10 July 2018

Amazon River impacted eutrophication of Atlantic Ocean

In the past, the Amazon exerted a major influence on the eutrophication in the westernmost equatorial portion of the Atlantic Ocean. This is the conclusion of an international research team including Francesca Sangiorgi from Earth Sciences, Utrecht University. Their findings will soon be published in the journal Palaeogeography, Palaeoclimatology, Palaeocology.

A study of fossilised marine plankton from a deep-water drilling site off the coast of Brazil reveals that starting 9 million years ago, a marked increase in specific kinds of ‘micro-algae’ – including heterotrophic dinoflagellates – occurred, which indicates a richer concentration of nutrients that promoted the plankton bloom. Species from before that time primarily included those typically found in more nutrient-poor waters.

The location where the drilling on which this study is based took place. (Lammertsma et al. In press.)

Although a ‘rudimentary version of the Amazon’ existed much earlier than 9 million years ago, the connection between the Andes and the Atlantic Ocean was not established until around 9 Ma. As the area impacted by the main body and tributaries of the Amazon increased in size, the transport of sediment and nutrients from the Andes and the Amazon region rose sharply as well. Once in the ocean, this resulted in algal blooms and potential changes to (and increased numbers of) marine organisms.

Crucial role in regulating CO2

The Amazon region plays a crucial role in regulating CO2 because its tropical forests absorb CO2 and convert it into organic material such as leaves, humus and wood. A large portion of this organic material is then broken down again in turn, ending up in the ocean via the Amazon.  In the ocean this organic material and other nutrients like nitrogen and phosphorus stimulates the algal bloom of mostly photosynthetic plankton and influenced in doing so the food chain and the CO2 life cycle.

The study also demonstrates that the most significant increase in plankton began 2.6 million years ago, at the start of the Quaternary Period, and coincided with an equally significant increase in the quantity of sediment transported by the river.

Today, the region’s wet and dry seasons affect changes in the plankton bloom. This phenomenon, which can be seen clearly in satellite images, supplies additional food to the entire ecosystem. Researchers have now placed it in a historical perspective for the first time.

The mouth of the Amazon, where sediment and nutrients are distributed. Credits: European Space Agency (ESA)

Project Clim-Amazon

The study is part of the Clim-Amazon project, a joint Brazilian-European scientific initiative aimed at studying the climate and geodynamics of Amazon river basin. This project is supported by the EU via its FP7 (Seventh Framework Programme for Research and Technological Development). See: www.clim-amazon.eu

Publication details

E.I.Lammertsma, S.R.Troelstra, J.A.Flores, F.Sangiorgi, F.ChemaleJr, D.A.do Carmo, C.Hoorn. ‘Primary productivity in the western tropical Atlantic follows Neogene Amazon River evolution’, in: Palaeogeography, Palaeoclimatology, Palaeoecology 6 June 2018. DOI: 10.1016/j.palaeo.2018.05.048