Bio-optical properties of the Fladen Ground: "Meteor" - FLEX-75 and FLEX-76

Various daylight and light measurements for water-mass classification and for in situ photosynthetic studies were made on the Fladen Ground for more than three months during FLEX-75 and FLEX-76. It was demonstrated both experimentally and theoretically that the depth of the euphotic zone can be derived either from the colour of the sea being defined as a colour index or from the Secchi disc depth. A universal algorithm valid for all marine waters giving the depth of the euphotic zone in terms of channel 1 (443 nm) and channel 2 (520 nm) in NIMBUS-7, CZCS, Coastal ZoneColour Scanner is presented. Algorithms - based on signal ratios from the available channels in NIMBUS-7, CZCS - for the retrieval of chlorophyll a and phaeophytin a have no real potential for the area in question nor for other selected areas with similar biological and/or optical characteristics. During FLEX-76 the evolution of two plankton blooms was observed by optical means which revealed considerable changes in the concentration and composition of the suspended matter. The disintegration of this suspended matter was accompanied by only insignificant changes in the concentration of yellow substance. This suggests no in situ production of yellow substance but rather that its presence in the North Sea is a result of the freshwater river influx from the bordering countries. Rapid changes in the optical parameters were observed in the area on certain occasions. They were caused by either advection, internal waves, or biological activity.