The fragility of plankton

When there is a natural disaster at sea, such as the Exxon Valdez in Alaska (1989) or the Prestige in the NW Spanish coast (2002), we all immediately worry about seabirds, turtles, and dolphins and how the disaster will affect fisheries. On television, we are flooded with images of black birds, covered in oil, and dead fish on beaches. All of this is relevant and important, of course. However, no one (apart from three or four specialist scientists in the field) stops thinking about what effect it can have on the base of the food web, the plankton. We are not aware of the fact that if the plankton fails, the house of cards collapses, and we can forget about the birds, turtles, dolphins, and fish because nothing will reach them to eat and they will die irretrievably. Fortunately, although certain groups of plankton are very sensitive to hydrocarbon pollutants, the dilution effect of seawater and the mostly superficial zoning of the crude play in their favor.

It does not take a natural catastrophe to harm plankton. Fully accepted anthropogenic activities also have an effect. For example, if someone proposed cutting down an oak forest to plant exotic fruits, such as passion fruit, papaya or others, we would react by calling it an ecological monstrosity. Yet, if the case was a foreign species of oyster (like those that populate aquaculture facilities, mostly originated in the Pacific), in a bay where we do not swim, it probably would alarm no one. However, these crops destroy planktonic diversity and damage the entire marine ecosystem around them. Besides filtering a good part of the plankton present, they pollute the water and the sediments to the point of producing anoxia. If that was not enough, they are also hotspots for jellyfish and harmful algal bloom proliferations. All in all, a real ecological disaster that we completely ignore.

Mediterranean plankton

Plankton are very sensitive to pollutants, and many studies support this. I do not want to get into technicalities and flood you with data on the effect of mercury, cadmium, chromium, etc., or what are the consequences of sunblock creams or medicines that end up in the sea. What I will tell you are two anecdotic examples that I have witnessed and which were not part of any specific study, but which undoubtedly reflect the fragility of plankton.

The first example dates back to my time as a Ph.D. student, when I started growing copepods in the laboratory. To keep these small crustaceans, we used seawater filtered through cartridges with different filters. I remember once the plastic connecting screw between two cartridge holders was damaged, and we replaced it with a copper/bronze one. Almost immediately, the copepods, despite presenting a healthy appearance, stopped laying eggs. We checked the food, temperature, salinity, etc. However, we did not find any problem. Finally, one day, after turning it over a thousand times, it occurred to us that the only different thing was that screw. We installed a plastic one, and the copepods returned to lay eggs as usual. Consider that the water was only in contact with this piece for a few seconds, but it still affected them sublethally. Today, we know that copper inhibits vitellogenesis (egg yolk formation) in copepods and therefore negatively affects egg production.

Copèpod Paracartia grani from the permanent culture at the Institute of Marine Sciences, CSIC.

I witnessed another curious example during my postdoc at the University of Hawaii (USA). At that time, we carried out monthly oceanographic campaigns in Pacific waters, in which I had the privilege of participating. On those scientific cruises, we took seawater with Niskin bottles (plastic cylinders with caps at both ends) mounted in a rosette around a CTD (a device that measures conductivity, temperature, and depth). In two of those Niskin bottles, the O-ring, which ensures they close properly, and no water is lost, broke. As there were no more O-rings of the same high quality (they were free of trace metals), the technician in question replaced them with standard ones made of black rubber, which are common in plumbing. During the following cruises, in those two bottles, there was always a drop in primary production (algae production) measurements of approximately 20-30%. New, better-quality O-rings arrived, and the problem went away. Such an insignificant detail, because the rings are almost not in contact with the water, affected the algae negatively and forced us to throw away all the data corresponding to those samplings. Let me add that most Niskin bottles in use today still have the typical black plumbing O-rings. Because this was never published, it has not reached the scientific community. To publish it, it would be necessary to do more tests with different communities and more replicates. However, in the end, the problem is there.

Rosette of Niskin bottles mounted around a CTD

Many pollutants can affect plankton, some of them classic and well-known, such as hydrocarbons or heavy metals, and others emerging, which have seldom been taken into consideration, such as nanoparticles, hormones, medicines, plasticizers, etc. We know little about what effect they will have on plankton. Nor do we know if the increase in temperature will act synergistically with them to the detriment of plankton. We still have a lot to learn, but, in the meantime, I would ask you to take care of our precious and delicate sea and the creatures that live in it, including the plankton. I would also ask you to think that, although invisible, the planktonic food web makes up an ecosystem as complex and precious as an oak grove or a pine forest.

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