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-dm3 polyethylene cylinders. Nutrients were added to produce two different molar ratios of nitrogen to silicon (N/Si 3.2/3.2 μM = 1 and 12.8/3.2 μM=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 N/Si 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 μm) became dominant and diatoms never constituted more than 20% of total phytoplankton biomass.