Dual effects of estuarine dams on summertime phytoplankton productivity in temperate river-estuary transition zones

Abstract

The river transports dissolved and particulate matter to the estuary, where physical and biogeochemical transitions occur. In the river-estuary transition zone (RETZ), phytoplankton response is an effective indicator of environmental variability with their immediate responses. Here, we examine summer-monsoonal variability in productivity and nitrogen uptake of phytoplankton in three major Korean rivers and their estuaries: the Han, Geum, and Yeongsan. Results showed that summer-monsoonal variability of primary productivity in dam-constructed RETZ divided them into riverine zone governed by nutrient-phytoplankton interactions and estuarine zone associated with nitrogenous substrate dynamics. In contrast, free-flowing RETZ exhibited lower variability in phytoplankton productivity due to simultaneously enhanced nitrogenous nutrients and turbidity. Our generalized additive model analysis indicated that composition of particulate organic matter is primarily correlated with phytoplankton biomass associated with light availability. Our findings emphasize the complex interplay of physical and biogeochemical processes in RETZ, mainly influenced by monsoonal variability and river-estuary connectivity. © 2025 Elsevier Ltd