Dilution effects on microzooplankton in dilution grazing experiments

In dilution experiments, filtered seawater is used to create a gradient of grazing pressure on phytoplankton. Microzooplankton grazing is estimated by examining phytoplankton growth within the gradient. However, the dilution series also represents a resource gradient for microzooplankton. Here we re port the effects of dilution on grazers. In 2 standard dilution experiments , using communities from the eutrophic (chlorophyll a = 12 to 15 mu g l(-1) ) Rhode River Estuary, we examined the effects of dilution on different gro ups of microzooplankters: rotifers, tintinnid ciliates, oligotrich ciliates , predacious ciliates, and Mesodinium rubrum. Apparent growth rates of tint innids and oligotrichs varied with prey concentration, decreasing with the dilution factor from about +0.5 d(-1) in undiluted whole water to about -1 d(-1) in the 5% whole water, closely resembling numerical response curves. Among tintinnids, there was an increase in the relative abundance of larger tintinnids in the time 24 h samples of dilute treatments compared to the l ess dilute treatments. No consistent dilution effect was shown by rotifers or predacious ciliates. The growth rates of the photosynthetic ciliate M. r ubrum increased with dilution, resembling the typical pattern of chlorophyl l a and autotrophic nanoplankton. Grazer growth in undiluted waters and gra zer mortality in dilute water may be common and result in uncertainty in me asured grazing rates. We urge that grazers be examined in grazing experimen ts not only to assess possible artifacts in grazing rate estimates, but als o to provide information, beyond a simple grazing rate, on the grazer popul ations.