We examined the preferences of phytoplankton groups for waters of different trophic status by comparing the distribution of 8 main phytoplankton groups during the spring bloom, post-bloom, and late stratification periods in the northwestern Mediterranean. Pigment chemotaxonomy (using the CHEMTAX computer program) was applied to estimate the contribution of Prymnesiophyceae, Pelagophyceae, Synechococcus spp., Prochlorococcus spp., Prasinophyceae, Cryptophyceae, Dinophyceae, and Bacillariophyceae to the chlorophyll a (chl a) stock. Particulate organic nitrogen (PON) concentration was used as an indicator of trophic status. PON at the surface was 1.7 +/- 1.4, 0.57 +/- 0.02, and 0.37 +/- 0.04 mu mol l(-1) in the bloom, post-bloom, and stratification periods, respectively. During the bloom period, there was a weak stratification and a large chl a biomass. Bacillariophyceae dominated during the bloom period, with a substantial contribution of Prasinophyceae. Prymnesiophyceae and Synechococcus spp. dominated during post-bloom and stratification periods, and Prochlorococcus spp. was a major contributor to biomass in the deep chlorophyll maximum (DCM) during the stratification period. Vertical segregation was also evident for Pelagophyceae, Prymnesiophyceae, and Cryptophyceae, which preferred the DCM to surface layers in non-bloom conditions. The relative distribution of each group combined with PON concentrations in these 3 periods allowed us to calculate a group-specific trophic preference index, which showed its highest values (more eutrophic) for Bacillariophyceae, Prasinophyceae, Cryptophyceae, and Dinophyceae; medium values (mesotrophic) for Prymnesiophyceae, Pelagophyceae, and Synechococcus spp.; and a very low value (oligotrophic) for Prochlorococcus spp. The pigment-group diversity (Shannon index) and evenness were lower during the bloom period.