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

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