Semidiurnal Dynamics of Phytoplankton Size Structure and Taxonomic Composition in a Macrotidal Temperate Estuary

The semidiurnal dynamics of phytoplankton in a temperate macrotidal estuary were explored by monitoring phytoplankton size structure and taxonomic composition at a fixed station on a seasonal basis. Phytoplankton chlorophyll a (chl a , a proxy for biomass) varied out of phase with the tidal height in February, May, and November, but varied in phase with the tidal height in July. The in-phase pattern in July was observed because the horizontal gradient of chl a differed from other seasons. The horizontal chl a gradient was affected by salinity decrease resulting from the regulated freshwater discharge that occurred 1 day before the diel monitoring in July. This suggests that phytoplankton chl a varies with the rapid change in salinity resulting from anthropogenic freshwater inputs from sea dikes, in addition to varying with tidal currents in the estuary. This season-dependent pattern of response to tidal forcing has not been observed in other temperate estuaries. The relationship with tidal height was enhanced as cell size increased from pico- to micro-size, as the horizontal chl a gradients of larger cells were more evident than those of pico-sized cells. The abundance of diatoms, a dominant group throughout all seasons, fluctuated with tidal height, as did the large cells, whereas the dominant diatom species shifted quickly, and tidal effects on phytoplankton species were not evident. The intra-tidal variations in the phytoplankton community were substantial, but were explained mainly by tidal currents (except at the species level) and as the effect of regulated freshwater input, even when freshwater was no longer being discharged. This suggests that tidal phase and salinity are important parameters to consider in determining long-term trends and seasonal variations in the phytoplankton communities of macrotidal estuaries altered by engineered structures such as sea dikes.