New insights into the day/night cycle of the global ocean

Phytoplankton are the foundation of life on Earth. Understanding how these organisms react to the ocean environment can improve our knowledge of the rest of the food web. The day/night light cycle – “diel cycle” – is critical for photosynthesis among the ocean’s primary producers.

Biologists at the Marine Biological Laboratory in Massachusetts incorporated the diel cycle into a new global ocean model to investigate its effects on phytoplankton. The study is the first to investigate how the day/night cycle affects the biogeography and diversity of these primary producers. 

The new model applied natural light and dark cycles over the global ocean to 15 simulated phytoplankton types. These simulations were compared to a control simulation to see how diel light cycles affected productivity and nutrient concentration dynamics.

The simulated phytoplankton were separated into two groups: “gleaners,” or simulated smaller cells with high nutrient affinity (capturing nutrients out of the water column even if those nutrients were in low quantities) but slow growth; and “opportunists,” or simulated larger cells with higher growth rates but low nutrient affinity (meaning they did better in nutrient-rich water). 

The researchers found that the diel cycle did matter to the simulated phytoplankton. Diel cycles were associated with higher concentrations of limited nutrients.  At lower latitudes, the simulated opportunists were more abundant than the gleaners. This became less important at higher latitudes, where the effects of the seasonal light cycle were stronger than the day/night cycles.

If scientists cannot understand how phytoplankton get their energy as primary producers, it’s hard to understand the rest of the ocean food web –  all the way up to humans.

By gaining insight into how the ocean works, experts can better predict how global warming will further affect our planet. Scientists make better ocean models to investigate possible solutions to climate change, while minimizing unintended consequences.

“This model contributes to advancing our understanding of how the ocean works,” said Vallino. “Being able to predict how the distribution of phytoplankton will change has repercussions higher up the food web. If you can’t get that base change right, you can’t get anything that’s connected to that above it right.”

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By Katherine Bucko, Earth.com Staff Writer