SEAWATER EXCHANGE AND RIVER WATER INFLOW ESTIMATED BY PHYTOPLANKTON COMMUNITIES IN NAGATSURA-URA LAGOON, MIYAGI PREFECTURE

Abstract

 Nagatsura-ura Lagoon, a brackish lake in Miyagi prefecture, is the main sea area producing oysters in Miyagi, Japan. However, death of the oysters is a concern because of the poor oxygen water mass is likely to be formed owing to the closed topography. In this study, we estimated the seawater residence time using a numerical model and the phytoplankton community composition in and around the lagoon to examine the seawater exchange and river water inflow in the lagoon. The numerical model showed the lower layer of the seawater in the lagoon has a longer residence time, especially in summer when the density stratification is remarkable. On the other hand, the phytoplankton community composition in the middle and lower layers of the lagoon at that time were also different from the surroundings. This fact indicates that the similarity of phytoplankton community was related to the residence time of seawater. In addition, the proportion of freshwater phytoplankton contained in the upper layer of the lagoon does not always coincide with the decrease in salinity. Hence, salinity fluctuations in the upper layer of the lagoon may be influenced not only by river water but also by the inflow of stream water. These results indicate that it is useful to examine the phytoplankton community to understand the inflow situation from each water source in water areas entered from a variety of sources such as sea, rivers and streams.