Blue Algae - Nitrogen Fixation in the Ocean

The phytoplankton is composed of various microalgae. These tiny plants produce organic compounds by the process of photosynthesis and form therefore the basis of the food chain in the Benguela ecosystem. A specific fraction of these primary producers are cyanobacteria, also called “blue-green algae”. In contrast to the ordinary algae, some cyanobacteria are able to convert the atmospheric nitrogen into bioavailable nitrogen compounds – therefore they are called “nitrogen fixers”. For understanding the Benguela ecosystem it is decisive to figure out where and to what extend these nitrogen fixers occur. The growth potential of nitrogen fixers are investigated on board in special experiments.

In order to measure the process of nitrogen fixation, the samples are supplied with a rare nitrogen isotope 15N. As the cyanobacteria use also this unusual isotope together with the common 14N, the incorporation of the 15N gives information on the rate of nitrogen fixation. Applying these measurements in a station grid, they give an overview on the spatial patterns and the general magnitude of nitrogen fixation.

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Universität Hamburg
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GENUS (Geochemistry and Ecology of the Namibian Upwelling System) aims to clarify relationships between climate change, biogeochemical cycles, and ecosystem structure in the large marine ecosystem of the northern Benguela/Namibian Coast (sout-west Africa). The coastal upwelling system has high seasonal and interannual variability in atmospheric forcing, in properties of water masses on the shelf offshore the Republic of Namibia, and in oxygen supply and demand on the shelf. In consequence, concentrations and ratios of nutrients in upwelling water and their CO2-content have steep gradients in space and time. In the past, significant and economically severe changes in ecosystem structure have occurred which are in part attributed to changes in physical forcing, translated to the ecosystem by oxygen dynamics.

The GENUS project is funded by the German Federal Ministry of Education and Research (BMBF) and is an endorsed project of the Integrated Marine Biogeochemistry and Ecosystem Research (IMBER)

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