Phytoplankton boosted by eddies supplying nutrients, per University of Hawai‘i study

Beyond colorful coral reefs and diverse nearshore ecosystems, Pacific Ocean waters surrounding the Hawaiian Islands have comparatively little marine life and low biological productivity.

New research published by University of Hawai‘i at Mānoa oceanographers showed that eddies (circular currents 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 stimulate blooms of phytoplankton, microscopic plant life that lives in the surface ocean.

“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 Kate Feloy, lead author of the study.

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, University of Hawai‘i 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.

“This work identifies a new mechanism that can deliver nutrients around Hawaiʻi,” said Feloy.

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