Consumer-induced changes in phytoplankton: inducibility, costs, benefits and the impact on grazers.

While inducible defenses of terrestrial plants against herbivores have long been known, only recently have corresponding responses of phytoplankton to their consumers been reported. Several chlorococcal algal species are able to adjust their phenotype (colony form, spines) to grazing pressure within hours or a few generations. The responses are triggered by information chemicals based on an immediate perception of consumer density. Several zooplankton species induce spines or colony formation in various species of Scenedesmus and Coelastrum. Induced changes in algae confer grazing resistance against small zooplankton and can be interpreted as an adaptive antiherbivore strategy. Reduced algal palatability results in life history effects (lower growth and fecundity) on small Daphnia species, but not on large ones. Forming colonies does not seem to lower algal growth rates, but algal colonies may have higher sinking rates. Inducible phenotypic responses may be a consequence of a trade-off between higher sinking rates and grazing resistance of colonial forms. They may have evolved in response to the variable conditions in the lake pelagic zone