Dilution experiments were performed during two transects across

M~a s u r e m c n t s of natural marine phytoplankton communities were carried out in a mesocosm e x p e r i m~n t by means of pigment analysis by HPLC to identify phytoplankton composition, and using mirroscop~c ~dentification of species and measurement of cell volume to estimate the carbon content of the phytoplankton groups. The enclosures werc manipulated with additions of nutrients and mussels and reduction of light to induce changes in the phytoplankton communities. The trophic status of the individual enclosures was illustrated by F,, ratios deflned as the diagnostic pigments for diatoms and dinoflagellates d~v i d e d by the total concentration of all d~agnostic pigments. The F,, ratios were significantly correlated to F, ratios, defined as the carbon content of dinoflagellates and dlcitoms divided by total phytoplankton carbon, suggesting that the phytoplankton diagnostic pigments can be used as both a qualitative and quantitative indicator of the respective phytoplankton groups.

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