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Phytoplankton is the foundation of all life on the planet. Understanding how these photosynthetic organisms react to their ocean environment is important to understanding the rest of the food web.

In spite of that, computer models of the global ocean biogeochemistry typically don’t include the day/night (diel) light cycle, even though that cycle is critical for photosynthesis in the ocean’s primary producers.

For the first time, scientists from the MBL Ecosystems Center have incorporated the diel cycle into a global ocean model in order to investigate its effects on phytoplankton. Their study, published in , is the first to investigate how the day/night cycle affects the biogeography and diversity of these primary producers.

The model offered natural light and dark cycles over the global ocean to 15 simulated phytoplankton types. It was then compared to a control simulation using the same plankton model, but illuminated with light averaged over 24-hour periods. The goal was to see how diel light cycles affected phytoplankton productivity and changed nutrient concentration dynamics.

The simulated phytoplankton were all different cell sizes and separated into two different groups with two broad ecological strategies. “Gleaners” simulated smaller cells with high nutrient affinity (meaning they could capture nutrients out of the water column even if those nutrients were in low quantities) but slow growth, and “opportunists” simulated larger cells with higher maximum growth rate but low nutrient affinity (meaning they did better in nutrient-rich water). These were representations of real phytoplankton based on parameters from lab cultures. 

The researchers found that the diel cycle did indeed matter to the simulated phytoplankton.

“We know that a lot of traits of different phytoplankton are based on the day/night cycle. Some dinoflagellates go deeper [in the water column] to get more nutrients and then go up to photosynthesize. Some store carbon during the day, so they can use it at night,” says Ioannis Tsakalakis, postdoctoral researcher in the Ecosystems Center and first author on the paper.

The model showed that diel cycles are associated with higher concentrations of limited nutrients, which meant that at lower latitudes (−40° to 40°), the simulated opportunists were more abundant than the gleaners compared to the control simulation. This includes phytoplankton like diatoms. This mechanism became less important at higher latitudes, where the effects of the seasonal light cycle were stronger than the day/night cycles.