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For plankton, size matters

Atualizado: 25 de jan. de 2021

Today, I want to discuss a subject that has fascinated me since I started my PhD. We are often asked “What is you PhD about?” and the general reaction of grad students is simply to avoid the subject or to just reproduce the title (some long and complicated name that nearly nobody, let alone ourselves sometimes, is able to understand). Or we simply say that it is too hard to explain with simple words. Notice how this sounds like we think too much of ourselves: we are very smart and outsiders will never be able to understand what took us so long to embrace.

Well, that is exactly the kind of attitude that the grad student should avoid. This blog is designed to be a place where academia may connect with society. I had a beloved professor that used to say that every grad student should be able to explain his/her project to his/her grandmother, and only once we accomplish that, would we finally be confortable with the theory behind our research. So, I’ll try to do exactly that, a little late I confess since I have already finished my PhD. I’ll explain in a simple – but not simplistic – way the work I developed during my PhD.

I am interested in plankton, more specifically, the zooplankton! No, I’m not referring to SpongeBob’s villain, but they are nonetheless, interesting creatures worth knowing a bit more about. Zooplankton are tiny aquatic critters, usually invisible to the naked eye. They are traditionally described as organisms that travel with the currents because they don’t have enough “strength” to swim against it, due to their small size. But that does not mean they are lazy guys. On the contrary, many of them are able to vertically migrate large distances through the water column, sometimes hundreds of meters, on a daily basis.

Watch video in:

Zooplankton are very important in marine food webs, and they are also fundamental to other important processes in the oceans (we can discuss that in another post). These organisms feed on phytoplankton (the photosynthetic plankton that are to the oceans, what the trees are to the Amazon Forest) and are eaten by fish, which are ultimately eaten by larger fish, marine mammals including whales, and even us humans. So you can imagine that if there are few plankton in the area, there will also be less fish and other organisms in higher trophic levels. This includes a limited supply of fish for us, which means less sushi, and I love Japanese food!

If zooplankton are so important in mediating the transfer of biomass and energy from primary producers (phytoplankton) to higher trophic levels (fish, birds, whales, man) then we must understand these feeding relationships very deeply, don’t you agree? Well, one of the golden rules in the ocean is that organisms always (or almost always) feed on organisms that are smaller than themselves. That is why size matters when zooplankton choose the dinner menu. Many researchers have studied the flow of biomass and energy through the trophic levels. For example, it has been calculated how much of a “dinner” is actually absorbed by a zooplankton and how much is left to the fish, birds and whales that feed on the same guy. This information can potentially explain a lot of things about the oceans. peixes, aves e baleias que se alimentam desse mesmo plâncton. E essa informação aparentemente tão simples tem potencial explicar um monte de coisas sobre nossos oceanos.

But how? Well, if you measure the size of organisms, calculate their weights, and plot this information in a graph, such as the one in this page, you will notice that there is always more biomass accumulated in the small organisms than in the bigger ones. By accumulated biomass I mean the biomass of all organisms in that particular size range. What does that mean? It means that to satisfy the hunger of one big guy, it is necessary to have a whole bunch of small guys. You must remember there is energy loss in every “meal” because total nutrition is never absorbed with everything that we, or any other organisms, eat.

Based on these facts, the biomass size spectra theory was developed. This theory relates the shape of the biomass distribution through size classes (and also the mathematical indices associated with it) with properties of the ecosystems. Personally, I think it is absolutely amazing how a simple mathematical index can be used to determine the energy transfer efficiency in an ecosystem, taking into account productivity, predator-prey interactions, and the number of trophic links in the oceans.

My PhD was based on this theory with a scary name (spectra tends to conjure images of ghosts, no?), but the theory is not as complex as it seems. To get my data, I collected zooplankton samples with a simple net (as seen in the photo) aboard several cruises. When back in the lab, all I had to do was to scan my samples with a waterproof scanner (the ZooScan), and very useful software automatically classified, counted, and measured the size of each organism. I also learned how to program in R and Matlab to analyze the enormous amount of data for me, because life is short and I have other hobbies in addition to science to dedicate myself to, such as this blog!

The results I found for the coast of Ubatuba, Sao Paulo and Abrolhos Bank revealed that the mathematical indices associated with the biomass size spectra theory can be used to detect differences in the zooplankton community caused by seasons and local features (water column stratification, depth, proximity of the coast). That means these indices are useful for monitoring oceanic ecosystems because they are easily calculated – granted you have technology to help – and there is no need to identify species, which is usually a time-consuming task when we are talking about plankton.

If you are interested in the subject, my PhD dissertation is available at this link:

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