Influence of salt-wedge intrusion on ecological processes at lower trophic levels in the Yura Estuary, Japan

To examine the influence of salt-wedge intrusion on ecological processes at lower trophic levels in estuaries, we investigated seasonal variability in the nutrients, phytoplankton biomass (chlorophyll a), zooplankton density, and composition in the Yura Estuary, Japan. Phytoplankton composition was determined under two discriminative regimes: salt-wedge regime (summer) and freshwater regime (winter). Phytoplankton from two groups bloomed simultaneously under the salt-wedge regime. Freshwater and brackish phytoplankton dominated the upper freshwater layer (∼16,100 cells mL−1) while marine phytoplankton were dominant below the halocline (∼12,200 cells mL−1). Both phytoplankton groups grew using riverine nutrients. Marine phytoplankton increased closer to the tip of the salt-wedge, indicating that marine phytoplankton grew in situ in the river. The residence time of salt water increased due to the low river discharge from spring to summer, enhancing mineralization in the salt-wedge. Regenerated nutrients in the salt-wedge contribute to subsequent marine phytoplankton blooms. Copepods were more abundant in the salt-wedge (∼80.9 ind. L−1) than in the upper freshwater layer (∼14.2 ind. L−1) during summer. Phytoplankton and zooplankton densities under the freshwater regime (winter; ∼360 cells mL−1, ∼39.7 ind. L−1) were lower than under the salt-wedge regime (summer). The high river discharge under the freshwater regime flushes out the salt-wedge and discourages biological production in the river. Our results showed that salt-wedge intrusions change the physical conditions and nutrient dynamics, enhancing biological production in the salt-wedge estuary.