Calanoid copepods and nutrient enrichment determine population dynamics of the appendicularian Oikopleura dioica: A mesocosm experiment

Abstract

Appendicularians, an important group of marine gelatinous zooplankton, are highly efficient filter feeders of small phyto- and bacterioplankton. It is not known which factors regulate their abundance and biomass in the field. In a mesocosm experiment, we manipulated nutrient loading and initial densities of calanoid copepods (almost exclusively the genera Temora sp., Centropages and Pseudocalanus) and followed plankton dynamics over 2 wk. Peak appendicularian densities were inversely related to initial copepod densities. We observed more than 100-fold variation in Oikopleura dioica density among treatments, even though our experimental variables were kept within ecologically relevant magnitudes. The differences between copepod-reduced and copepod-enhanced treatments were much larger at high and moderate nutrient loadings than in treatments without nutrient enrichment. Thus, appendicularians are under strong pressure from calanoid copepods. Calanoid copepods appear to structure the plankton community via direct negative effects on large algae, ciliates and appendicularians and indirect positive effects on small algae resulting from the release of these from predation by ciliates and appendicularians.