Photo montage of marine life for the exhibit “Océan, Une Plongée Insolite” at the Muséum National d’Histoire Naturelle in Paris.
As I started my internship in a Marine Observatory, I knew broadly that I was going to work on water quality. Why broadly? Because my background is in environmental studies and coming into a Master’s program in biology means I had a lot of catching up to do. As you might imagine, words like “terrestrial organic matter” and “phytoplanktonic communities” had very little resonance in my mind. My internship was a great push towards catching up. While classes were a good start to mastering my new field, it turns out that there’s nothing like writing a scientific research report to really force yourself to start engaging more deeply with new concepts and material. This internship was a fascinating introduction to learning about plankton, their effects on aquatic ecosystems, and potential sources for their decline.
When you think of oceans and lakes, you often think of big mammals such as cetaceans – and I don’t blame you! They’re constantly in the news as we learn more and more about their human-like traits and intelligence. These large mammals are highly visible and much easier for us to relate to. And yet, since my internship focused on the impact of climate change and input of terrestrial matter into aquatic ecosystem, I had to acquaint myself with the much smaller creatures who are often the first to be impacted by environmental changes: plankton.
The first important distinction you have to make is between phytoplankton and zooplankton. Zooplankton are heterotrophic creature, meaning that they need exterior matter to feed themselves. They feed on phytoplankton. Most phytoplankton species are autotrophic, meaning that they get their energy through synthesizing mineral matter into organic matter – that is, through photosynthesis and mineral salts.
Over the course of my internship, I focused on phytoplankton, as they are quite affected by rising temperatures and brownification of aquatic systems. Brownification comes from terrestrial organic matter running off in water and affecting the amount of light that can penetrate the water. As extreme rain episodes become increasingly common, terrestrial matter is more likely to end up in water. And, since phytoplankton use light to produce energy, this phenomenon has a negative impact on their biomass. Furthermore, plankton typically live at the surface of the water, which is more affected by rising temperatures.
So why should we care?
Phytoplankton play – among others – two essential roles for ecosystems (and for humans!).
Figure 1.
Firstly, phytoplankton occupy an important role at the bottom of the food chain. As previously mentioned, zooplankton feed on phytoplankton, which are then eaten by fish larvae, who are eaten by even larger fish (Fig.1). These critters will then be eaten by progressively bigger sea creatures, and so on, ultimately reaching top predators such as killer whales and sharks. Plankton are also consumed directly by baleen whales. A decrease in phytoplankton biomass therefore means an entire food chain becomes compromised – not to mention the marine life humans feed on.
Secondly, despite their size, which ranges from femto to macro (but never bigger than the head of a needle), plankton constitutes a very large biomass in aquatic ecosystems. As such, their role in the photosynthesis and carbon cycle is of utmost importance as they account for more than 50% of all photosynthesis on the planet. Not insignificantly, this also means about 50 billion tons of carbon captured each year (source: Blue Planet Society). As with every other plant, plankton release oxygen and capture carbon. The carbon they capture ends up in the depths of the oceans when they die or pass through other creatures that eat them. A change in their biomass can thus have an impact on the concentration of atmospheric carbon, which is already on the rise because of anthropic activity.
This fact served as a backdrop to my internship as I learned more about phytoplankton and their vital role in ecosystems and tried to understand what phenomena and environmental changes can disturb their viability and biomass. During our experiment I learned how to measure phytoplankton’s presence and characterize major algal groups. These measurements, when they are completed at a larger scale, have revealed that phytoplankton have been declining at alarming rates since the 1950s. I also learned that, despite the fact that it seems like such a large-scale challenge, we must not underestimate the impact of climate change (and its innumerable ancillary effects) on even the most intricate and small parts of ecosystems, as they will often snowball into drastic and profound consequences.
This article was written by Clémentine Gallot, a Master’s student in the Museum’s “Ecology Biodiversity Evolution” (EBE) specialization.
Muséum national d'histoire naturelle 