The fluorescence measurements were performed in situ by the combined VARIOSENS fluorometer/turbidity measuring device (Impulsphysik, Hamburg, Germany) mounted on the CTD probe. Vertical profiles were taken down to the bottom during day time.
The characteristics of the equipment are:
projector flash rate: 10 per second.
Measuring interval: 6 seconds.
Excitation: flashbulb FX 124, continuous emission spectrum, wide-band filter to prevent the interference of the fluorescence signal by scattered light.
The fluorescent radiation emitted by chlorophyll-a was recorded by the optical receiving system, applying a narrow cut filter. The fluorescence spectrum of chlorophyll-a is largest at 680 nm (filter of projector: 400/530 nm, filter of the receiver: 677 nm). The detection limit ranged between 10-9 and 10-5g of (crystalline) chlorophyll-a (in distilled water). The signal was measured by the photodiode and converted to voltage. This voltage corresponds to a certain amount of chlorophyll-a, determined from in situ calibration.