Fish food: UH researchers study circular ocean currents’ role in phytoplankton blooms

Circular ocean currents, called “eddies,” play an important role in supplying nutrients for marine life and can stimulate blooms of phytoplankton, microscopic plant life that lives on the ocean surface, according to new research published by University of Hawaiʻi at Mānoa oceanographers.

The study is significant because — beyond colorful coral reefs and diverse nearshore ecosystems — Pacific Ocean waters surrounding the Hawaiian Islands have comparatively little marine life and low biological productivity.

But the research shows that eddies (circular current of water) on the leeward side of the Hawaiian Islands can supply nutrients, not only locally, but also to the opposite side of the island chain and feed phytoplankton blooms.

“While these eddies are known to impact biological productivity locally, our study reveals that nutrients upwelled by these eddies can also be transported around the islands, counter to the background flow,” said study lead author Kate Feloy, an Uehiro graduate fellow and doctoral candidate in the Department of Oceanography at the UH Mānoa School of Ocean and Earth Science and Technology “These results demonstrate how eddies can have far‐reaching, remote impacts on productivity around the Hawaiian Islands.”

Nutrient availability is vital for phytoplankton, which form the base of the marine food chain. With waters around Hawaiʻi typically very low in nutrients, growth is limited. Feloy and co-authors, oceanographers Brian Powell and Tobias Friedrich, observed in satellite data previously unreported blooms of phytoplankton off the northern coasts of some Hawaiian Islands.

The researchers used a computer model of the region to simulate the ocean around the Hawaiian Islands and conducted a series of experiments to determine the source of the nutrients driving these anomalous events. Initially, they expected to uncover a mechanism that caused local upwelling, on the north side of the island chain. The model accurately reproduced the bloom events. However, the results indicated that the blooms were driven by nutrients supplied from upwelling eddies around 100 miles away.

“Our study reveals that nutrients from the eddies can be transported in waters below the sunlit layer around the islands where local upwelling can lead to phytoplankton blooms,” Feloy said. “This work identifies a new mechanism that can deliver nutrients around Hawaiʻi.”

These blooms are significant events for biological productivity in the region — productivity that can be transferred through the food chain, potentially impacting fisheries near Hawaiʻi. This same mechanism may also impact productivity around islands in other nutrient-poor regions.