Short-term variations of phytoplankton communities in response to anthropogenic stressors in a highly altered temperate estuary

Data for phytoplankton size classes, taxonomy, and water properties were collected through an episodic freshwater discharge event (4 days) in the temperate Youngsan River estuary, which is highly disturbed by manually regulated inputs of freshwater from a sea dike, to investigate the effects of an acute change in anthropogenic stressors on the short-term dynamics of phytoplankton and their surrounding environments. The salinity of the well-mixed saline water (33.2–33.5) decreased to as low as 4.0 and water temperature increased to 24.0 °C during the freshwater discharge, resulting in a stratified water column in the upper region of the estuary. During the discharge, chlorophyll a (chl a) concentrations increased to as much as 15.66 μg L−1 with micro-sized phytoplankton being dominant due to the presence of micro-sized freshwater phytoplankton, mostly Aulacoseira ambigua (98% in cell abundance), transported from the reservoir. Primary production decreased to as little as 87.9 mg C m−2 d−1, although nutrients such as NO2− + NO3− were supplied by the freshwater inputs of the discharge. Following the discharge, dinoflagellate blooms, dominated by Heterocapsa sp. (>88%), a nano-sized red tide species, developed in the upper regions of the estuary with peaks in chl a concentrations reaching as high as 30.33 μg L−1. Another red tide species, Prorocentrum micans, was also dominant in the estuary, suggesting that harmful algal blooms (HABs) are associated with anthropogenic stressors related to the freshwater inputs. The Shannon diversity index decreased to 0.18 while the Simpson dominance index increased to 0.94 during the discharge, but the diversity increased again following the discharge. The phytoplankton communities and diversity changed along the salinity gradient, corresponding to an “ecocline” pattern. The results of multivariate statistical analysis suggested that phytoplankton species and size structure were controlled mainly by salinity, water temperature, turbidity, and PO43−, which were affected by the regulated freshwater discharge. This study indicates that rapid changes in anthropogenic stressors related to the operation of an engineered structure may impact the eutrophication and biotic integrity of an estuarine system by generating short-term variations in phytoplankton biomass, size, and species composition, especially of harmful algae.