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 Available to Purchase

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.