The simulated Phaeocystis and diatom trends (yearly average and maxima reached) in BCZ, over the past 50 years are discussed with respect to changing nutrient loads, especially NO3 and PO4. Historical reconstruction back to the fifties suggests that Phaeocystis colonies were already blooming in BCZ in the early 1950's and were sharing almost equally with diatoms, the bulk of annual primary production. Over the 1960–1992 period, the increased loads of both N and P were beneficial to both Phaeocystis colonies and summer diatoms with however a little advantage to the latter. Since 1989, the decrease in P loads subsequent to the removal of PO4 in washing powders and the maintenance of elevated NO3 loads had a positive feedback to both diatoms and Phaeocytis production with however a significantly larger impact on diatoms than on Phaeocytis. This is due to the fact that Phaeocystis has strong affinity for low PO4. Consequently Phaeocystis development is mainly controlled by NO3 loads. We conclude that future management of nutrient emission reduction aiming at decreasing Phaeocystis blooms in BCZ without impacting on diatoms would target decrease of NO3 loads by both the rivers Seine and Scheldt.