Seasonal variation of primary productivity in the Shinkawa River estuary, eastern Seto Inland Sea, Japan

Abstract

Seasonal variations in the increase rate of chlorophyll (Chl.) a and the primary production rate were measured by in situ seawater incubation in the Shinkawa River estuary in the Seto Inland Sea, Japan. Nutrient concentrations in the seawater were always sufficient for the growth of phytoplankton. The study site was very shallow, but photo-inhibition, which occurs at high light intensities, such as > 1,000 μmol/m²/s, was not observed in photosynthesis test samples. Therefore, light intensity was a minor factor to explain primary production in this area throughout the year. Chl. a concentration reduced a little or was almost constant at low water temperatures. In contrast, Chl. a concentration increased significantly in the period of high water temperature (July to September), showing a maximum specific increase rate of 0.3-0.4/h. The primary production rate was also high while water temperature was high. Monthly primary production rates, estimated from water temperature and PAR, varied from 0.01±0.00 to 1.13±0.24 gC/m²/day, which was similar to that reported for other estuaries and coastal seas. The most influential factor in primary productivity in this study area was water temperature, with peak temperatures associated with extremely high primary productivity, with up to 460μgC/l/h recorded in summer.