Eutrophication is a global environmental issue and has been studied for more than 40 years in Atsumi Bay, Aichi Prefecture, Japan. The distribution of nutrients in watersheds and estuaries is strongly influenced by meteorological factors, especially rainfall. Our observations in summer 2010 found that high river discharges during rainfall altered nutrient concentrations throughout the bay, and furthermore resulted in a red tide due to nutrient uptake by phytoplankton. In order to study nutrient dynamics and phytoplankton growth in estuaries, with a focus on the effects of river inputs during rainfall, we developed a depth-averaged two-dimensional ecological model. The model was conceived with the ability to simulate nutrient dynamics, in the form of dissolved nutrient concentrations, and phytoplankton growth throughout an estuary. The model provided reasonable results and agreed well with observed data. Our evaluation showed that the increase in availability of dissolved nutrients and favorable irradiance post-rainfall stimulated production of phytoplankton in the bay, compared to pre-rainfall.

Agricultural rivers are often silent receivers of microplastics (MPs) from diffuse, non-point sources; however, their pollution dynamics during rainfall events remain poorly understood. In this study, MP transport was investigated at three sampling points in an agricultural river catchment, where mulching films are used, and sewage sludge is not applied. Sampling was conducted in the Umeda River and its tributaries during six sampling events. MP flux exhibited a strong positive correlation with river discharge (L–Q relationship; n = 1.49–1.61, R² = 0.67–0.87). The L–Q model indicates that a tenfold increase in discharge results in approximately a 600-fold increase in MP flux and a 1000-fold increase in total suspended solid flux. MP abundance during rainfall was up to four times higher than that during baseflow, ranging from 73 ± 64 to 200 ± 111 particles/m³, while peak flux reached 6736 particles/s, with an MP mass of 811 mg/s. Regarding particle characteristics, rainfall enhanced the heterogeneity of MPs, although fragments and polyethylene/polypropylene polymers remained consistently dominant across all hydrological stages. First-flush behavior was observed at HU, with more than half of the total MP mass exported within the initial 50% of the event flow volume. These findings help to inform mitigation strategies that should prioritize a reduction in upstream plastic inputs in order to effectively manage MP transport in agricultural rivers.

Eutrophication in an estuary occurs as an effect of the enrichment of nutrient inputs from rivers. This condition has become one of the most common environmental issues experienced around the globe and especially in Japan. Atsumi Bay is a eutrophic coastal area in Japan. The objective of this research was to analyze the influences of nutrient inputs from the Umeda River into Atsumi Bay on pre- and post-rainfall water quality conditions. This study was conducted from July to October 2010. The results showed a decrease of surface salinity after rainfall indicating that huge freshwater inputs had overlaid the surface layer of Atsumi Bay rather than the bottom layer. Moreover, post-rainfall conditions showed an increase of chlorophyll a as an effect of phytoplankton growth, followed by an increase of particulate nutrients. On the other hand, dissolved nutrients decreased due to uptake by phytoplankton and dilution by freshwater.