Limnol. Oceanogr., 44(2), 1999, 357–370

The effects of herbivorous zooplankton on the sedimentation of particles out of the euphotic zone are examined with mathematical models, a large-scale field experiment, and descriptive data from a eutrophic lake. The theory is rooted in the population dynamics of phytoplankton and zooplankton and so explicitly accounts for the potential effect of zooplankton grazing on primary production and the connection between the rate at which phytoplankton cells sink and sustainable zooplankton biomass. The models predict positive, negative, or unimodal zooplankton effects, depending on the values of four parameters: rate of direct phytoplankton sinking, fraction of zooplankton fecal material exiting the euphotic zone, zooplankton assimilation efficiency, and system productivity. Models were parameterized with data from a eutrophic lake to make a priori predictions about the shape and direction of zooplankton effects. Predictions were tested against the results of an independent experiment in which a gradient of Daphnia biomass was established in large enclosures. Daphnia negatively affected sedimentation rates of carbon, nitrogen, and phosphorus in the enclosure experiment, which confirmed model predictions. Daphnia had a strong negative effect on phytoplankton biomass, and phytoplankton biomass was positively correlated with sedimentation in the enclosures. Experimental results were congruent with relationships between Daphnia biomass and sedimentation rate in the lake. Successful application of the theory suggests that these models may be of utility for assessing the direction of zooplankton effects on vertical flux in other systems. More generally, the models help to identify parameters that should be measured in studies of zooplankton effects on downward particle flux. The rate at which particles sink out of the euphotic zone is an important process driving the seasonal dynamics of nutrients, as well as benthic–pelagic coupling, in stratified water columns (Hinga et al. 1979; Angel 1984; Guy et al. 1994). In addition, the sinking flux of particulate organic carbon fixed by marine phytoplankton is an important component of global carbon cycles (Michaels et al. 1994; Sarmiento and Bender 1994). Consequently, there is considerable interest in understanding and predicting sedimentation flux in both lakes and oceans (Suess 1980; Pace et al. 1987; Wassmann 1991; Baines and Pace 1994). Of particular interest in recent years is the role that primary consumers (zooplankton) play in sedimentation (Noji 1991). This interest has grown out of recognition that marine flux is generally dominated by large particles with high sinking velocities (Angel 1984; Fowler and Knauer 1986) and that fecal pellets of zooplankton are an important class of rapidly sinking particles (Honjo and Roman 1978; Bathmann et al. 1987; Bodungen et al. 1987; Morris et al. 1988). Most research to date addressing the role of zooplankton in sedimentation has been directed at determining the potential contribution of zooplankton-derived fecal material to the total downward flux of particles (Pilskaln and Honjo 1987; Dilling and Alldredge 1993; Lane et al. 1994). Such studies, however, do not indicate the overall effect of zooplankton