Phytoplankton diversity in Lake Norrviken 1961–1975

Five common indices were compared to see which of them would be most suitable in the study of long time changes in a phytoplankton community. Margalefs index d showed the same course during the year as did the number of species. This is also valid for Brillouin's, Shannon's, Simpson's and McIntosh's indices when based on number of individuals. Brillouin's and Shannon's indices based on cell number were found to differ from the diversity values obtained by Simpson's and McIntosh's indices depending on their different sensitivity to the two components of diversity, species richness and species evenness. Diversity indices including species richness and evenness (e.g. all except Margalef's) are recommended, but their different sensitivity to their species richness and evenness component should not be overlooked. A description of the phytoplankton composition should always be given together with diversity indices. This is very important since any index used gives no information on the species and their ecology.