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What is Blue Carbon?
According to the website of NOAA, blue carbon is the term used for carbon that is captured by the world's ocean and coastal systems. Atmospheric carbon is sequestered by ecosystems like mangroves, tidal salt marshes and seagrass meadows at a rate that is up to three times higher compared to terrestrial carbon sinks, like tropical rainforests.
This is visualised in the figure on the right, showing the total CO2 equivalents per hectare (tCO2eq/ha) for the different habitats that is stored in.
Despite the relatively small area these habitats cover in the world (about 2%), they are considered essential in climate change mitigation.
As mentoined, the Ria Formosa Natural Park consist among other habitats of tidal salt marshes and seagrass meadows.
source: https://www.iucn.org/resources/issues-briefs/blue-carbon
Analysing Blue Carbon
Now that you know what blue carbon is and why it is important, we will look into what I did in the weeks I spent in Faro. Estimating the blue carbon in seagrass habitats was the main goal of my professional practice and the section that took up most of my time. After reading up on the theory and shadowing my supervisor to see how it's done, I got to complete an analysis by myself from beginning to end. I'll show you how I did this.
TAKING SAMPLES
As in most scientific designs, the first step was to take samples. We set out to a clear spot in the salt marsh and took three sediment cores. These cores are long PVC tubes that we hammered into the ground, collecting the sediment that is captured inside the core. The core we used here was 150 cm long, and has a diameter of 4,6 cm.
On the picture to the right, you can see me hammering the core into soil, with the help of my two lovely assistants, Lorena and Simon, both IMBRSea colleagues.
It's important to take measurements before, during and after taking the cores. When hammering the core into the soil, the sediment can get compressed. This way, the soil inside the core is compacted, and this can cause some trouble for the analysis of blue carbon. In this case, the soil came out very compacted and this is definitely something to take into account in the later stages.
The compaction was very high in these samples. After exactring them, the cores was filled with only 35 cm of sediment, despite the fact that we hammered the core more than 1 meter into the sediment.
When the sampling is finished, the cores are stored in the freezer until we can start processing them.
To perform the analysis, the PVC core has to be cut open exactly in the middle so that we can access the sediment and have an accurate estimate of the volume of the slices (remember the length, the diameter and all the measurements before, during and after sampling?).
This time, it was my turn to be the assistant. On the picture to the left, you can see Marcio saw through the PVC with a circular saw. He had done this before and knew exactly how to saw through the material without contaminating the sediment inside.
Below you can see the core that was cut in half. You can clearly see how the sediment changes color depending on whether is it more superficial (left) or has been buried longer (right). Also of note are the roots, visible in the top (left) part, which might have been the reason this core is so compacted as they pushed the sediment down while we were hammering the core, and some contamination in the deeper (right) part. This contamination can be recognised by a ring on the outside of the sediment with a lighter, brown-ish color compared to the grey-ish sediment in the inner part (of course, we see this only as two thin lines along the outer part of the sediment). The origin of this contamination is probably sediment that is positioned closer to the surface and was pushed down when the core was inserted into the soil.
PROCESSING THE CORE
Next up, I divided the sediment in slices of 0,5cm intervals, as you can see here on the rigth. This interval was choosen because of the high compaction. If the interval between slices would have been larger, a lot of the information would have been lost.
Each slice was then put in a separate zip-lock bag and placed in the balance to determine the wet weight.
All the slices were then dried in the oven at 60°C for 48 hours, so that all the water would evaporate and the dry weigth could be determined by placing all the zip-lock bags on the balance again. With this information, we can determine the amount of water in the sediment along the depth (see below).
LOI (Loss-On-Ignition)
To determine the organic matter in the sediment at each interval, we used the LOI method. LOI stands for Loss On Ignition, in this technique, the sediment is fired at 450°C for 4 hours. At this temperature, all the organic matter will evaporate in the form of CO2. I took a fraction of the dried sediment from each interval from the previous step and homogenised it with a mortar and pestle, transfered the grit in a small aluminium cup and put them in the oven.
Above you see the aluminium cups, filled with homogenised sediment, before (left) and after (right) firing them in the oven at 450°C. You can clearly see how the color is different before and after firing, changing from brownish to almost red. With the weigth before and after ignition, we can calculate the percentage organic matter (%OM) in each slice, using this equation:
%OM = [(Weight pre LOI) - (Weight post LOI)]
(Weight pre LOI)
From the organic matter, we can calculate the organic carbon. This is done by in an equation that is specific for the Ria Formosa, calculated in a previous study done by my supervisor. When all of the results were in, it was time to get started on R!
Below you can see two graphs that I made in R based on my results. You can clearly see how the top layer has higher percentages of organic carbon and lower percentages of water, whereas near the bottom, we see that the percentage organic carbon is very close to 0 and a percentage of water that is near 80%. These results are in agreement with previous studies.
As I mentioned before, obtaining results was not one of the main goals of this professional practice, so I will not go into detail about this. If you want to know more, you might want to visit the website of the ALGAE research group (linked in the page ALGAE). Thank you for reading all the way through, I hope it was halfway interesting and that you learned something!
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