AME 45:207-218 (2006)  -  doi:10.3354/ame045207

Trophic cascades mediated by copepods, not nutrient supply rate, determine the development of picocyanobacteria

Line E. Sundt-Hansen1,5, Yngvar Olsen1, Herwig Stibor2, Mikal Heldal3, Olav Vadstein4,*

1Norwegian University of Science and Technology, Department of Biology, Trondhjem Biological Station, 7491 Trondheim, Norway
2Department of Biology II, Aquatic Ecology Group, Ludwig-Maximilians-Universität München, Karlstrasse 23-25, Grosshaderner Strasse 2, 82152 Planegg, Germany
3University of Bergen, Department of Biology, Jahnebakken 5, 5020 Bergen, Norway
4Norwegian University of Science and Technology, Department of Biotechnology, 7491 Trondheim, Norway
5Present address: Norwegian Institute for Nature Research, Unit of Fish Ecology, Tungasletta 2, 7485 Trondheim, Norway
*Corresponding author. Email:

ABSTRACT: We studied the effect of nutrient supply rate (resources) and concentrations of copepods (top predator) on picocyanbacteria (PCY) in experimental ecosystems (mesocosms) using gradients of both nutrients and predators. The biomass and production of PCY, and the biomass and predation rates of their predators were determined by epifluorescence microscopy, X-ray microanalysis (XRMA), the 14C method, and cell counting by microscopy. PCY biomass did not change significantly with increased supply of nutrients, but it was positively correlated with copepod biomass. A lack of variation in PCY growth rates, and nutrient saturation of cells as suggested by nutrient analysis and XRMA measurements, indicate that PCY grew close to their maximum growth rate during the experiment. Appendicularia and ciliates were the main predators on PCY when copepod biomass was normal or low, whereas heterotrophic nanoflagellates were significant predators on PCY when copepod biomass was high. Based on a negative effect of copepod biomass on the biomass of PCY predators, we conclude that predation had a stronger effect on the development of PCY than did nutrient supply rate. The only effect of nutrient supply was negative and indirect, implemented through stimulation of predatory ciliates. Copepods had a profound effect by structuring the food web, and thus directly and indirectly regulating the abundances of predators of PCY (top-down control). Therefore, copepods indirectly controlled the abundance of PCY through trophic cascades.


KEY WORDS: Trophic cascade · Resource limitation · Nutrients · Ciliates · Appendicularia · Top-down · Bottom-up


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