Effects of Humic Acids on Size and Species Composition of Phytoplankton in a Eutrophic Temperate Estuary

Abstract

The Yeongsan River estuary was divided into freshwater and seawater zones by a sea dike constructed at its mouth in 1981. The freshwater zone, which flows through a metropolitan area, is eutrophic, causing frequent algal blooms with an expected increase in the concentration of refractory organic compounds such as humic substances (HS). Herein, the in situ freshwater zone phytoplankton community size and taxonomic composition were investigated in response to the addition of humic acids (HA) using seasonal mesocosm experiments. Phytoplankton (chlorophyll a) were fractionated into nano-(<20 µm) and net-size (>20 µm) classes and identified by species or genus. Their response to HA treatment was examined by repeated measures analysis of variance (RM-ANOVA). With the addition of HA, the concentrations of total and nanosized chlorophyll a increased significantly (p < 0.05), whereas that of net-sized chlorophyll a did not change significantly through the seasons. The abundance of Stephanodiscus sp. (diatoms) also increased significantly when this genus dominated the phytoplankton community. This suggests that the management of HS may be crucial in mitigating algal blooms in estuaries, such as in the Yeongsan River estuary, that are subjected to anthropogenic disturbances by engineered structures.