MEASURING OCCURRENCE OF ALGAE WITH DELAYED FLUORESCENCE EXCITATION SPECTROSCOPY / Mätning av algförekomst med fördröjd fluorescensexcitationsspektroskopi

Increased nutrient load often causes algal blooms. An algal bloom lowers the water quality which is a problem in many fresh water catchments. Today, we have too little information about how the nutrient supply influences the algal composition. With a new method (still under development), called ”delayed fluorescence (DF) excitation spectroscopy”, the goal is to be able to determine both concentration and species of phytoplankton continuously (on-line). The different algae species that were determined are green algae, diatoms algae, blue-green algae and cryptophytes algae. This method is to be used to study the effects of nutrients on the phyto-plankton composition and in this way be able to control and improve the water quality.

When phytoplankton (algae) have been exposed to light and thereafter are carried into dark, they emit photons. The emission of photons is called fluorescence. Delayed fluorescence (DF) is a special kind of fluorescence. By measuring the intensity of the emission (= fluorescence) after exposure to light it is possible to both determine the species of phytoplankton and their concentration.

My work is part of the EU-project ”Phytoplankton On-Line” which has two main purposes.
1. To develop delayed fluorescence excitation spectroscopy, to be able to measure the concentration of phytoplankton and composition of the phytoplankton community, continuously.
2. To use the new method to study the effects of different chemical and physical processes on the development of the phytoplankton community.

During a period of five weeks I have studied how well the conventional measuring method (acetone extraction) of chlorophyll a, agrees with the measurement carried out with the new DF-method. I have also compared the chlorophyll content in enclosures with water from outside the enclosures and studied the phytoplankton composition in both of these environments. The enclosures are two big circular pools without bottoms. The intention of the enclosures is to screen off a little piece of the lake.