Edinburgh Napier University

  In order to study the effect of the nitrogen:silicon (N:Si) ratio on temperate microplankton food webs, mesocosm experiments were conducted in Trondheim (Norway) using two different ratios (molar ratios of 1:1 and 4:1). With the exception of diatoms, the increase in abundance of all microbial groups [phototrophic nanoflagellates, autotrophic dinoflagellates, bacteria, heterotrophic nanoflagellates (HNAN), heterotrophic microflagellates and ciliates] was significantly greater in the high N:Si treatment. Midway through the experiment, HNAN biovolume exceeded that of bacteria in the high N:Si treatment, indicating strong top-down grazing control. Heterotrophic microflagellate biovolume exceeded ciliate biovolume under both nutrient regimes. However, hetero-trophic plankton failed to respond rapidly to increased diatom biomass. The heterotrophic:auto-trophic biovolume ratio remained