Samples from 2 l of sea water were filtered through polycarbonate Millipore filters. The filters were kept deep frozen and analyzed according to Holm-Hansen et al. (Holm-Hansen, O., Lorenzen, C.J., Holmes, R.W., Strickland, J.D.H. 1965. Fluorometric determination of chlorophyll. Journal du Conseil Permanent International Pour l'Exploration de la Mer 30, 3-15).