Multiple environmental variables related to ocean currents, freshwater runoff, and upwelling in a coastal area have complex effects on the phytoplankton community. To assess the influence of environmental variables on the phytoplankton community structure during the summer of 2019, we investigated the various abiotic and biotic factors in Korean coastal waters (KCWs), separated into five different zones. Summer environmental factors in KCWs were strongly influenced by Changjiang Diluted Water (CDW) in St. SO (Southern Offshore) 1 and 2, upwelling in St. SI (Southern Inshore) 2–4, and Nakdong River discharge in St. SI 12. In particular, low–salinity water masses (p < 0.05 for nearby locations) of CDW gradually expanded from the East China Sea to southwestern KCWs from June to July. In addition, there were high levels of nutrients following freshwater runoff from the Nakdong River in southeastern KCW, which led to the dominance of Cryptomonas spp. (81%), a freshwater and brackish water algae. On the other hand, upwelling areas in southwestern KCW were dominated by diatoms Skeletonema spp., and are characterized by high phosphate concentrations (p < 0.05) and low temperatures (p < 0.05) compared to nearby locations. Leptocylindrus danicus (20%) was dominant due to the effect of water temperature in the SE (Southeastern area) zone. Low nutrient concentrations were maintained in the East Sea (dissolved inorganic nitrogen (DIN) = 0.39 ± 0.40 μM; dissolved inorganic phosphate (DIP) = 0.09 ± 0.03 μM) and the Yellow Sea (DIN = 0.40 ± 0.07 μM; DIP = 0.04 ± 0.02 μM), which were characterized by low levels of chlorophyll a and dominated by unidentified small flagellates (35, 40%). Therefore, our results indicated that hydro–oceanographic events such as upwelling and freshwater run–off, but not ocean currents, provide nutrients to the euphotic layers of the coastal environment and play important roles in determining the phytoplankton community structure during summer in the KCWs.

Ship biofouling is recognized as a significant pathway for the introduction and spread of invasive organisms. The in-water cleaning of ship hulls generates wastewater that includes antifouling paint residues and biofouling organisms, which inevitably leak into the marine environments, resulting in substantial adverse effects on marine ecosystems. To assess the impact of hull cleaning wastewater (HCW) on microalgae, we conducted microcosm experiments using HCW including attached microalgae. The experiments consisted of a total of 12 combined trials, including the following groups: ambient seawater as a control, the 5% HCW group (HCW), and the 5% HCW + nutrient addition group (HCW+N), conducted at temperatures of 15 and 20 °C, respectively. The Chl. a concentrations in the water column in the control group exhibited maximum values on day 1 (5.24 μg L⁻¹ at 15 °C and 12.37 μg L⁻¹ at 20 °C), but those of the treatments were at low levels, below 2 μg L⁻¹ at both temperatures. On the other hand, the Chl. a concentrations on plastic plates were higher in the treatments than in the control group. Specifically, the Fv/Fm ratio in the water column, which indicates photosynthetic activity, was significantly higher in the control group compared to both the HCW and HCW+N groups at 15 and 20 °C (p < 0.05). This suggests that the growth of water column phytoplankton was inhibited following HCW inoculation. However, there were no significant differences in the Fv/Fm on plastic plates between the control and HCW treatment groups, implying that the periphyton maintained a high photosynthetic capacity even in the presence of HCW treatments. The elution of particulate copper in HCW was observed, which was considered as the main reason for the growth of phytoplankton. Our study results suggest that the runoff of HCW in the marine environment has a greater negative effect on phytoplankton than on periphyton, which can lead to changes in microalgae community composition and a decrease in productivity in the marine environment. Therefore, it is crucial to manage HCW runoff based on scientific assessments to minimize the ecological risks associated with the removal of biofilm or slime from ship biofouling during in-water hull cleaning.