased MMLD. Their net primary productivity (NPP) and peak biomass decrease but their annual biomass increases due to decreased grazing. Dinoflagellates are always doing better when MMLD decreases. Taken together, if warming continues and winter vertical mixing decreases, the total NPP and peak biomass will decrease although the yearly biomass of flagellate, pico-, and dinoflagellate will increase.