Flood-driven nutrient influx and phytoplankton succession in the subtropical waters of the Matsu Archipelago, Taiwan

Abstract

Flood-driven nutrient influxes are a major driver of harmful algal blooms (HABs) in subtropical coastal systems, yet baseline phytoplankton abundance data are often lacking. We conducted a four-year (2021-2024) time-series study in the Matsu Archipelago, Taiwan, to examine the background abundance and succession dynamics of diatoms and dinoflagellates in relation to Minjiang River flooding events. Salinity was used to define flooding (salinity <29; March-June) and non-flooding seasons. The results of our study show that the distribution of phytoplankton consisting of diatoms (1.8 x 10(6) cells/L) and dinoflagellates (4.8 x 10(6) cells/L) has the highest concentration peak in the May and July range. In those months, flooding generally occurs in the Minjiang River. The new background abundance of diatoms is 250,000 cells/L while dinoflagellates are 60,000 cells/L. In general, the abundance each year ranges between these numbers and increases drastically during the flooding season. Diatoms peaked during flooding periods, exhibiting strong correlations with silicate, nitrite, chlorophyll a, temperature, and pH, whereas dinoflagellates were positively associated with dissolved oxygen and chlorophyll a. Canonical Correspondence Analysis revealed that Pseudo-nitzschia, Rhizosolenia, and Chaetoceros were strongly linked to temperature and chlorophyll a during flooding events, while Eucampia dominated in non-flooding seasons. These results provide the first baseline abundance estimates for key phytoplankton groups in this region and highlight the role of freshwater nutrient inputs in shaping community dynamics. This ecological framework offers a foundation for early-warning systems to mitigate HAB risks in subtropical estuarine ecosystems.