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.
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| Watch video in: http://laps.io.usp.br/index.php/en/projects/81-english/laps/projects/97-samba |
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| Example of food chain. Source: http://lifeadrift.info/ |
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.
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:
