Otago Harbour heating up, study shows

The number of days Otago Harbour experiences marine heatwave conditions each year has more than doubled over the past 50 years. Photo / DunedinNZ

The number of days Otago Harbour experiences marine heatwave conditions each year has more than doubled over the past 50 years.

University of Otago research has found the number of days marine heatwave conditions were recorded had grown from 19 to 40 days per year between 1967 and 2020.

The research represents one of the few in-depth evaluations globally on marine heatwaves in near-shore ecosystems, such as rocky reefs, harbours and estuarine environments, and highlights the significant role of local atmospheric conditions in modulating the occurrence of extreme temperature events in these ocean ecosystems.

Lead researcher and University of Otago marine science PhD student Felix Cook said this modulating role of local atmospheric conditions could cause temperature dynamics in these ecosystems to become decoupled from those in the nearby open-ocean environments.

“We found that marine heatwaves in near-shore coastal waters typically occur over time scales consistent with large-scale weather systems of 9 to 13 days and that their onset is associated with a combination of factors that include blocking high-pressure systems, low wind speeds and reduced heat loss to the atmosphere.”

The research also found a complex pattern of long-term trends in marine heatwave properties at two long-term coastal monitoring stations - one at Portobello Marine Laboratory in Otago Harbour, the other at Leigh Marine Laboratory, north of Auckland.

He said while marine heatwave conditions had doubled in Otago Harbour between 1953 and 2020, there was no evidence of an increase in exposure to marine heatwave conditions in coastal waters at Leigh over the same time period.

The temperature of Otago Harbour from October 19-November 4. Photo / ODT

“This pattern is consistent with broad-scale warming trends previously documented at these two locations, with differences related to changes in large-scale ocean circulation patterns around New Zealand.”

Cook said the research was part of the Moana Project, which aimed to improve understanding of coastal ocean circulation, larval connectivity and marine heatwaves to support sustainable growth of New Zealand’s seafood sector.

Marine heatwaves could have devastating effects on ecosystems, industry and communities, such as the summer 2021-22 event in the Marlborough Sounds that killed about 40 per cent of salmon in a series of aquaculture facilities, contributing to a $73 million loss for the company involved and the loss of 139 jobs in the area.

“Improving our understanding of the drivers of coastal marine heatwaves is supporting the development of forecasting tools in the Moana Project, which will provide our important ocean industries and coastal communities with more certainty and warning, to prepare for the impacts of future marine heatwaves.”

University marine science oceanographer Dr Robert Smith said the long-term coastal records from Portobello were now available online, providing real-time and historical data to the world.

“Improving access to ocean data can help scientists, fishers, resource managers and the public to monitor and prepare for the impacts of these extreme temperature events.

“As climate change affects our oceans, marine heatwaves are expected to become more frequent, intense and longer-lasting.

“Our ocean life around New Zealand has evolved to thrive in cooler seas, so the increasing incidence of marine heatwaves could have devastating effects on these sensitive near-shore ecosystems.”