The results of Bioassay 4 (Fig. 4) indicate that almost all the animals, regardless of the type of particle they were exposed to, survived throughout the bioassays, if food was added, even at low concentration. However, when no food was added, all the organisms died, except for those in the NP and NPn treatments. Thus it may be affirmed that the natural particles provided some nutrition for the animals so that the animals showed dose-dependent survivorship for NP and NPn. This is a novel and unexpected finding, in spite of the existence of some supporting findings from previous studies. In these previous studies, B. longirostris, C. dubia, D. ambigua, and D. pulex sometimes showed increased population growthrates with low concentrations (10 mg L1) of coarse clay (<2 lm), and high concentrations (50–100 mg L1) of fine clay (<1 lm).With limited food concentrations, both the inhibitory effects ofhigh concentrations of coarse clay and the stimulatory effects oflow concentrations of clay were greater (Kirk and Gilbert, 1990).Moreover, Cyclidium sp. showed slightly higher yet significantgrowth rates in the presence of clay, as did most flagellates. Thelargest difference in growth was found for Spumella sp., whichshowed respective rates of 0.19 and 0.14 h1 with and without clay(Boenigk and Novarino, 2004). Suspended particle concentrationsof 5, 10, 25 and 100 mg L1, which are present in Lake Brienz, didnot reduce the fitness of the Daphnia hyaline, even at very low food
concentrations, and sometimes even increased their fitness when
in suspension with a low concentration of suspended particles
(Rellstab and Spaak, 2007).
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