Cladocerans versus copepods: the cause of contrasting top-down controls on freshwater and marine phytoplankton.

Top-down control of phytoplankton by crustacean mesozooplankton is a cornerstone of freshwater ecology. Apparently, trophic cascades are more frequently reported from freshwater than from marine plankton. We argue that this difference is real and mainly caused by biological differences at the zooplankton-phytoplankton link: cladocerans (particularly Daphnia) in the lakes and copepods in the sea. We derive these conclusions from recent literature and a number of own, similarly designed mesocosm experiments conducted in a lake, a brackish water and a marine site. In all experiments, phytoplankton were exposed to gradients of experimentally manipulated densities of zooplankton, including freshwater copepods and cladocerans, and marine copepods and appendicularians. The suggested reasons for the difference between lake and marine trophic cascades are: (1) Both copepods and cladocerans suppress only part of the phytoplankton size spectrum: cladocerans the small and copepods the large phytoplankton. (2) If not controlled by grazing, small phytoplankton may increase their biomass faster than large phytoplankton. (3) Copepods additionally release small phytoplankton from grazing pressure by intermediate consumers (protozoa) and competitors (predation on appendicularian eggs), while cladocerans do not release large phytoplankton from grazing pressure by any functional group. (4) Cladocerans sequester more of the limiting nutrient than copepods, leaving fewer nutrients available for compensatory growth of ungrazed phytoplankton.