Growth of three plankton diatom species in Oslofjord water in the absence of artificial chelators

Cultures of the plankton diatoms Skeletonema costatum (Greville) Cleve, Thalassiosira nordenskioeldii Cleve, and Lauderia annulata Cleve were grown in sea-water samples collected in the inner Oslofjord on 11 occasions between February, 1976 and January, 1977. The growth experiments were carried out in silicone-coated culture flasks, with nitrate, phosphate, and silicate added in excess, and under standardized conditions of a temperature of 10 °C and a saturating light intensity. Growth rates, as measured during exponential growth over a period of 5–6 days, were, with very few exceptions, close to the maximum rates obtained in a complete medium. The addition of ethylenediaminetetra-acetic acid (EDTA) to the water samples did not lead to improved growth, nor did EDTA have an effect on the initial lag phase. The lack of a seasonal variation in the observed growth responses was in marked contrast to the large fluctuations in the phytoplankton populations in the fjord. These findings give no support to the hypothesis that species successions among marine plankton diatoms are influenced by organic chelators or other extracellular substances produced by planktonic algae.