Toxicant exposure increases threshold food levels in freshwater rotifer populations

Food limitation and toxicant stress may be important community structuring factors in zooplankton such as rotifers. We hypothesized that food limitati on increases rotifer sensitivity to toxicants as measured with a standard t wo day population growth test and that toxicant stressors shift population growth thresholds, potentially altering competitive relationships. Populati on growth rates of the rotifer Brachionus calyciflorus were determined at h igh (18.3 mg of the green alga Nannochloris oculata dry weight per liter) a nd low (1.83 mg/L) food levels under mercury or pentachlorophenol toxicant stress. Rotifer sensitivity to toxicant stress was increased by as much as 7 times when populations were exposed to toxicants at the low versus high f ood level. Rotifer populations required up to 6 times as much food to maint ain their size when exposed to a toxicant concentration causing no statisti cally significant reduction in r (NOEC). (C) 1999 by John Wiley & Sons, Inc .