Project CONNECT

How similar are lakes that are more or less strongly connected by rivers and how do algal blooms spread along lake chains? These are the questions investigated by the CONNECT project.

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CONNECT – Connectivity and synchronisation of lake ecosystems in space and time

Monday, 12.08.2019

Light or heavy oxygen?

Researcher of the University of Erlangen analyse isotope signature

In order to characterise a lake comprehensively, it is also necessary to understand the material flows. Dr. Alexander Frank from the GeoZentrum Nordbayern at the University of Erlangen-Nuremberg is particularly interested in oxygen fluxes, i.e. how much oxygen aquatic algae release during photosynthesis and how much of this oxygen is directly reused by the algae and other aquatic organisms. Since both processes affect the isotope composition of the dissolved oxygen in different ways, the microbiologist uses a method to analyse the isotope signature.

As primary producers algae are at the beginning of the food chain. They use photosynthesis to build their biomass from carbon dioxide and water, thereby releasing oxygen. Part of this oxygen is immediately reused during respiration by the aquatic organisms, i.e. not only by microorganisms and to a lesser extent by animals, but also by the primary producers themselves, which in turn leads to a decrease of the net photosynthesis.

 

In order to determine the effective algal photosynthesis in a lake, it is hence necessary to distinguish between respiration and photosynthesis. Therefore, Dr. Alexander Frank from the University of Erlangen-Nuremberg applies a method that is already widely used in marine research. During the LakeLab experiment, he takes weekly water samples in which he determines the oxygen isotope composition of the dissolved oxygen. This allows estimating the photosynthesis in each of the 24 enclosures, which differ due to the varying algal densities caused by the experimental set-up.

 

The freshly taken water samples are immediately processed in the laboratory.

 

The technique is based on the fact that many chemical elements occur as stable, non-radioactive isotopes with different atomic weights. Oxygen (O) comprises the lighter 16O isotope and the heavier 17O and 18O isotopes. In the atmosphere they are present in a constant isotope ratio: 99.76 % 16O, 0.037 % 17O and 0.20 % 18O. Since the oxygen produced as a waste product during photosynthesis is photolytically split from ambient water (H2O), it contains fewer 18O isotopes and is thus isotopically lighter than the oxygen in the atmosphere. This makes it possible to distinguish the amount of oxygen formed during photosynthesis from the amount of oxygen dissolved in water from the atmosphere. In contrast, the oxygen-consuming processes lead to the enrichment of the two heavier oxygen isotopes in the water, because the lighter 16O isotopes are preferentially incorporated during respiration. Such an altered oxygen isotope signature can be determined by mass spectrometry. It provides information on the effective photosynthesis performance of the lake.

 

 

Dr. Alexander H. Frank

studied Microbial Ecology and Marine Biology in Vienna and also received his PhD degree there. After a postdoctoral stay at the Woods Hole Oceanographic Institution, he now works at the Faculty of Applied Geology at the University of Erlangen-Nuremberg.

 

 

Text and photos: Dr. Martina Bauchrowitz, IGB

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