In order to test the response of phytoplankton to high nitrogen inputs occurring in the coastal Bay of Brest, subsurface water samples were enriched at varying ammonium concentrations (0, 10, 20, 30, and 40 μm-N) and a fixed concentration of nitrate (40 μm-N), other nutrients being in excess. Results indicated that natural phytoplankton populations could perform simultaneous uptake of both nitrogen forms at ammonium levels previously considered as preventing nitrate uptake by oceanic phytoplankton. Nitrate uptake commenced at a lowered rate (range: 0·15–0·23 μm-N h−1) after ammonium concentrations have decreased to a first threshold value (range: 8·0–16·0 μm-N). Below a second ammonium threshold level (range: 2·3–6·2 μm-N), maximum rate of uptake of nitrate was observed (range: 0·31–0·56 μm-N h−1). Results are discussed in terms of adaptation of microalgae to the mode of fertilization they experience in the coastal ecosystem.