River input is one of the most important sources of nutrients for coastal waters. Compared to normal conditions, flood events can deliver several times the amount of freshwater and nutrients to estuaries in a short period, potentially exerting a substantial influence on primary production in coastal waters affected by the river plume. However, these potential impacts depend on the local conditions of each estuary and its adjacent coastal waters. To clarify these influences, we selected all the major rivers around Japan, spanning a latitude range from 31°N to 45°N, and conducted a comprehensive analysis of the effects of flood events on coastal chlorophyll concentrations using satellite data (GOCI).

The annual mean discharges of selected rivers range from 100 to 800 m³/s. Flood events are defined as the 98th percentile of discharge, which is generally several times the mean discharge. Flood discharges occur on a small number of days but can contribute as much as 6.7% to 40.6% of the annual discharge for different rivers. This ratio exhibits a significant meridional increasing trend from north to south. The timing of flood events also shows meridional variation. Flood events in northern rivers typically occur in spring and summer, whereas those in southern rivers are more likely to occur in late summer.

The relationship between satellite chlorophyll data and flood discharge is relatively complex. To address the significant gaps in chlorophyll data before and after each flood event, we calculated the mean chlorophyll concentration for periods with and without flood influence and assessed the flood effects by comparing the differences between these two periods. For rivers with a high flooding discharge ratio, the relative increases in estuarine chlorophyll concentrations are also high. This correlation is more pronounced in estuaries with shorter residence times. During spring floods, both a high flooding discharge ratio and shorter residence time contribute to the increase in chlorophyll after flooding. In contrast, during summer floods, shorter residence time plays a more significant role than river discharge.

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