New model sheds light on day/night cycle in the global ocean
Phytoplankton is the basis of all life on Earth. Understanding how these photosynthetic organisms respond to their ocean environment is important to understanding the rest of the food web.
Nevertheless, computer models of global ocean biogeochemistry usually do not include the day/night (diel) light cycle, although that cycle is critical for photosynthesis in the primary producers of the ocean.
For the first time, scientists at the Ecosystems Center at the Marine Biological Laboratory (MBL) have incorporated the diel cycle into a global ocean model to investigate its effects on phytoplankton. Their study, published in Global Ecology and Biogeographyis the first to investigate how the day/night cycle affects the biogeography and diversity of these primary producers.
The model provided natural light and dark cycles across the global ocean to 15 simulated species of phytoplankton. It was then compared to a control simulation using the same plankton model, but illuminated with light averaged over 24-hour periods. The aim was to see how diel light cycles affected phytoplankton productivity and changed nutrient concentration dynamics.
The simulated phytoplankton all had different cell sizes and were divided into two distinct groups with two broad ecological strategies. “Gleaners” simulated smaller cells with a high nutritional affinity (meaning they could capture nutrients from the water column even if those nutrients were in small amounts), but slow growth, and “opportunists” simulated larger cells with a higher maximum growth rate but a low nutritional affinity (meaning they did better in nutrient-rich water). These were representations of real phytoplankton based on parameters from laboratory cultures.
The researchers found that the diel cycle did indeed matter for the simulated phytoplankton.
“We know that many traits of different types of phytoplankton are based on the day/night cycle. Some dinoflagellates go deeper [in the water column] to get more nutrients and then proceed to photosynthesis. Some store carbon during the day so they can use it at night,” said Ioannis Tsakalakis, MBL postdoctoral researcher and lead author of the paper.
The model showed that diel cycles are associated with higher concentrations of limited nutrients, meaning that at lower latitudes (−40° to 40°), the simulated opportunists were more abundant than the foragers 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.
If scientists don’t understand how phytoplankton gets their energy as primary producers at the base of the food web, it’s hard to draw conclusions about the interactions of the rest of that global ocean food web — all the way down to humans.
So why hasn’t anyone included the diel cycle before?
The global ocean is vast, and so are the models that represent it. To cope with the complexity of what happens in the ocean, modellers often simplify certain processes. Typical models only include seasonal light shifts rather than adding the more fine-grained details of the day/night cycle. This is mostly a computational decision, says MBL Senior Scientist Joe Vallino, senior author on the paper. “If you don’t solve fine time detail in general, [the models] walk faster.”
“You’re pushing against the hardware limitations,” Vallino says. “You don’t want a 10-year simulation to take 10 years to simulate.”
But as climate change progresses, understanding how the ocean works is vital to understanding how global warming and increased carbon dioxide affect it.
“This model helps improve our fundamental understanding of how the ocean works,” Vallino says, adding that scientists who create better ocean models can ultimately use them to explore potential solutions to climate change and minimize unintended impacts. to limit.
“Being able to predict how phytoplankton distribution will change will have consequences higher up in the food web,” Vallino says. “If you can’t get that basic change right, you can’t get anything connected to it above it.”
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Ioannis Tsakalakis et al, Diel light cycles affect phytoplankton competition in the global ocean, Global Ecology and Biogeography (2022). DOI: 10.1111/b.13562
Quote: New model sheds light on global ocean day/night cycle (2022, 9 August), retrieved 9 August 2022 from https://phys.org/news/2022-08-daynight-global-ocean.html
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