INFLUENCE OF DIFFERENT NITROGEN TO SILICA RATIOS AND ARTIFICIAL MIXING ON THE STRUCTURE OF A SUMMER PHYTOPLANKTON COMMUNITY FROM THE SWEDISH WEST-COAST (GULLMAR FJORD)

A natural summer phytoplankton community from the Gullmar Fjord (Skage rrak, Swedish west coast) was diluted 10 times with filtered seawater in twelve 300-dm(3) polyethylene cylinders. Nutrients were added to pr oduce two different molar ratios of nitrogen to silicon (N/Si 3.2/3.2 mu M=1 and 12.8/3.2 mu M=4). Three cylinders of each nutrient treatmen t were carefully mixed while the other three of each nutrient treatmen t remained unmixed. Diatoms (Nitzschia and Chaetoceros species) were f avoured by low dissolved NISI molar ratios (N/Si 1) and by mixing. Dia toms increased as a percentage of total phytoplankton biomass from 11% initially to about 32% in the unmixed and to 46% in the mixed cylinde rs. At high N/SI ratios (N/Si 4) small flagellates (1-6 mu m) became d ominant and diatoms never constituted more than 20% of total phytoplan kton biomass.