Lampitt, Richard S, and Martin V. Angel

Southampton Oceanography Centre, Empress Dock, Southampton, SO14 3ZH, UK, Tel: 44 (0)23 80596347, Fax: 44 (0)23 80596247, E-Mail: R.Lampitt@soc.soton.ac.uk

The forgotten mesopelagic – a synthesis of our understanding at the end of JGOFS

The mesopelagic zone (100-1000m) was, for good reasons, not the main focus of JGOFS. Nevertheless with the current increasing interest in the responses of the oceans to climate change and feedback to the earth system, significant focus is now directed at the processes that occur in this part of the water column. Most of the particulate material that settles out of the euphotic zone is remineralised in the mesopleagic although there is substantial uncertainty about the relative importance of the various processes responsible for this. In spite of the fact that physical changes here are often small over large areas, there are significant geographic trends in the characteristics of the biosphere. This is not always the case as evinced by dramatic oxygen minimum layers in the Indian Ocean and warm temperatures in the Red Sea both of which exert a profound influence. With increasing depth the biomass, size distribution, species composition, physiology, behaviour and diversity of the fauna demonstrate trends that will have a direct bearing on the way in which organic matter is remineralised and nutrients made available for primary production. Here we present a synthesis of the properties and processes of the mesopelagic that affect remineralisation and the downward transport of organic matter. We summarise current insights and relate these to upper ocean processes such as primary production. There is much to be learned about this important zone but this should be built upon the knowledge that has been painstakingly gained over the past decades.