16.05.2018 – ICBM
By: Dr. Sibet Riexinger

METEOR cruise M147 comes to its end

(from left to right) Prof. Dr. Thorsten Dittmar, Melina Knoke and Dr. Michael Seidel at work in a RV METEOR lab [Photo: Clive Maguire]

The 147th RV METEOR expedition led scientists of ICBM, amongst team members of other research facilities, to the Amazon estuary. On whit sunday, 20 May, cruise M 147 will come to its end in Belém („Bethlehem“), Brazil. What made the estuary of the biggest stream on earth such attractive to the scientists?

Local consultant Clive Maguire aboard RV METEOR not only kept in view the contacts with regional authorities as well as logistics to go off without a hitch. In his blog, he also lets us hear some vivid reports of the scientists. Thus, we come to know from co-organizer of the cruise, Prof. Dr Thorsten Dittmar, not only about an excellent team spirit on board. Dittmar also reports about the on-site confirmation of theories, originally conceived so to speak in the small space of a study room.

Dead material is washed to the sea from far away
Scientists are looking at the Amazon estuary from different angles. Marine geochemist Dittmar and his ICBM colleagues are particularly interested in the molecular traces left by decaying creatures in the marine water. The researchers call those carbon compounds dissolved organic matter, shortly D.O.M. In this context, the Amazon estuary is especially exciting, because the huge stream of about 7,000 kilometres length carries vast amounts of it from the rain forests to the sea, mixed up with minute mineral particles from the Andes of countries as far away as Peru and Colombia.

Play of colours of different water bodies
Bacteria feed on these carbon compounds and eventually release the carbon dioxide, originally captured by rainforest plants. By-product of this bacterial digestion process: Nutrients. They would be lost forever, if it were not for the microorganisms. The fluvial waters virtually form a fertilizer solution on which algae could multiply on a huge scale, would it not be for the vast D.O.M. load, giving the river waters a tea-brown colour, thus preventing algal growth for the lack of light. Not until the sea meets the river in the estuary, the marine water acts as a flocculent: The suspended matter starts to stratify at the bottom of the Amazon estuary. This is one reason for the mud and the sand banks in the river mouth. While the sediment falls out, the water gets clearer and brighter. Now, time has come for the microalgae to grow – and the water starts to turn green. Where the nutrients become depleted, the algae get into a stalemate – the water becomes darker green. Finally, in the open ocean, where there are no more nutrients and algae, it appears blue.

Knowledge Gaps
„There is still a lot we don‘t understand about what is happening in this mixing process,“ Dittmar explains. Iron is one of the key elements, needed by all living things. And the D.O.M. keeps it in suspension and available to the algae. Just when the organic matter is going to flocculate – and light is going to be available to the algae sufficiently – the iron is locked in. Thus, the scientists want to now, what exactly happens to it, how and when it flocculates and where it is deposited. Another very interesting aspect to the Dittmar team is the changing carbon dioxide levels in the water.

For more, see: http://amazongeotraces-m147.com/dissolved-organic-material-2/