Suspended Clay and the Population Dynamics of Planktonic Rotifers and Cladocerans

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Abstract
The direct and indirect effect of suspended clay on the population dynamics of several species of planktonic rotifers and cladocerans were investigated using long—term laboratory experiments. Life table and population growth experiments showed that high, but naturally occuring, concentrations (50—100 mg/L) of coarse, suspended clay (m) of four cladoceran species (Bosmina longirostris, Ceriodaphnia dubia, Daphnia ambigua, and D. pulex). Juveniles were more susceptible than adult to suspended clay. Low concentrations (10 mg/L) of coarse clay, and high concentrations of fine clay (<1 μm), did not decrease, and sometimes increased, cladoceran population growth rates. Both the inhibitory effects of high concentrations of coarse clay and the stimulatory effects of low concentrations of clay were greater at limiting food (Cryptomonas) concentrations. The population growth rates of four rotifer species (Brachionus calyciflorus, Keratella cochlearis, Polyarthra vulgaris, and Synchaeta pectinata) were not affected by high concentrations of coarse or fine clay, even at very low food levels. The population growth rates of the rotifer K. crassa was decreased by coarse clay. The threshold food concentration (the food concentration, measured as dry mass, at which population growth rate equals zero) of the cladoceran D. ambigua was increased from 0.15 to 0.40 mg/mL in the presence of 50 mg/L coarse clay, indicating a reduced ability to compete in exploitative contests. The threshold food concentration of the rotifer K. cochlearis (0.05 mg/mL) was unaffected by suspended clay. Suspended clay reversed the outcome of two rotifer—cladoceran competition experiments, between K. cochlearis and D. ambigua, and between K. cochlearis and C. dubia. In the absence of clay, the cladocerans dominated; in the presence of clay, the rotifer dominated. The presence of suspended sediments in natural ecosystems, such as turbid lakes and reservoirs, should favor rotifers over cladocerans and thus influence the structure of zooplankton communities.