New model sheds light on day/night cycle in global ocean

Phytoplankton is the foundation of all life on the planet.

Despite this, computer models of global ocean biogeochemistry generally do not include the day/night (diel) light cycle.

For the first time, scientists from the Laboratory’s Ecosystems Center of Maritime Biology (MBL) integrated the daily cycle into a world wide ocean model in order to study its effects on phytoplankton. Their study, published in Worldwide Ecology and Biogeography, is the first to investigate how the day/night cycle affects the biogeography and diversity of these primary producers.

The model offered cycles natural light and dark over the world ocean to 15 simulated phytoplankton forms. It was then compared to a control simulation using the same plankton model, but illuminated with light averaging over periods of 24 hours. The objective was to see how daytime light cycles affected phytoplankton productivity and altered nutrient focus dynamics.

The simulated phytoplanktons were all different cell sizes and separated into two different groups with two major ecological strategies. The “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 supply) but slow growth, and the “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 laboratory culture parameters.

Researchers found that the daily cycle did indeed matter to the simulated phytoplankton.

“We know that many characteristics of different phytoplankton are based on the day/night cycle. Some store carbon during the day, so they can use it at night,” says Ioannis Tsakalakis, postdoctoral researcher at MBL and first author of the paper.

The model showed that daily cycles are associated with higher concentrations of limited nutrients, which means that at lower latitudes (?40° to 40°), simulated opportunists were more abundant than gleaners compared to the control simulation. This includes phytoplankton like diatoms. This mechanism became less important at higher latitudes, where seasonal light cycle effects were stronger than day/night cycles. phytoplankton obtains its energy as a primary producer at the base of the food web, it is difficult to make inferences about the interactions of the rest of this global ocean food web – down to humans.

So why hasn’t anyone included the daily cycle before?

The world ocean is immense, just like the models that represent it. To cope with the complexity of what happens in the ocean, modelers often simplify certain processes. Typical models only incorporate seasonal light changes rather than adding the finer details of the day/night cycle. This is primarily a computational decision, says MBL lead scientist Joe Vallino, lead author of the paper. “If you don’t work out the fine details of time, in general, run faster.”

“You push away material constraints,” says Vallino. “You don’t want a year simulation to take 10 years to simulate.”

But as climate change progresses, understanding how climate works is essential. the ocean to understand how global warming and increased carbon dioxide are affecting it.

“This model helps advance our fundamental understanding of how the ocean works “says Vallino, adding that as scientists develop better ocean models, they could eventually use them to research possible methods of climate change while minimizing unintended consequences.

“Being able to predict how the distribution of phytoplankton will change is going to have repercussions further up the food web,” says Vallino. “If you can’t get this base change right, you can’t get anything connected to this above.”

This research is part of the CBIOMES project, funded by the Simons Foundation.

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