Abstract
The area near the subpolar front of the East Sea has high primary productivity during the spring season. We conducted two surveys, one in early spring and another in late spring, to assess environmental factors that influence phytoplankton community structure during these times. During early spring, vertical mixing supplied abundant nutrients to the surface. Due to the well-mixed water column, there were high nutrient levels, but total phytoplankton abundances and diversity were relatively low and were dominated by the diatom Chaetoceros spp. During late spring, the water column gradually stratified, with relatively high levels of nutrients in the surface layers near the coastal areas. Phytoplankton abundance and diversity at that time were higher, and there were diatoms (Pseudo-nitzschia spp. and Chaetoceros spp.), cryptophytes, and small flagellates. Pseudo-nitzschia spp. were especially abundant in re-sampled areas. The presence of a stratified and stable water mass and sufficient nitrate led to high phytoplankton growth, even in the open sea during late spring. These findings provide a better understanding of how phytoplankton population dynamics in the East Sea depend on water column stability during both early and late spring seasons.
Highlights
The waters surrounding the inner Jinhae Bay to the Nakdong River Estuary (Stn. 1) ranged from 7 to 9 ◦ C, the Busan coast and Pohang offshore area ranged from 12 to 15 ◦ C, and regions near Ulleungdo and Dokdo (Stns. 6–16) ranged from 9 to 12 ◦ C
We assessed surface water temperature gradients during early spring using sea surface temperature (SST) data from an onboard TSG sensor (Figure 2). These data clearly distinguished waters influenced by the East Korea Warm Current (EKWC) and southeastern Korean coastal waters, implying that the warm water from the Tsushima Warm Current (TWC) and EKWC had a great influence on the East Sea, in the Ulsan and Pohang offshore waters, the region included in our CS zone
The early spring was characterized by strong winds and vertical mixing of the water column, which led to relatively high nutrient concentrations and an undeveloped phytoplankton community
Summary
Phytoplankton are a foundational part of marine ecosystems and have a significant effect on overall ecological community dynamics. Many oceanographic features, such as temperature, salinity, and nutrient availability, influence the timing and distribution of phytoplankton blooms. An influx of coastal nutrients can trigger phytoplankton blooms and species succession [1,2]. The coastal, pelagic, and estuarine zones may differ in nutrient concentrations, phytoplankton species, and total species abundance. Many studies throughout the world have documented this general pattern of a nutrient gradient and its impact on phytoplankton communities and primary production [3,4]
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