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Uusitalo, L., Fleming-Lehtinen, V., Hällfors, H., Jaanus A., Hällfors, S., and London, L. 2013. A novel approach for estimating phytoplankton biodiversity – ICES Journal of Marine Science, 70: 408–417. Maintaining biodiversity is one of the main priorities in environmental protection. The biodiversity of phytoplankton, the key primary producers in the marine ecosystem, is, however, often very difficult to estimate, since the phytoplankton assemblage includes a vast number of taxa, many of which occur in such small quantities that they may not be recorded in routine sampling. Moreover, many taxa cannot be identified to species level by routine methods such as light microscopy of preserved samples, even by a skilled taxonomist. This means that, in general, we cannot assume to have a complete list of species in the ecosystem at any given point in time. In this paper, we present an approach for evaluating phytoplankton biodiversity in spite of this challenge. Since eutrophication, which increases phytoplankton biomass, has been identified as the most important factor causing degradation of the Baltic Sea ecosystem, the proposed approach was evaluated against total phytoplankton biomass. When phytoplankton biomass was low, both low and high biodiversity values were observed, and, as the phytoplankton biomass increased, the high biodiversity values disappeared. These results were consistent both using data based on individual samples and using yearly sampling station averages.