JOURNAL OF PLANKTON RESEARCH | VOLUME 25 | NUMBER 12 | PAGES 1485-1495 | 2003
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Response of temperate microplankton communities to N:Si ratio perturbation
1 Scottish Association for Marine Science, Dunstaffnage Marine Laboratory, Oban, Argyll PA34 4AD and 2 School of Life Sciences, Napier University, 10 Colinton Road, Edinburgh EH10 5DT, UK 3 Present Address: School of Biological Sciences, University of Wales Swansea, Singleton Park, Swansea SA2 8PP, UK
* Corresponding Author: e.roberts{at}swansea.ac.uk
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, heterotrophic plankton failed to respond rapidly to increased diatom biomass. The heterotrophic:autotrophic biovolume ratio remained <0.1 for the majority of the experiment, suggesting that, given similar nutrient concentrations and ratios, much of the autotrophic production would be lost from surface waters through diatom aggregation and sinking before the micrograzer community was able to respond. Measured differences in diatom physiology between treatments are discussed with respect to nutritional quality and consequences for planktonic grazers.