Optical water quality variables (Secchi disk depth and Forel-Ule colour indexes) observed during cruise M148/2 aboard RV METEOR

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).