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Phytoplankton are recognised as the primary source of food for cultured mussels. Small phytoplankton (<5 μm), however, are often not efficiently retained as food and therefore phytoplankton size can affect bivalve growth and condition. In Beatrix Bay, New Zealand, small picophytoplankton (phytoplankton <2 μm in diam.) sized cells contributed on average 29% of the phytoplankton biomass. They dominated the biomass (52%) in June (winter), when light was limiting, and (40%) during September (spring), when light levels were increasing but nutrients were becoming depleted. This substantial contribution of picophytoplankton to total phytoplankton biomass may affect the growth and condition of mussels. Our results highlight the need for a better understanding of prey selection by mussels and raise questions about the relevance of total chlorophyll a concentration as an accurate measure of phytoplankton food …