Abstract
A subpolar front (SPF) generated between the East Korea Warm Current (EKWC) and the North Korea Cold Current (NKCC) in the western margin of the East/Japan Sea has shifted northward in recent decades. This study investigated the biomass and composition of the phytoplankton assemblage in relation to hydrological and biogeochemical features in the shallow shelf and slope off the Korean coast from January to June in 2016 and 2017, to determine the mechanistic effects of SPF on spring–summer phytoplankton bloom dynamics. Monthly average depth-integrated chlorophylla(Chla) levels and the contribution of phytoplankton classes revealed bimodal diatom blooms in early spring and summer in the frontal zone. Canonical correspondence analysis showed that the distribution of high Chlawas associated with cold, low-salinity NKCC water in March 2016. No Chlapeak was observed in March 2017 when the warm saline EKWC water mass invaded. These results suggest that the NKCC intrusion acts as a forcing mechanism leading to enhanced phytoplankton biomass in the frontal zone. In contrast, positive correlations of Chlaconcentration with water density and nutrient concentrations suggest that summer blooms were fed by the subsurface chlorophyll maximum (SCM) driven by shoaling of the pycnocline and nitracline. Varying water-column stratification determined the thickness of the SCM layer, driving year-to-year variability in the magnitude of diatom blooms. These findings further suggest that seasonal/interannual variability in the timing of algal blooms affects regional trophodynamics and hence could be an important factor in explaining ecosystem changes in this region.
Highlights
Nearshore surface water (
Chlorophyll a concentrations peaked in the surface layer (0–20 m) along nearshore, shallow stations parallel to the coastline in March 2016 and in the subsurface layer (20–50 m) across cross-shelf transects in June 2016
A conceptual paradigm that explains the seasonal variability of primary production and phytoplankton biomass in the Ulleung Basin (UB) has been proposed based on water-column stability and its direct link to biogeochemical and phytoplankton-bloom dynamics (Kwak et al, 2013b)
Summary
Nearshore surface water (
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