A field study was carried out to determine the relative importance of both food quantity and food quality to reproduction of the copepod Euterpina acutifrons. The total protein concentration of the seston 3 to 20 µm size fraction was used as the indicator of food available for copepods in the field. Food quality was quantified by comparing the amino acid (AA) composition of females, eggs and the seston 3 to 20 µm size fraction, and by considering the species composition of the phytoplankton community. Egg production rate was mainly determined by food concentration. However, for the same food concentration, a reduced egg production rate was observed when there were more differences between the AA composition of eggs and females. Hatching success was correlated only with food quality. A higher hatching success was obtained when the similarity between the AA composition of eggs and females and between the AA composition of females and seston was higher. However, a reduced hatching success was observed when the proportion of diatoms in the phytoplankton was higher than 70%. Therefore, food quality limitation was due to an AA imbalance in the natural seston and, perhaps, due to the presence of inhibitory compounds. Copepods were food limited for long periods of the year. Food quality mainly limited copepod reproduction in spring and early summer, whereas food quantity limited reproduction at the end of autumn and early winter. In summer and early autumn the reproductive success of copepods was higher.

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