BLOOMS OF PHYTOPLANKTON INCLUDING EMILIANIA-HUXLEYI (HAPTOPHYTA) - EFFECTS OF NUTRIENT SUPPLY IN DIFFERENT N-P RATIOS

Enclosures containing natural phytoplankton communities were fertilized with nitrate and phosphate in duplicates in three different ratios (16: 5, 16 : 1, 16 : 0.2) in order to initiate blooms of the coccolithophorid Emiliania huxleyi (Lohmann) Hay et Mohler under controlled environmental conditions. The development of the phytoplankton community was in addition followed in two unfertilized enclosures and in the surrounding sea water. The phytoplankton succession during the experiment (22 April - 29 May 1992) was mainly dinoflagellates-diatoms-E. huxleyi/Phaeocystis sp. in all fertilized enclosures, but the importance of the different species/groups were different in enclosures with different N : P ratio. Diatom numbers decreased when the N : P ratio increased in the nutrient supply. From an initial concentration of 0.09 10(9) cells m-3 E. huxleyi increased to concentrations between 20 10(9) cells m-3 and 37 10(9) cells in m-3 in the enclosures supplied with nitrate and phosphate in a N : P ratio of 16 : 1 and 16 : 0.2. The growth of E. huxleyi in the four enclosures was remarkably synchronous with a specific growth rate mu almost-equal-to 0.3 day-1. The peak of the blooms differed only by a few days in time, and the blooms decayed immediately after maximum in cell numbers (decay rate mu almost-equal-to -0.2 day-1). Control factors of growth and decay of the blooms are discussed. The growth was apparently controlled by temperature and in periods also by low irradiance, while estimated viral mortality could explain the decay. In enclosures supplied with nitrate and phosphate in a N : P ratio of 16 : 5 E. huxleyi only reached concentrations of 5 10(9) cells m-3. The highest cell numbers of Phaeocystis sp. were observed in enclosures with low and balanced N : P ratios. An analysis of blooms of E. huxleyi and Phaeocystis sp. from enclosure experiments carried out every year between 1988 and 1992 illustrated that E. huxleyi grow well at low concentrations of phosphate, and was a better competitor than Phaeocystis sp. as long as the sea water temperature was about 10-degrees-C or higher and the surface irradiance was 20 mol m-2 day-1 or higher. Phaeocystis sp. had a higher phosphate demand, but was a better competitor than E. huxleyi at sea water temperatures lower than 10-degrees-C and at surface irradiance lower than 20 mol m-2 day-1, if nutrients were available.