Environmental Baseline Surveys in Deep Waters: the lessons learnt offshore West Africa

摘要

This paper describes the technical methods used during three deep offshore environmental surveys performed off Angola, Nigeria and Congo. The logistical means such as the choice of vessel, crane type, winch speed and cable length, adequate equipment and protocol for sediment and water sampling and measurements, and benthic samples preparation and biodiversity identification are described. The results show similar physical features for the three areas, consisting in 1) water column stratification, with three water masses (0 to 50 m deep ; 100 to 400 m and below 400 m) lowly enriched by nutrients, heavy metals and hydrocarbons, probably due to low inputs from the continent, and 2) sediment characterised by a very high rate of fine particles, a quite high organic enrichment and low heavy metals and hydrocarbons background levels. Such environmental conditions suggest over all that vertical mixing of waters is low, and that the horizontal water circulation is slow at such great depths. They provide constant conditions of life for benthic fauna communities, which are very rich, abundant and diversified, I.e. healthy and equilibrated. Main difficulties to be overcome regarded some technical issues, as identification of unknown benthic organisms. To conclude, the specific equipment and protocols deployed for performing the three great-depth surveys on Girassol, Akpo and Moho Bilondo fields were well adapted to deep-sea sampling, and allowed to significantly improve our knowledge of deep-sea environment.