Moderate increase in the biomass of omnivorous copepods may ease grazing control of planktonic algae

Abstract

Copepods and ciliates are 2 functionally different types of herbivores that dominate marine ecosystems. Ciliates have a higher reproductive capacity than copepods, but copepods can graze on ciliates. Hence we have an omnivorous system in which the inferior grazer can graze on its competitor. We conducted a 3 2 factorial mesocosm experiment with natural plankton communities to explore how the copepod-ciliate interaction affects algal development on the timescale relevant for algal blooms. We used nutrient load and copepod biomass as the experimental variables. Ciliate biomass was positively correlated to nutrient addition and negatively correlated to copepod biomass, with copepod biomass as the predominant factor at the highest densities tested. Both increasing copepod biomass and nutrient addition resulted in increased algal biomass, with the strongest effect on nanoalgae. The mechanism involved was carnivory by copepods and a resulting trophic cascade through ciliates to algae. We conclude that even a moderately high biomass of copepods may promote algal blooms when the initial size distribution of the algal community is appropriate (i.e. dominance of algae eaten primarily by ciliates).