Abstract
Mesocosm experiments were conducted in the Seto Inland Sea every summer seasons from 1987 to 1989. Effects of controlled conditions on phytoplankton succession in the mesocosm were examined. One control factor was a vertical mixing method, which included a direct circulation by pumps (run 1) and a new designed circulation system (run 2 and run 3). In the system, vertical jet entrained the surrounding water, making soft mixing in the mesocosm. The other factor was an enrichment pattern. Nutrients were added at the surface layer in run 1 and run 2, and introduced under the mixing layer in run 3. Phytoplankton species succession was observed every year from diatom to dinoflagellate. Red tide due to Chattonella antiqua was observed only in 1989, when the new circulation system was used and the vertical nutrient profile was set as similar to those for red tides in the field. The most important factors to control environmental conditions in mesocosms were found to be the vertical mixing power, the mixing depth, nutrient ratios (N : P : Si), and vertical distribution of nutrients.
