When I searched Santarem, Brazil on weather.com a few days ago, I was shocked to see that the daily rain forecasted for the next 4 days is less than or equal 10%. How can a city at the heart of the world’s largest tropical rainforest receive such little daily precipitation? I know one should always take weather forecasts with a grain of salt, and quickly validated that I had typed in the right city when I saw that predicted humidity was >75%. But perhaps the precipitation forecast is not so far-fetched. I have arrived at the onset of Santarem’s dry season.
And, perhaps contrary to intuition, now that we are here in Northern Brazil’s dry season, the waters of the Amazon River and its nearby tributary, the Tapajos, are still very high. The levels have just begun to fall as the rainy season transitioned to dry sometime during this past month, later than usual, according to Jose, our main collaborating scientist from Universidade Federal do Oeste do Para (UFOPA) – Santarem. But the transition in water level is more gradual than what one sees in the weather. Part of the disconnect between weather and river comes from the size of the Amazon floodplain, which occupies an area of 6.4 million km2, equivalent to almost 9 billion Manaus World Cup arenas. The main stem of the Amazon River receives not only the rain that falls directly on it, but also the cumulative precipitation over the entire Amazon floodplain. The time is takes for the Amazon River to realize the input of rainfall from directly above plus rain hitting the Andean headwaters plus rain navigating underground, beneath all 9 million soccer stadiums, creates the lag we see between season and river water level.
Views of the muddy Amazon River floodplain in flight between Santarem and Manaus.
The river is not the only body of water subject to seasonal cycles. Lining the Amazon River network are several floodplain or varzea lakes that are connected to river year round by small passageways. As detailed in an article published by Patricia Moreira-Turcq and her colleagues last year, these lakes feel the hydrological pulse of the Amazon River, as well. When river levels are falling, these lakes push water into the Amazon River. By contrast, when levels rise, which we also observed during our last trip in late March (during the rainy season), river water spills into these lakes, reversing the flow.
If we were to go back to questions about carbon, the motivation behind my frequent trips to this site with Woods Hole Research Center and UFOPA, seasonal patterns in the riverine carbon cycle reflect the ebb and flow of water between varzea lakes and the Amazon River main stem, as well. Using carbon isotope observations (see previous post “Clogging the Filters” for an example), Moreiera-Turcq et al. concluded that during falling river levels, most of the organic carbon flowing from lake to river originates from the bodies of algae growing in the lakes. In contrast, during rising river levels, most of the organic matter from rivers and flowing into the varzea lakes is mostly the degraded remains of soils and old vegetation washing in from land nearby. For a given lake, you can see this seasonal cycle in the lake’s bottom, plant and soil remains settle as sediments during the period of rising rivers.
The seasonal interchange between river and lake fits into a longstanding idea that there are two types of organic carbon moving through river water, and each type participates in a distinct recycling process through the river. The carbon coming from algae in the lakes is newly produced, and very susceptible to breakdown by bacteria. For this reason, this carbon does not persist for long in river water; it quickly returns to its building blocks as carbon dioxide gas that bubbles out of the river and back into the atmosphere. At the same time, there is older organic carbon washing in from the adjacent floodplain, less likely to degrade. This source of carbon is believed to survive the river-borne journey to the Atlantic Ocean, where some fraction eventually gets buried in coastal sediments.
Many scientists continue to return to this two-piece carbon model of the Amazon River system, the question of whether the river is a “pipe or processor” (as summarized by Aufdenkampe et al. 2007) of organic carbon from the floodplain. Considering the size of this system, the answers to this question greatly impact how we understand the role of the Amazon River Basin in the global carbon budget. Some of the data we collect on our field trips will hopefully contribute to this understanding.
Aufdenkampe, A. K., et al. (2007). Organic Geochemistry, 38(3): 337-364.
Moreira-Turcq, P., et al. (2013). Global Biogeochemical Cycles 27: 119-130.
Richey, J. E., et al. (2002). Nature, 416(6881): 617-620.