Analyses of planktonic ecosystem structure in coastal seas using a large-scale stratified mesocosm: a new approach to understanding the effects of physicsl, biochemical and ecological factors on phytoplankton species succession

Abstract

Mesocosms (marine enclosures) have advantages in ecosystem analysis because they are mass-conserved systems, but with biochemical and ecological conditions similar to those of the surrounding water column. Furthermore, our mesocosm (5m in diameter and 18m in depth) developed in the Seto-Inland Sea, Japan (Watanabe et al., 1995) has vertical mixing system to control its physical conditions. Two independent experiments in 1989 and 1991 both in summer showed a high ability to reproduce the physical and biochemical conditions of the water column, underpinning the ability to design desired experimental yet near-natural conditions using the mesocosm. The dominance and population shifts of centric diatoms, pennate diatoms and flagellates in the experiments were explained by the effects of changing nutrient (N, P and Si) availability, the features of the phytoplankton species present and grazing by zooplankton. Two implications for nutrient management to control phytoplankton composition at the site were obtained: 1) Si depletion leads to shifts of dominant species from larger to smaller diatoms or flagellates; and 2) stocks of N and P below the nutricline remaining after depletion of those in the surface layer lead to the dominance of flaggellates, the latter being difficult to observe within small scale experimental systems.