All at sea - the surprising reach of river waters

Two self-driving underwater robots are making surprising discoveries about where river water ends up at sea, far from land.

Autonomous ocean gliders that measure New Zealand's coastal seas have detected eddies of river water more than one hundred kilometres away from their source.

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Rivers discharging plumes of sediment-rich freshwater into the ocean are a common sight around the New Zealand coast.

But it has taken two ocean robots, named Manaia and Betty, to reveal just how far offshore eddies of this freshwater can travel.

Joe O'Callaghan is a coastal oceanographer NIWA and the Sustainable Seas National Science Challenge. She says that in the past few years the underwater gliders have been carrying out repeated surveys of greater Cook Strait.

The gliders measure salinity, turbidity, temperature and chlorophyll, and Joe says they detected a distinct salinity signature much further offshore from Golden and Tasman bays than previously recorded.

Four rivers feed into these bays, with the Aorere and the Motueka being the largest.

Joe says that traditionally it was thought that water from these rivers would be moved by tides and wind, causing them to meander and shed different sized 'blobs' of rotating water or eddies that carry nutrients and sediment offshore. The idea was that it "would mix locally and just not have a very substantial signal outside of the bays."

"Instead we've got some quite significant far-field signals," says Joe.

The freshwater eddies are about four to ten kilometres across, and last from a few hours to a few days, depending on the strength of the D'Urville current which varies in intensity and direction.

University of Auckland PhD student Khushboo Jhugroo says these eddies "connect the land and the ocean."

"What's new with this study is that it's taking this connection much further out to sea than we previously know," says Khushboo.

The freshwater eddies are buoyant and float on the surface where sunlight can reach them, which means the often nutrient-rich freshwater may contribute to phytoplankton blooms and general ocean productivity.

"The glider observations are more effective at tracking rivers than satellites because they can map the ocean in all weather conditions and at much better resolution", says Joe.

"Betty and Manaia detect not only temperature, like satellites, but also salinity which was key to seeing these eddies."…

Go to this episode on rnz.co.nz for more details