Researchers from New Zealand think the first investigation into the biological reaction of the upper ocean following South Pacific cyclones could aid in predicting the effects of rising ocean temperatures.
Dr. Pete Russell, of the University of Otago’s Department of Marine Science, and Dr. Christopher Horvat, of the University of Auckland’s Department of Physics, have published a study on the oceanic biological effect of Cyclone Oma which passed near Vanuatu in 2019.
“While Oma was a relatively benign cyclone, it produced a massive phytoplankton bloom in its wake the single most abnormal event in the history of South Pacific chlorophyll measurements,” Dr. Russell says.
“Such an extreme event can produce a large amount of biomass in a part of the ocean that is typically a biological desert. We don’t yet know about the fate of this biomass, but one possibility is that it could end up on the bottom of the ocean, sequestering carbon.”
“The study, just published in Geophysical Research Letters, found the phytoplankton bloom produced by Oma was exceedingly rare, occurring just once every 1500 years in the same location.”
“Cyclones are one of the mechanisms that dissipate heat from the tropics. Warming oceans mean more heat to dissipate. This means more intense storms and perhaps longer storm seasons resulting in more storms.”
Phytoplankton blooms refer to the rapid and excessive growth of microscopic, single-celled photosynthetic organisms known as phytoplankton in aquatic ecosystems, particularly in marine environments. These blooms can be so massive that they are often visible from space and can cover large areas of water.
Along with these bloom events in the open ocean, cyclone activity results in both coastal upwelling and runoff from the land that also deliver nutrients into the photic zone, generating blooms. These blooms could be an integral part of the local marine ecosystems of our Pacific neighbours supporting higher food chains.
Dr. Christopher Horvat
“By examining sediment cores from the last inter-glacial period, we may get a heads up on what cyclone activity to expect with ocean temperatures 1+ degrees higher than today,” Dr. Russell says.
The researchers discovered that if a storm hangs over a region of ocean for an extended period of time, physical interactions between the ocean and cyclone winds would force water to rise towards the eye, bringing nutrient-rich water to the surface and triggering a phytoplankton bloom.
Dr. Horvat says “these events may be biological hotspots, causing large amounts of biological material to be produced in areas typically devoid of upper-ocean life.”
“These cyclones can do amazing things other than have strong winds, they can also dramatically affect the plants and animals living in the upper ocean and change the cycling of carbon by leading to blooms.”
“Along with these bloom events in the open ocean, cyclone activity results in both coastal upwelling and runoff from the land that also deliver nutrients into the photic zone, generating blooms. These blooms could be an integral part of the local marine ecosystems of our Pacific neighbours supporting higher food chains,” he says.
Phytoplankton blooms may have the capacity to support open ocean ecosystems that are nutrient-limited, but the researchers claim they don’t know enough about them to make that judgment.
“We hope to investigate this further, in particular the influence on fisheries for Pacific islands,” Dr. Horvat says.
*The researchers note recent cyclones in the South Pacific were unique to Oma.
Cyclone Gabrielle’s circular motion prevented it from producing a bloom, but thanks to interactions with coral reefs in the coral sea, it did cause one there.
Due to their circular path, Cyclones Judy and Kevin once more did not hover in the same area long enough to induce a phytoplankton bloom, but because both passed over bigger islands, there was indications of a bloom caused by run-off of nutrients from the land.
