Complement méthodologique à l'étude d'impact du Légué (Saint Brieuc)

The project to enlarge the port of Le Légué at St-Brieuc required conducting an environmental impact study. This took place in 1980 and 1981 (Merceron et al). In view of the project's importance and the almost complete absence of baseline ecological studies of this area, it has been necessary to proceed with a rather extensive investigation. In this way, the field studies focused on the following aspects: hydrobiology, benthos, contaminants in the water, the sediment and living matter, Yffiniac marsh, flat fish nurseries. Overall, the impact of the Le Légué port enlargement on the marine environment of the bottom of St-Brieuc's bay seemed to be relatively minor. However a qualification was made concerning the modes of discharge of the planned basin's waters. Indeed, the factors governing the future impact of the Le Légué's waters were numerous, some positive, others negative, and weighing them seemed risky in the absence of additional data. From a methodological point of view, it was interesting to try to go further. In fact, along the Manche-Atlantique shoreline, there are many sandy foreshores subjected to polluted water and likely to present a sanitation problem for shellfish farming and resort areas. We therefore wanted to increase the investigative methods focused on the hydrobiological data, on the bottom of St-Brieuc's bay, which is a good example of them. More directly, the foreshores of the Norman-Breton Gulf are comparable to the latter (large area, small water supplies) and the study of them, which is beginning, will most certainly profit from the gains realised at St-Brieuc. All the impact study's hydrobiological specimens were collected at sea. We therefore needed to know if, during the tide cycle, the various effluents from the bottom of the bay could be detected at sea and their trajectory monitored. The goal of this study is therefore to enable us to refine the hydrobiological study method of this type of zone. The impact study having made it possible to note the small flows of fresh water at the bottom of the bay, it was decided to sample when the fresh water was at its most abundant, that is, at periods of flood and neap tides. For the general presentation of the study zone, one may consult the impact study itself.