ASSESSMENT OF POTENTIAL TOXICITY IN ESTUA RY WATER AND SEDIMENTS IN THE BAY OF BREST USING 2 TYPES OF BIOASSAY

The aim of the study was to determine the potential toxicity of the ma in estuaries of the Bay of Brest as well as that of their sediments. u sing alternatively ''direct contact'' or their aqueous extracts. The e ffects were assessed by measuring bioluminescence inhibition in the ba cterium Photobacterium phosphoreum (Microtox(R)) and by determining th e success rate of the larval stage ''D'' in the mussel (Mytilus edulis ) and the Pacific oyster (Crassostrea gigas), when embryonic developme nt took place entirely in the tested media or in control seawater afte r pre-treating the sperm. The results show that neither Microtox nor p re-treated bivalve sperm give any significant responses to either rive r water or sediment aqueous extract: only a stimulation effect (hormes is) is observed. Experiments carried out using aqueous extract, on the development of the bivalve embryo (which is more sensitive) showed a North-South gradient. The results obtained by ''direct contact'' expos ure to sediments show that the two types of bioassay are correlated: t he most potentially toxic sediments are those found in the Northern ba sin of the Bay of Brest, which is subject to the influence of urban, i ndustrial and port activities, whereas those of the Southern basin are less toxic, corresponding to a more agricultural zone, Even though th ese tests may underestimate any long term effect, they do detect acute toxicity levels and give a rapid response. In this way it is possible to assess overall intrinsic toxicity in the tested medium (water or s ediment). This cannot always be detected or quantified using existing physico-chemical methods. The sites which present the highest levels o f potential toxicity furthermore correspond to those areas where the i ndices derived by benthic community analysis show the environment to b e disturbed. Moreover, the results suggest that ''direct contact'' exp osure to sediments is a sensitive and reliable test in evaluating pote ntial toxicity of estuary sediment which incorporates the synergistic effects of both hydrophilic and hydrophobic contaminants as well as po re water. A regular use of this type of bioassay combined with other b iological indices should enable the sources of contamination to be tra ced, to monitor environmental quality and to test the efficacity of ef fluent treatment systems around the Bay of Brest.