PARENT SESSION
Oral Session #56: Aquatic Ecology: Plankton. Presiding: N. Hairston.
Wednesday, August 8, 2001. 1:00 PM to 5:00 PM. Hall of Ideas I.

Generalist herbivores reduce prey community diversity in nutrient-poor systems.

VANDER MEULEN, MICHAEL¹, MCCAULEY, EDWARD¹, ¹

ABSTRACT- It is well known that the freshwater herbivore Daphnia can suppress the biomass of edible algae and that their dynamics are tightly coupled energetically. These generalist herbivores can promote the biomass and diversity of inedible algae, but few studies have tested how these herbivore dynamics affect the diversity of the edible algal community. Mechanisms, related to grazing losses and nutrient recycling, suggest that increases in grazing pressure should promote the species diversity of edible algae. We tested the hypothesis that Daphnia dynamics promotes the diversity of edible algal communities under conditions of either pulsed, pressed, or ′packaged′ phosphorus additions (i.e. perturbations that would modify algal competition in phosphorus-limited systems). Predictions were tested in four mesocosm experiments using natural phytoplankton assemblages enclosed in situ. Each experiment lasted 40 days and documented the dynamics of phosphorus, Daphnia, other herbivores, and edible algal biomass and community composition in 16 replicate 700 L systems. As expected, Daphnia significantly suppressed edible algal biomass and increased phosphorus supplies for algae relative to controls. Contrary to expectations, increases in Daphnia biomass resulted in significant decreases in edible algal species diversity, but promoted diversity among major algal taxa (i.e. class level). Phosphorus additions did not influence our results. Diversity reductions due to grazing losses by Daphnia outweigh the effects on diversity caused by reduced algal competition.

KEY WORDS: herbivore dynamics, prey diversity, Daphnia, phytoplankton