Science Journal Club resources (1) – Nutrient Deposition in the Atlantic Ocean

Science Journal Club is now in its second year. I wrote here about how it has helped widen our students’ understanding of interdisciplinary science, and how it has improved scientific literacy. One of our regular attendees wrote about how and why she reads scientific papers here.

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In our most recent meeting, we read a paper about nutrient deposition in the Atlantic Ocean, and one of the authors came along to the session. I thought it might be helpful if I shared the paper we read, along with some background information, and the questions I wrote to go with it. Please feel free to use any of the resources linked to here.

Background

Dr Alex Baker is a Researcher in the School of Environmental Sciences at UEA. In his session, we looked at this paper: Estimation of the Atmospheric Flux of Nutrients and Trace Metals to the Eastern Tropical North Atlantic Ocean

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Alex explained that he’d never actually expected to be a marine scientist. Chemistry was his favourite subject at school, and he chose to study Chemistry with Oceanography at university because it looked interesting.

The research in this paper looks at nutrient input into the Atlantic Ocean, especially within the gyres (the blue/ purple regions either side of the equator on the map below). These regions are oligotrophic, which means that they have very low levels of nutrients. The blue/purple colouring on the map indicates that chlorophyll A levels are low, reflecting the sparse phytoplankton coverage in these nutrient-limited regions, often referred to as the “ocean deserts”.

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Plumbago at English Wikipedia [CC BY-SA 3.0  or GFDL via Wikimedia Commons

 

Alex found that dust from the Sahara Desert is a significant source of nutrients to the North Atlantic gyre and this has a much larger effect here than it would do within regions that are already high in nutrients.

Why do we care?

The oceans cover over 70% of the Earth’s surface. We expect the ocean to act as a “sink” for carbon dioxide and, left alone, the ocean and the atmosphere are kept in balance. However, human activity has upset this natural balance. Scientists are trying to model the effect this will have on the environment in the future. To do this, they need to understand what is happening now.

In the “ocean deserts” where Alex was sampling, a small change can have a large effect. For example, phytoplankton in these regions has adapted to survive on limited nutrients, so even a small input (which would take them above the levels they are used to) can really perturb the natural balance. This needs to be measured and taken into account when modelling change and making predictions about future climate.

Sampling

The figure above shows a map of chlorophyll data for the entire world ocean. This data is gathered via satellite, but it has to be calibrated via “in-situ” measurements. In other words, you have to physically sample some sea water, measure the concentration of cholorophyll in it, and then compare it to satellite measurements. This is the principle behind all on-ship sampling. You need to be there to measure it (whatever “it” happens to be). And you need to measure it as often, and as widely, as time and money will allow.

I sailed on 5 of the cruises where Alex’s data was collected, and there were a number of ways that my fellow scientists collected their samples: optical rigs, bottles filled with water at specific depths, nets to collect plankton… buckets over the side of the ship…

 

Alex collected aerosol samples by sucking them onto a huge piece of filter paper as the ship travelled along. He described how these papers would change colour from white (in clean air) to brick red when they’d collected Saharan dust. He joked about how he was always the least popular person on any ship, because his sampler was on the “monkey island” right at the top of the ship. This is a prime sunbathing spot, but he couldn’t risk having his air samples contaminated by people (or smoke).

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Alex also collected rainwater but (fortunately for his fellow scientists), it didn’t always rain onboard. So Alex would ask the officers on the bridge to ring him if they thought it was going to rain… whatever the time of day or night. If he got “the call”, he would have to dash up to the monkey island (several decks up) and set up the rain sampler.

Questions

These are the questions I asked my students. If you would like me to sketch out some answers to them, I can do. Please just ask! I’ve also put them on a word doc here.

  • What do the following words from the abstract mean? anthropogenic, oligotrophic, deposition, atmospheric deposition, trace (metals), intertropical convergence zone.
  • Which nutrients are mentioned in the first paragraph of the introduction? What do we mean by “fixed” nitrogen?
  • Why is it so important that the eastern North Atlantic is relatively close to the Sahara Desert?
  • How does N and P get into the Atlantic (via the Sahara)? What is the original source of these nutrients?
  • Which factors affect how much dust is deposited on the ocean?
  • Why do scientists use modelling and remote sensing? What are the advantages? What are the limitations?
  • What are the advantages and disadvantages of taking direct measurements from the sea (from ships) compared to modelling and remote measurements?
  • Two Baker papers are mentioned in the penultimate paragraph of the introduction (papers from 2010 and 2013). Why? What’s the difference between the two papers?
  • In the methods section, they distinguish between the N and S Atlantic Ocean. Why not just calculate a value for the whole Atlantic?
  • How did they analyse their samples?
  • Why was it important to distinguish between dry and wet deposition? (and what do we mean by these?)
  • Can you use the final sentence of the abstract and the Conclusion to explain why the results of this paper were significant? What’s the significance of the ratio mentioned in the abstract?

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