Zooplankton effects on vertical particulate flux: Testable models and experimental results

Abstract

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.

Contribution of algal sinking and zooplankton grazing to downward flux in the Lazarev Sea (Southern Ocean) during the onset of phytoplankton bloom: a lagrangian study

“…The results of the drogue study show that the contribution of grazing-mediated POC flux was insignificant during the onset of the MIZ bloom. Apparently, large meso-and macrozooplankton densities and feeding activities were insufficient to exert either positive (via faecal pellets) or negative (via reducing suspended phytoplankton concentration) effects on sedimentation (Sarnelle 1999). As a consequence of the classical mismatch between phytoplank- Table 7.…”

Kinetics of phosphorus in Daphnia at different food concentrations and carbon:phosphorus ratios

“…However, direct excretion is not the only pathway in P recycling by zooplankton. Zooplankton can feed on phytoplankton in surface waters and produce sinking fecal materials, which are remineralized by bacteria or contribute to the vertical flux of particulates (Sarnelle 1999). In addition, the P flux associated with living crustaceans may occur by molting, since a considerable percentage of body P is carapace bound (Vrede et al 1999).…”

Influence of zooplankton stoichiometry on nutrient sedimentation in a lake system

“…In this lake, the zooplankton community was dominated by cladoceran species (Elser et al 1998). In Lake 110, there was no significant effect of zooplankton on primary production, but had a negative effect on sedimentation, resulting in a net decrease of the export ratio (Sarnelle 1999, with data from Elser et al 1995). The zooplankton community was dominated by calanoid copepods (Elser et al 1998).…”

“…7. In Lake 240, zooplankton negatively influenced both sedimentation and production rates, but with greater effects on production than on sedimentation, resulting in a net increase of the export ratio (Sarnelle 1999, with data from Elser et al 1995). In this lake, the zooplankton community was dominated by cladoceran species (Elser et al 1998).…”

“…In lakes, the vertical flux of small particulate matter essentially comprises two components: the sinking of ungrazed phytoplankton cells (e.g., Reynolds et al 1982) and the sinking of the feces or fecal pellets of planktonic primary consumers (Sarnelle 1999). Both types of particles will settle from the upper layer below the thermocline if their sinking rates are higher than their mineralization rates.…”