###### Abstract  
The Youngsan River estuary, located on the southwest coast of South Korea, has transitioned from a natural to an artificial estuary since dike construction in 1981 separated freshwater and seawater zones. This artificial transition has induced changes in the physical properties and circulation within the estuary, which has led to hypoxia and algal blooms. In this study, an artificial neural network (ANN) model was employed to simulate phytoplankton variations, including algal blooms and size fractions based on chlorophyll \(a\), using data obtained by long-term monitoring (2008-2018) of the seawater zone of the Youngsan River estuary. The model was validated through statistical analyses, and the validated model was used to determine the contribution of the environmental factors on size-fractionated phytoplankton variations. The statistical validation of the model showed extremely low sum square error (SSE \(\leq\) 0.0003) and root mean square error (RMSE \(\leq\) 0.0173) values, with R\({}^{2}\)\(\geq\) 0.9952. The accuracy of the model predictions was high, despite the considerable irregularity and wide range of phytoplankton variations in the estuary. With respect to phytoplankton size structure, the contribution of seasonal environmental factors such as water temperature and solar radiation was high for net-sized chlorophyll \(a\), whereas the contribution of factors such as freshwater discharge and salinity was high for nano-sized chlorophyll \(a\), which includes typical harmful algae. Notably, because the Youngsan River estuary is influenced by a monsoon climate--characterized by high precipitation in summer--the contribution of freshwater discharge to harmful algal blooms is predicted to increase during this period. Our results suggest that the ANN model can be an important tool for understanding the influence of freshwater discharge, which is essential for managing algal blooms and maintaining the ecosystem health of altered estuaries. harmful algal blooms; artificial neural network; size-fractionated phytoplankton; Youngsan River estuary; South Korea +
Footnote †: journal: Applied Physics Letters  
1
Footnote 1: _Applied Sciences_  
25 July 2021  
Published: 27 July 2021
Thus, analyzing the variations in phytoplankton size structure along with environmental factors is critical to understand and manage HABs as well as the entire aquatic ecosystem. Turbulence occurs in estuaries due to their binary water layer structure, where freshwater meets seawater in a transition zone [8]. Estuaries, thus, harbor diverse species and are known to be highly productive systems due to nutrient loads from the adjacent land [9]. The production of organic matter per unit area is estimated to be 4-10-fold higher than in grainfields, and the ecological value per unit area is reported to be the highest among Earth's ecosystems, at USD 22,832 ha\({}^{-1}\) yr\({}^{-1}\)[10]. The Youngsan River (YR) estuary, the target estuary in the present study, is located on the southwest coast of South Korea and has a temperate and monsoonal climate (E 126\({}^{\circ}\)26\({}^{\prime}\)-126\({}^{\circ}\)18\({}^{\prime}\), N 34\({}^{\circ}\)49\({}^{\prime}\)-34\({}^{\circ}\)44\({}^{\prime}\)) [11]. A dike was constructed in 1981, and the resulting division between the freshwater and seawater zones transformed the YR estuary from a natural to an artificial estuary, with corresponding changes in water quality and environmental properties [12]. Since dike construction, the freshwater zone has experienced an increase in nutrient concentration and accelerated organic pollution [11], with recent incidences of hypoxia [13]. In the case of the seawater zone, although the normal state (negligible freshwater discharge) allows the physical properties and circulation to be controlled by tidal forcing, the opening of the sluice gate lets in a large quantity of freshwater that alters the physical properties and circulation in the estuary [14].