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in drift

Screen Shot 2017-08-24 at 12.46.44 PM copy.jpgAllie, Kyle, Jesse, Ron, Melissa, Alysia and me. Sampling water from the Niskin bottles after a CTD cast into upwelled waters along the Oregon coast. Above, Patrick drives the line that lowers the bottles down to a 100 meters and reminds us to wear safety vests. Photo credit: Frank Gonzalez

Yesterday ended our drifter journey following upwelled water, and it is refreshing now to have finally experienced 6 hours of sleep without interruption. The three day blur included a 2-day shift of waking up every 4 fours to take a surface water sample. Many thanks to Steve from the crew for cookies and snacks during this time.

Outside the wake-up-every-4-hours period, the whole science party also participated in CTD (conductivity, temperature, depth) deployments every 6 hours. My continuous, second-by-second optical measurements and those samples I took every 4 hours for Nina will resolve the surface ocean layer very, very well. The data fill in a rich, 2-dimensional picture of the water our ship has meandered through, and how it may have evolved in the last three days. CTD deployments, by contrast, illuminate a third dimension.  With each one, we cast water collection bottles and instruments that measure temperature and seawater optical properties down to a few hundred meters below surface.

Some immediate data that these CTD casts offer is the structure of the water “column” below our continuous surface measurements. Because the ocean has three dimensions, these vertical data help us understand our surface measurements as part of a greater volume of seawater rather than just a slice. Light profiles, for instance, tell us how deep sunlight penetrates this “column”, giving us an idea of how deep photosynthesizing algae can persist. Fluorescence profiles allude to concentrations of the algal pigment chlorophyll at each depth, sometimes revealing the famous “deep chlorophyll maximum”, which results from algae adapting to deeper, darker waters by packing more chlorophyll into their cells. The extra chlorophyll boost in their cells helps them harvest more light to continue photosynthesis at such depths.

From drifter round #1, we have grown wiser to the challenges of observing upwelling. Our drifter was no more than four orange Styrofoam balls, a GPS locator, and a submerged net that fanned out into the upwelled water mass. So, one purely logistical challenge was steering behind the windy path that this water pulled the drifter; R/V Oceanus could never stray too far if we wanted to be sure that were analyzing the right seawater. Moreover, while we shifted directions, we were certainly not alone. The ship’s steering crew were very busy alerting nearby fishing vessels not to run over our drifter device.

Today, we dropped drifter #2  into a new water mass, and this time the water we track is not upwelling water. But, it is another unique packet of water that differs from its surrounding waters. It has many fewer algae living in it that the water we tracked with drifter #1. It will be our natural, “controlled” laboratory for the rest of our cruise at sea, and hopefully it will allow us one more rare glimpse of how ocean life and conditions change day to day.

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We found upwelling

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Kyle spotting Frank’s genomics experiment. Fire extinguisher never far from flame.

This time of year, prevailing summer winds push surface seawater away from the North American west coast, leaving room for deeper waters to rise to the surface. This upwelled water from the deep Northeast Pacific Ocean is chemically unique: cold, salty, and old. If you ever swam at the beach in California, Oregon, Washington or Vancouver Island, consider that some of this seawater has not seen the surface for a thousand years.

Most likely, when this seawater left the surface ocean, it brought with it some of the organic matter produced by phytoplankton living there. And, since residing in the deeper sea, bacteria decomposed this organic matter, releasing its building blocks, a lot of dissolved nitrogen and phosphorus, back into the water. These building blocks are the nutrients that fuel photosynthesis. Deep in the ocean, there are no algae to use those nutrients, but when this water finally resurfaces on the west coast of North America, it delivers the nutrients to the algae waiting there, stimulating spurts of population growth, sometimes massive blooms. Because algae are the foundation of the ocean food chain, we can also thank upwelling for the rich fisheries of the west coast.

Algae blooms are so transient, rising and falling within weeks, that few chemical oceanographers get the chance to find them in the field and stay there long enough to observe all the biological and chemical changes in seawater that result from these blooms. In our last phase on Oceanus, we will use all of our remaining time to follow a parcel of recently upwelled water, easily noticeable by its cold, salty nature, as it slowly spreads out through the ocean surface. When we found that water today, we quickly dropped a drifter, a 30 foot net that caught onto the water mass for us and now relays its GPS location so that we know to stay close by. By entraining our ship and all our instruments on top of this water mass, we hope to witness the onset of an algae bloom and monitor it through the course of its life.

With this opportunity, there are many hypotheses to test: the algae species that grow, the amount of biomass that dies and sinks into the deeper ocean (again), the concentration of cells that remain at the surface, the amount of DMS production. All these hypotheses demand several types of measurements, and we have to collect them all frequently through the next six days—sometimes once every four hours. Now, the main lab on Oceanus overflows with color-coded spreadsheets, Nina’s creations, organizing people’s water sampling burdens and implicitly, all of our sleep schedules.

The only time we stop is tomorrow morning, which Ron reminded me is when we get to see the first total eclipse in all of North America! But, even then, we aren’t really stopping, as several scientists on board are using the eclipse to learn about marine life’s sensitivity to changes in light.

DMS tags

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Melissa and Alysia getting ready to measure uniquely tagged DMS compounds in their algae cultures on the Oceanus back deck. Let’s be happy for them — today they weren’t drenched by any waves while working by  the starboard!

The vein of surface seawater continuously pumped into the ship’s wet lab splits across several pathways before analysis. One branch goes to my optical meters, another goes to a system that tells us how salty and warm the water is, and some other branches enter gas chromatographs and mass spectrometers to measure specific chemical constituents of seawater. The wet lab is noisy; every sound exists in that room, and Alysia’s music library will never overcome the mechanical symphony in the background (but it gets close).

Inside this symphony, our team can compare how several properties of the ocean are changing alongside each other in real time. And, last night, we saw something: my back-scatter meter registered higher numbers, while seawater DMS levels increased. When you look at thousands of data points per day, and try not to get seasick doing so, these shifts are exciting—even before you know what they mean!

When we woke up today with Vancouver Island in the distance, it was clear that the changes last night signaled our entry into more productive waters. From deck, we could see greener hues around us, indicating more algae, and thus more cells in the water to scatter light, produce DMS, and feed the higher ocean food chain. I like to think this is why today was full of dolphin and whale sightings, too.

Returning to the data and a lot more DMS chemistry: there are at least two processes in an algal cell that produce DMS. Each involves breaking up an even bigger molecule that is DMS plus either an oxygen or phosphorus atom (let’s call them DMSO and DMSP) to release DMS. And, I just witnessed Alysia’s clever strategy to compare how much and how quickly each process contributes to an algae cell’s DMS production. The approach: (1) Alysia and crew (aka Melissa) collect algae in seawater every morning, (2) grow them throughout the day in a tank on deck, and (3) monitor production of DMS from either DMSO or DMSP over time using Ross’s giant mass spectrometer on board. What is clever about it: how they derive separate DMSO-to-DMS and DMSP-to-DMS production rates by only measuring DMS, not DMSO/DMSP.

A lot of oceanographers that are not me, who have arguably cooler jobs, add tags to marine mammals to monitor their individual movements in a population. Hopefully, chemical oceanographers are about to get cooler because, actually, we do something very similar on the molecular level! Alysia differentiates DMS production pathways in an algae cell by making her own DMSO and DMSP molecules with different DMS’s in them and adding these tagged DMSO/DMSP molecules to the growing algae. In this case, the different DMS’s are the unique tags, DMSO/DMSP are the dolphins. Through time, the algae that Alysia and Melissa cultivate in all weather conditions break down the tagged DMSP/DMSO molecules, releasing the tags as free DMS. The DMS tags inside the DMSO molecules have eight extra neutrons, while the DMS tags inside the DMSP molecules have six. And so, the DMS tags released from DMSP are slightly “lighter” in mass than the DMS tags released from DMSO.

Fortunately, Ross’s mass spec can see these differences. And thus, knowing the release of each DMS tag through time from Alysia and Melissa’s algae incubations reveals distinct rates of DMS production from each precursor molecule.

Adapting

2017-08-12 21.28.40 copy.jpgWaves crash over the starboard side as we steam southeast through the Gulf of Alaska. The CTD rosette, pictured here, is a staple for chemical oceanographers because it allows us to collect water in up to twelve Niskin bottles as deep as the sea floor.

Five days at sea and the Oceanus is beginning to feel spacious. It will never feel large, but it feels spacious in a sense that my daily interests and necessities have adapted to the size of this vessel. When on land, you can’t imagine this happening. Yet, somehow, with less than a week in a bobbing steel capsule behind you: no more need for sea sickness pills; your limbs recalibrate the distances to dinner, the bathroom, the bedroom, the workplace (just one flight up and twenty feet forward); and you kind of feel okay about jogging on a treadmill at the bottom of the ship with just pictures of forest trails taped to the front of it.

Why do we adjust so fast? One reason is the nature of fusing lab and home. Sometimes, especially at night, because of the way I’ve scheduled instrument maintenance, I think about nothing else but how my optical meters are doing, what their numerical displays mean, and how to overcome the unending battle against faulty plumbing. I twist lids tight, climb up tables to screw hose clamps into awkward corners, clean up surfaces that get splashed again a minute later, and stare at clear tubes, searching for tiny bubbles in the seawater flow that could render my numbers erroneous.

Another reason is the high swells, typical for the subarctic Pacific Ocean, that confine us most of the day, unless we go outside for water collection/experiments. Thus, we see the ocean out of small portholes and do not get to appreciate the infinity of the sea in a way that you would on a vacation ferry in the warm summertime.

(If I’m allowed to take a stab at a grand simile) We are like the algae growing in several ongoing incubation studies on Oceanus! For example, Erin from University of Southern California is sampling algae from different stops along our cruise track, then growing them in capsules on board at conditions that are similar enough to the ocean but different enough to elicit measurable responses in their healthiness index (that’s what Nina does), and production of DMS/related compounds (that’s what Tara does). This week, she added vitamin B12 to these ocean algae.

Later in the month, Erin will switch things up: she will add seawater from the ocean to just one algae type, diatoms, that she grew in purity back home, and see how introducing bacteria from the seawater outside shift the DMS production of those pure diatoms. Thus, she’ll compare how both “wild” mixed algae and cultured diatoms interact with the DMS cycle.

In these confined and controlled spaces, my fellow scientists can hope to nail down the specific processes (e.g., what algae do) that add or subtract from the pool of DMS in the ocean.

Seeing light underwater

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My lab and port hole to the horizon outside! A maze of secured, non-leaking tubing and wires that go from seawater to ship to instruments to computer files.

See? I must have over three million data points now, and less than 3 days at sea have passed. In part, I owe this quantity to the relative “easiness” of characterizing light’s behavior in seawater- i.e., the fact that the measuring process can be automated by two key instruments that monitor light through the ship’s steady seawater intake. Hopefully, by the end of this cruise, you’ll agree with me that measurements like these are essential to understanding life in the ocean, and relatedly, the planet’s carbon cycle.

Visible light, like the ultraviolet radiation you block with sunscreen or heat emanating from a warm sidewalk in the city, is made of energy packets, or photons, that travel as waves. Waves with different energies have different crest-to-crest distances and humans see those differences as colors. So lower energy red light, for instance, would have the longest wavelengths, while violet light has shortest. Natural sunlight is a mix of light waves with many wavelengths between red and blue, each of which behaves differently in the ocean.

Our team seeks three parameters for light behavior in the ocean: absorption, scatter and attenuation. I am interested in all of these parameters, but in these last four months on the new job, absorption has been the most intuitive to learn. For our ac-s meter to measure the light absorption in the ocean, it has to shine its own beam of light into a tube full of incoming seawater from one end, and then monitor the amount of light that exits the other side. With absorption, less light exits the tube than had entered, as is always the case with seawater.

I am most fascinated not by absorption itself, but by what in seawater causes light to get absorbed. The options we have are: terrestrial minerals kicked up from the ocean floor or delivered by rivers, mineral shells produced by algae like coccolithophores, and organic carbon in the cells of living and dead algae (yes finally the connection to carbon!). At 5 meters in the Gulf of Alaska, we can rule out the first two things, which means that our absorption numbers primarily tell us about algae in the water.

Moreover, the ac-s meter measures absorption at >150 wavelengths per seawater sample (per second)! With so many wavelengths, we can start to tease apart absorption by different pigment groups in algae cells, which in turn mark distinct species that are made of different pigments. You have heard of pigments: the most famous is probably beta-carotene, but among oceanographers, chlorophyll makes the top for its role in photosynthesis.

It is challenging to take a cup of seawater and say how many of what types of algae are living there. It is also challenging to look at enough cups seawater and make global interpretations, how the algae fuel the food chain or turn the carbon cycle. These rich measurements that optical instruments make at sea take us one step closer to overcoming both challenges.

We will swim in numbers

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Sideways image of Oceanus harbored in Resurrection Bay, Seward, Alaska.

The R/V Oceanus is my new home for nearly the rest of August. It is ~170 feet in length, roughly 2/3 that of a Woods Hole commuter ferry or (because I now live in Vancouver) half the length of a Vancouver Island ferry. And, as we steam out of the fjord that is Seward Alaska, where every direction is a Chugach mountain, it is easy to feel dwarfed by the world on Oceanus’s main deck.

Nonetheless, from now until August 27th, a remarkable quantity of science per square foot will happen on this ship. Most of thirteen scientists on board are interested in a chemical compound in seawater called dimethylsulfide (DMS is the acronym). So, for the first time, I travel among company that cares primarily about the movement of DMS in the ocean – not the carbon cycle (though the two cycles are related)! Like carbon, DMS transforms into other chemical forms in seawater. Sometimes, life in the ocean – mainly small things like algae and bacteria – drive these changes. Other times, abiotic factors, like UV radiation from the sun, can add an oxygen atom to the DMS molecule. These transformations are complex, and it makes sense that you’d need at least ten people per ship to study them; more details will come later as I get to know my all of my scientist roommates.

What I mostly know so far is this: the DMS molecule is responsible for the ocean’s smell. Which some of us love! As a gas, it easily escapes the sea into the atmosphere. Scientists have long posited that if DMS travels high enough in the atmosphere, it can trigger clouds to form, playing an important role in our planet’s climate. Thus, if something shifts the production of DMS in the ocean, it can ultimately affect the climate we experience on land, as well.

Coming from Philippe Tortell’s diverse team at University of British Columbia, I can still measure those constituents of seawater that tell us about the marine carbon cycle. Between here and Newport Oregon, I will continuously monitor how light at just 5 meters (~16 feet) below the ocean surface gets absorbed or scattered in seawater. These optical properties can tell us about the solid particles of organic carbon in the water, and more specifically, the different types of algae that produce these carbon particles. More later on that too.

And for a relatively small research vessel, the volume of numbers that we will get in just sixteen days is massive. My team alone, a group of five, specializes in the development and deployment of equipment that analyzes seawater incessantly by taking seawater that the ship pumps from 5 meters below surface to the main deck. One instrument I will use can provide me several seawater absorption numbers per second. Recall that this is quite different from graduate school, where I would spend weeks acquiring the equivalent of thirty data points on a graph. After two weeks, that is tens of thousands of numbers and 4-5 orders of magnitude more coverage of what is going on in the ocean’s surface.

O Fim

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Looking down over the meeting of the Rio Solimões and Rio Negro from the boat

Memorable things happen when two rivers of very different provenance collide. The meeting of the Rio Solimões and Rio Negro is one of the most highly famed colliding points of the Amazon River network. The system is so large here that, at times, you are better off seeing some things happen from an airplane. Height is sometimes the only way you can appreciate how certain things work in this region: where water channels connect, or the quantity of forest that covers the drainage basin.

But, the meeting of the white and black water Solimões and Negro rivers, just downstream of Manaus, is one of the key events that you can experience a few feet from the river, i.e., on a boat right on top of it.

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Google map image of the black Rio Negro and white Rio Solimões meeting to form the Rio Amazonas. Note that the rivers do not fully mix for about 50 miles after meeting, at roughly the red marker. (Ignore the blue driving path.)

The most useful comparison that comes to mind in describing this ongoing and never-ending event is adding milk to coffee. But only in the immediate aftermath of adding milk, because as we all know, coffee and milk mix quickly and create a new, uniform color within seconds after the mixing event. Unlike what we drink, the Rio Solimões and Rio Negro do not mix that quickly; their physical disparities exceed the differences between coffee and milk. The white water river, cold and sediment-loaded, rushing down from the Andes, is much denser than the black water river, sediment poor, flowing in from the lower-lying, warm rain forest of the Guayana shield. Because of this density difference, the white Rio Solimões and black Rio Negro proceed downstream side by side, unmixed for about 50 miles (according to a rough Google map calculation, shown above) after their first meeting event.

Depending on which country you are in, the mixing of these tributaries marks the official start of the main Amazon River. Other mixing events on this river boat trip have yielded similarly fantastic outcomes. To extend the metaphor, the mixing of scientists and non-scientists our river boat has paved the way for unique learning opportunities and new intellectual results, as well. Just as it was rare for scientists to have the undivided attention of a public audience, it was similarly rare for the non-scientists to witness the procession of field work and scientific debate.

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Meeting room of the Premium, where group discussions and presentations took place.

For graduate students on board, it was a rich opportunity to observe their mentors step outside of typical meeting rooms and laboratories, and present results in a way that expressed their own sentiments towards why this research was important. Equally insightful was the opportunity to hear the audience react, how their attention gravitated towards certain scientific details more than others. For a week, scientists had to tell stories. And for a week, the audience had to draw from their diverse backgrounds to respond, ask insightful questions, and provide useful feedback.

For what is probably my last trip to the Amazon region for graduate school, it was fitting to end with something that was both so reflective and forward-facing at the same time. Everyone stepped off of the Premium with slightly more practice in addressing key challenges in research: how one prioritizes earth science research when funding and time are not infinite, how objective and subjective this field of science should or can be. To get better at addressing these issues surely benefits everyone involved. What awaits further “downstream” of the mixing event that occurred aboard the Premium is perhaps a better platform for communication between science and the public.

8 December 2014: “Shaping” the river depth

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First meeting before disembarking

Because it takes a long time to process samples from the field, I just got my first major data points from the river sediments we collected this past March and July. The data measure (1) what percentage of the sediment is and (2) the composition of carbon isotopes in each sample which I will represent by the symbol δ13C. These metrics are useful for making broad statements about where the river organic carbon comes from (e.g., algae vs. various plant types), how these vary with season, and what further analyses of greater specificity need to be done.

Once you start plotting the data in different graphs, you can see different shapes and forms of it emerge across the times and in locations of the river’s cross-section that we sampled. The most obvious shape so far is how the percent organic carbon and δ13C values of sediments between the river surface and depths closer to the river bed. The same shift by depth occurs in March and July.

We are not the first to see this, but it somewhat comforting to verify previous scientist’s observations that the Amazon River is more complicated than it looks. It means that you can’t just take one measurement at one time – one sample – and say that this data point scales with all the carbon moving through the river system towards the ocean. It means that the solid material which the river carries in the surface can be significantly different in composition from the material it carries at depth. What we see in the surface can come from different sources or parts of the drainage basin, and can also interact with the river environment differently on its way towards the Atlantic Ocean.

This is a typical complexity of all large, deep rivers. The volume of water in flux is so great that the denser (which tends to be heavier and larger) sedimentary material always ends up deeper in the river. Scientists have tried to fit all this mathematically into one equation, called the Rouse profile equation, named after the scientist who formulated it (Rouse 1950). The equation allows you to calculate the amount of sedimentary material (or a specific chemical measurement in it) at any depth of a river so long as you have one known measurement at one depth. The unique Rouse number, a constant in the equation, encompasses all change with depth expected to occur in that specific river.

Like all scientific models of the real world, the Rouse profile is a simplification. It does not always work. But it is more accurate and practical than assuming that large rivers are all the same with depth. And when it does, it means that one need not take measurements at all depths of a large river because you can calculate, or “model” them, instead (Bouchez et al. 2010).

Carl Johnson, our lab manager, emailed us the data just this past Friday. It’s perfect timing to have fresh data to discuss as we disembark on the boat today. Here’s our course for the coming week. I will reveal the sequence of these sites later on.

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from: http://www.globalrivers.org/expeditions/2014-amazon-river-expedition/

References:

1. Bouchez, J., Metivier, F., Lupker, M., Maurice, L., Perez, M., Gaillardet, J., & France‐Lanord, C. (2011). Prediction of depth‐integrated fluxes of suspended sediment in the Amazon River: Particle aggregation as a complicating factor.Hydrological Processes25(5), 778-794.

2. Rouse H. 1950. Engineering Hydraulics. Wiley: New York.

7 December 2014: Faces of the Amazon Dream

IMG_3682Harbor by Universidade Federal do Oeste do Pará – Santerem, July 2014

It is probably no coincidence that boats on the Tapajos River in Santarem are named Amazon Dream. Now on my fourth visit to Brazil in the past 13 months, I have come to realize that the concept of the Amazon dream is more than a sporadic phenomenon in my world. It would be an over-statement to compare it to formal anthropological patterns like the American Dream. Nonetheless, it pervades more facets of my life, and captivates more people, colleagues and friends in my community than I was aware of before it became part of my own graduate thesis.

Starting today, I will join thirty explorers in Manaus, the largest city of the Brazilian Amazon, to discuss what comprises the Amazon Dream and navigate through the confluence of two major Amazon tributaries, the Negro and Solimões Rivers. Our team largely consists of faculty from Woods Hole Oceanographic Institution, Woods Hole Research Center and collaborating institutions in Brazil and Siberia: from light-hearted and passionate students to researchers like Dr. Bernhard Peucker-Ehrenbrink, who was one of the early pioneers of the Global Rivers project at Woods Hole Oceanographic Institution, and Dr. José Mauro Sousa de Moura, who leads monthly river sampling efforts from Santarem. The group also includes people who support earth science from several other angles: affiliates of the Woods Hole organizations who have been drawn in different ways to the research there, and Chris Linder, who has mastered the skill of conveying the excitement of earth science through the still (and sometimes moving image).

Dream is such a non-scientific word because it sounds vague and theatrical, an over-dramatization of emotions over logic. With that said, knowing myself and my advisors in graduate school, who have made it their career to study the global significance of river systems on Earth, scientific dreams may be dramatic, but they are also very specific and testable. I have only just scraped the surface of understanding why the diverse group of scientists I am joining in Manaus could share a similar attraction to the Amazon River Basin.

Given the size of the drainage basin, there is a lot of room to dream here. Depending on how you count, over ten tributaries meet the eastward flowing Amazon River on its way to the Atlantic Ocean. They come from a wide expanse of South America: the Andes in the far West, the elevated Guyana and Brazilian shields to the North and South, and the tropical lowland forests that the hug the main stem right in the middle. By the time the Amazon River meets the Atlantic Ocean farther to the East, it has carried the unique imprints of each tributary. If you manage to look at the discharge with just the right lens (e.g., stable isotope chemistry, traces of lignin or sediment load), you can find the diverse natural and human histories that river water from distinct tributaries has absorbed during its journey towards the main stem.

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The geographical extent of the Amazon River tributaries (Gaillardet et al. 1996)

At the same time, there is nothing like a biodiversity hotspot and a massive organic carbon pool to prove that one need not travel far, or function at the large scale, to actually dream big in a place like this. The Amazon rainforest and river systems have a way of concentrating a lot in a small space. If you ever find yourself in primary rainforest, try to tally each distinct flora you can see, or pick apart every unique sound you hear coming from the tree canopy. Or, re-visit all the questions one can ask by visiting one site on the river, like Óbidos. These challenges are comparable to counting all the stars in the night sky; they can occupy people for their entire careers, or for their entire lifetime.

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Rain forest in the Xingu River basin, one of the Amazon River’s most Eastern tributaries

In the coming days, as I traverse the largest river in the world among new company, it will be our shared learning experience and challenge to characterize what it means to dream about the Amazon River and, more broadly, other river systems in the world that compare and contrast with the Amazon in significant ways. Notably, this boat is not named Amazon Dream; perhaps that would be thematic overkill.

Reference:

Gaillardet, J., Dupre, B., Allegre, C. J., & Négrel, P. (1997). Chemical and physical denudation in the Amazon River Basin. Chemical geology142(3), 141-173.

24 July 2014: Starting the dry season

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.

 

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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.

 

Sources:

Aufdenkampe, A. K., et al. (2007). Organic Geochemistry38(3): 337-364.

Moreira-Turcq, P., et al. (2013). Global Biogeochemical Cycles 27: 119-130.

 Richey, J. E., et al. (2002). Nature416(6881): 617-620.