Tidal Influences on Biotic and Abiotic Factors in the Seomjin River Estuary and Gwangyang Bay, Korea

Abstract

To assess changes in abiotic and biotic factors between flood and ebb tides, we investigated the seasonal phytoplankton dynamics and environmental conditions along a salinity gradient at 14 stations in the Seomjin River estuary (SRE), Korea, and conducted bioassays to investigate the effect of nutrient addition (+N, +P, and +NP) on phytoplankton growth. Saltwater intrusion upstream was greatly dependent on the amount of freshwater discharge resulting from seasonal rainfall. There was a strong negative correlation between salinity and the nitrate+nitrite concentration (p<0.001), and between salinity and the silicate concentration (p<0.001), but no clear correlation between salinity and the ammonium concentration, or salinity and the phosphate concentration (p>0.01). This indicates that the N and Si loading increased as a result of freshwater input. The algal bioassays showed that high phytoplankton growth rates were usually recorded in response to the +NP treatment, but in the saltwater zone, the phytoplankton community also responded rapidly to the +N treatment, and to the +P treatment in the freshwater zone. The range of nutrient limitation depended on freshwater discharge. The seasonal and horizontal distribution of phytoplankton communities changed along the salinity gradient. The significant differences in abiotic factors between flood and ebb tides play important roles in controlling the biotic factors, including the occurrence of aquatic organisms including microalgae.