When you think you finally know how the plankton ticks and what’s going to happen next, there is another big surprise around the corner. We are so used to the concept that upwelling of nutrient rich deep water fosters the growth of diatoms, those tiny phytoplankton thriving in their glassy shells. Because they take up nutrients very effectively and reproduce quickly, they outcompete all other phytoplankton groups when there are plenty of nutrients around. This makes them the acclaimed queens and kings of the ocean, accounting for more than 40% of marine primary production and being the main drivers of marine elemental cycles. And that concept was reinforced by the plankton community responses in our mesocosms upon deep water addition. Diatoms all over!
Out of the blue – or rather the green – appeared the secret queen of the ocean and turned the water in our mesocosms with continuous deep water addition whitish. Anyone who has seen it before will immediately know when looking at the water from above, even though the secret queen is so small that it’s hard to recognize her under the light microscope. The water takes on this characteristic milky tint. Our suspicion was quickly confirmed when placing some drops of the milky water under the microscope. Coccolithophores everywhere! It’s their chalky tiny platelets with which they cover their entire surface that turns the water whitish. It is difficult to tell from the light microscope which species it is, so samples were quickly prepared for later scanning electron microscopy. But I bet ten bars of Kinderschokolade that it is Emiliania huxleyi.
But what allowed the tiny secret queen to win against the large and powerful diatoms? Well, hopefully the many different variables measured throughout our experiment will tell us. My guess: The continuous supply of nutrient-rich deep water did not only nourish the diatoms, it also provided the zooplankton feeding on them sufficient time to catch up, reproduce themselves and eventually keep the diatoms in check. This paved the way for the secret queen to multiply underneath the radar of the grazers, as these are specially equipped to chew on the glassy and bulky diatoms, but are likely to miss the chalky dwarf.
Whatever caused the rise of the coccolithophores in our mesocosms, it means that a new set of rules applies to our enclosed plankton worlds. It not only changes the structure and functioning of the food web, it also affects nutrient uptake and elemental cycling. And also the exchange of climate relevant gases with the atmosphere is likely to be influenced, as the production of the chalky platelets releases CO2. And coccolithophores are among the most prolific producers of the climate cooling agent dimethylsulfide. The plankton universe, a world full of surprises.
The chalky beauties: Emiliania huxleyi, Michaelsarsia elegans, Acanthoica quattrospina, Calciosolenia murrayi, Discosphaera tubifera, Florisphaera profunda, Pappomonas sp. Poricalyptra isselii, Umbellosphaera irregularis, Rhabdosphaera spinifera, Calcidiscus leptoporus, Braarudosphaera bigelowii, Helicosphaera carteri (from top left to bottom right). The 5 micron scale bar at bottom right applies to all the images. Table from Monteiro et al. 2016
Milky waters in M8, the mesocosm receiving the highest continuous input of deep water
Monteiro, F.M., Bach, L.T., Brownlee, C., Bown, P., Rickaby, R.E.M., Poulton, A.J., Tyrrell, T., Beaufort, L., Dutkiewicz, S., Gibbs, S., Gutowska, M.A., Lee, R., Riebesell, U., Young, J., Ridgwell, A. (2016) Why marine phytoplankton calcify. Science Advances 2, e1501822