Rivers run farther than we think
New research shows that New Zealand river currents run up to 100 kilometres out to sea. This was unexpected because the four rivers studied are short and don’t carry large amounts of water.
But their effects could be detected many kilometres out to sea.
‘‘As rivers enter the sea at the coast, they meander and shed different sized ‘blobs’ of rotating water called eddies,’’ wrote University of Auckland PhD student Khushboo Jhugroo and Niwa’s Dr Joanne O’Callaghan.
These ‘‘unique eddy whirls and swirls’’ travelled more than 100km from Tasman and Golden Bay into Cook Strait. The rivers the water came from are the Aorere and Ta¯kaka rivers which flow into Golden Bay, and Motueka and Waimea rivers which flow into Tasman Bay.
‘‘This research is providing critical information for understanding how the health of Aotearoa New Zealand’s land and rivers affects the ocean,’’ says Sustainable Seas Challenge director Dr Julie Hall.
The research also shows rivers can be a primary source of nutrients that can kick-start new ocean food chains.
This is because the eddies stay close to the surface, rather than being mixed down into the water column. Being near the surface can allow photosynthesis (primary production) to occur, if the river water is loaded with nutrients from the land, said the Sustainable Seas Challenge.
To collect the data, the researchers used small submarine drones, which they call gliders. The torpedo-shaped vehicles, named Betty and Manaia, were launched from boats and operated mostly underwater.
Betty and Manaia steered themselves to GPS waypoints – or pretty close – collecting data on temperature, salinity, oxygen and chlorophyll as they went.
‘‘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 O’Callaghan.
Indeed, three-quarters of these river eddies are invisible to satellites because they don’t have a detectable difference in surface temperature.
Similar observations about river eddies have been made overseas, Jhugroo said in an interview. The article cited the Bay of Bengal, northern Gulf of Mexico (under the influence of Mississippi-Atchalafaya river system), the Chukchi Sea (north of Bering Strait between Alaska and Siberia) and north of the Amazon River as examples.
Now that New Zealand scientists are aware that river eddies are travelling far out to sea, they can start planning the next phase of research. That might include learning more about the change in primary production the eddies produce.
And if the eddies are carrying nutrients, they may also be carrying contaminants.
The Sustainable Seas Challenge is a funded 10-year research programme that asks ‘‘how can we best develop our marine economy, while protecting the taonga of our marine environment?’’ It is multidisciplinary, taking in biophysical science, economics, ma¯tauranga Ma¯ori, social science and policy, and hosted by Niwa.
It’s one of 11 Ministry of Business, Innovation and Employment-funded challenges aimed at taking amore strategic approach to science investment. Funding for the challenges was allocated for 10 years in two, fiveyear periods.
The research was published last month in the journal
not behind a paywall. and