Tank experiments for the fluorescence of phytoplankton 
Peter Gcgc 
DLR, Institute of Optoelectronics, 0-8031 Oberpfaffcnhofcn 
Abstract 
In a tank experiment the changes of reflectance spectra around the fluorescence wavelength 
of in vivo chlorophyll a at 685 nm have been studied for two different algae species. The 
wavelength dependent, non-linear increase of the fluorescence intensity measured at increas 
ing chlorophyll concentration was used to derive spectra of absorption and emission effi 
ciencies. It was found that especially the fluorescence intensity depends considerably on algae 
species. 
1. Introduction 
Each method of human technique harnessing the sun power 
is far away from the efficiency realized by the biological 
method photosynthesis. Nevertheless, the photosynthetic 
quantum efficiency is less than 100 % because of several loss 
mechanisms in the complex reaction chain. A part of the lost 
energy is emitted as fluorescence light. The wavelength of a 
fluorescence peak is characteristic for the corresponding loss 
mechanism. For photosynthesis, chlorophyll a is a "finger- 
print-molecule", thus the chlorophyll-a-specific fluorescence 
at 685 nm is a fingerprint for photosynthetic activities. 
Because water is transparent for radiation in the visible, the 
685 nm fluorescence is suited for chlorophyll monitoring in 
the water as well as on the land [ESA86, Nev77, Gow81, 
Kim85]. 
In remote sensing, the chlorophyll content in water was 
estimated by the blue/green-ratio in the past, taking advan 
tage of the different absorption coefficient of chlorophyll at 
440 nm (blue) and 550 nm (green). If no particular sub 
stances absorbing in these regions are dissolved, this method 
is well suited (case-I-waters, i.e. usually open ocean areas). 
But in many areas of interest (coastal zones, lakes), the water 
color is dominated by sediments and yellow substances, so 
that the blue/grecn-ratio is no longer a measure of chlorop 
hyll. For these cases the characteristic 685 nm peak is the 
only information for passive remote sensing which allows a 
relatively exact estimate of chlorophyll contents. 
The fluorescence peak shows a great variation in intensity. 
Only when these variations are understood or at least quan 
tified, an algorithm correlating the fluorescence intensity 
with chlorophyll concentration can be applied. In the fol 
lowing, experiments will be reported which were made to 
investigate these variations by spectral measurements at dif 
ferent algae species under controlled conditions. 
2. Experimental 
Three different algae cultures were cultivated in tanks. Two 
cultures consisted of a single species (tank 1: Biddulphia 
sinensis, tank 2: Prorocentrum micans), the third culture was 
a mixture of several species living in the North Sea. In order 
to have reproducible light conditions, the tanks were in an 
artifically illuminated room. 
After the tanks were filled with desalted seawater, the algae 
as well as nutients were added. The algae growed about two 
weeks, then nutients were exhausted, and the algae concen 
tration decreased. After this, the tanks were cleaned and 
filled again for a second period of observation. 
Several biological parameters were measured periodically: 
chlorophyll a, phaeophytin (both photometric), cell num 
bers, scston, phosphate, nitrate, nitrite, ammonium, silicate. 
The spectrometric measurements were made with a TRA- 
COR Northern spectrometer. It disperses the light by a 
grating on a 512-element silicium diode array which records 
the spectrum from 398 to 811 nm, thus one diode integrates 
over 0.8 nm. The spectral resolution is about 3-5 nm. 
The spectrometer looked perpendicular to the water surface, 
measuring it's light energy output (radiance). Since the 
energy output depends on the energy input (irradiance), 
which shows a great variation in nature, it is common prac 
tice to normalize the radiance with the irradiance, that gives 
the reflectance. Irradiance was measured by recording the 
spectrum of the illuminating light reflected from a white card 
(barium sulfate) which was inserted into the light path. 
Cell Numbers (1/1) TANK 2 
Figure 1: Chlorophyll concentration and Cell Numbers of Prorocentrum micans (tank 2) as a function of time.