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 (Skagerrak, Swedish west coast) was diluted 10 times with filtered seawater in twelve 300-dm(3) polyethylene cylinders. Nutrients were added to produce 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 treatment were carefully mixed while the other three of each nutrient treatment remained unmixed. Diatoms (Nitzschia and Chaetoceros species) were favoured by low dissolved NISI molar ratios (N/Si 1) and by mixing. Diatoms increased as a percentage of total phytoplankton biomass from 11% initially to about 32% in the unmixed and to 46% in the mixed cylinders. At high N/SI ratios (N/Si 4) small flagellates (1-6 mu m) became dominant and diatoms never constituted more than 20% of total phytoplankton biomass.