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.

Non-linear effects of Daphnia on a phytoplankton community: a large-scale, grazer-gradient experiment.

Sarnelle, Orlando¹, ¹

ABSTRACT- Most of what is known about the effects of consumers on their resources in nature has come from field experiments with two or a few consumer-density treatments. Consequently, we know little about the functional forms of consumer effects in the field. To delineate the form of herbivore effects on phytoplankton species abundance, I maintained a gradient of Daphnia via periodic removal in 12 large enclosures suspended in a eutrophic lake. For comparison, mortality rates of individual phytoplankton species were assessed with separate 24-hour dark incubations with and without Daphnia. Comparing the net responses of phytoplankton species to the Daphnia gradient after 5 days, with estimates of relative vulnerability from the mortality rate experiments suggested that species-specific variation in net responses was largely driven by variation in mortality over the short term. Over longer time scales (25-30 d), Daphnia's net effects on the biomass of phytoplankton species were strongly non-linear, most commonly approximating a decelerating negative function. After 25-30 days, the effect of Daphnia on Anabaena, a filamentous bluegreen, was parabolic: Anabaena biomass decreased as Daphnia density increased from 40 to 600MISSING CHARACTER ENTITY: mug/L, then increased as Daphnia density increased from 600 to 1100MISSING CHARACTER ENTITY: mug/L. Alternative explanations for these non-linear effects will be discussed.

KEY WORDS: phytoplankton, zooplankton, herbivory, non-linear