Optical water quality variables were recorded using a black and white Secchi disk with a diameter of 40 cm and a Forel-Ule colour scale. The Secchi disk depth was recorded as a relative measure of water clarity at each in situ station. At half the Secchi disk depth the apparent colour of the water above the submerged Secchi disk was determined using the Forel-Ule colour scale. A Forel-Ule colour scale is a classic tools used to differentiate the percieved color of water based on a scale from 1 (indigo blue) to 21 (cola brown). The measurements were conducted as proposed in literature (Garaba and Zielinski, 2015; Wernand, 2011; Wernand and van der Woerd, 2010).

We would like to thank RV Captain Detlef Korte and crew for supporting our research activities aboard RV METEOR during cruise M148/2. We are grateful of all the shore-based technicians, administrative personnel and scientists involved in the cruise. The German Federal Ministry of Education and Research (BMBF), Meteor Leitstelle, German National Science Foundation (DFG), Max Planck Society contributed towards the organization and funding of Meteor Expedition M148/2 EreBUS: Processes Controlling Greenhouse Gas Emissions from the Benguela Upwelling System.