Sundt-Hansen LE, Olsen Y, Stibor H, Heldal M, Vadstein O
trophic cascade, Resource limitation, Nutrients, ciliates, Appendicularia, top down, bottom up, mesocosm, XRF, Hopavangen, Norway, 4 m3
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 ofboth nutrients and predators. The biomass and production of PCY, and the biomass and predationrates 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 significantlywith increased supply of nutrients, but it was positively correlated with copepod biomass. A lack ofvariation in PCY growth rates, and nutrient saturation of cells as suggested by nutrient analysis andXRMA 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 normalor low, whereas heterotrophic nanoflagellates were significant predators on PCY when copepodbiomass 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 supplyrate. The only effect of nutrient supply was negative and indirect, implemented through stimulationof predatory ciliates. Copepods had a profound effect by structuring the food web, and thusdirectly and indirectly regulating the abundances of predators of PCY (top-down control). Therefore,copepods indirectly controlled the abundance of PCY through trophic cascades.