Evaluating decline parameters of rotifer Brachionus plicatilis populations as an interstitial water toxicity bioassay

Population decline of the rotifer Brachionus plicatilis has been evaluated as a sensitive and reliable bioassay for assessing toxicity of marine sediment interstitial water. Three environmental conditions were examined using both interstitial and seawater cultures: (a) adverse effects from dissolved contaminant concentrations; (b) effects from particulate organic matter; and (c) increase or decrease of bacterial biomass from addition of mixtures of antibiotics. Three different parameters were measured to characterize decline: (i) time required for 50% of inoculated population to die (TL(50)); (ii) curve of the decline rate of Brachionus plicatilis (mu(BP)); and (iii) graphical area contained below plot of the egg:female ratio curve against time (A(BP)) Results indicated that, for each of the different assays, the three parameters of the Brachionus plicatilis population decline test were sensitive to concentrations of contaminants dissolved in interstitial waters (principally: total ammonia, Cu, Cr and alkylbenzenesulphonates) but not to particulate organic matter. Nevertheless, the presence or absence of mixed antibiotics with the contaminants may influence the Brachionus plicatilis population decline test, principally by retarding the hatching of eggs. Based on these results, Brachionus plicatilis is confirmed as an appropriate organism for use as an indicator of interstitial water contamination, using either decline rate, TL(50) or both parameters. The presence of particulate matter has no effect on these parameters, but the bacterial population may be an influence, although to a lesser extent than the toxicants.