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Published 13 September 2018

New Zealand's ocean productivity projected to decline

Oceanic phytoplankton

In an article in the New Zealand Journal of Marine and Freshwater Research, a NIWA research team led by Professor Cliff Law suggests we will soon see the effects of global warming on fish food supply.

Reducing our carbon emissions is becoming more urgent as the impact of climate change on the ecosystem is becoming more pronounced.

In the article ‘Climate change projections for the surface ocean around New Zealand’ a NIWA research team used two Earth System Models and four emissions scenarios to create a forecast of what changes might occur in our oceans as climate change progresses.

Changes are projected in phytoplankton concentrations—the foundation for the aquatic food web. Microscopic phytoplankton are photosynthesisers. They use chlorophyll to capture sunlight which they turn into chemical energy.

The concentration of chlorophyll in oceanic surface waters is used an indicator of phytoplankton biomass, and subsequently provides a representation of primary productivity. The model projections suggest a decrease in both chlorophyll and productivity of 2-7.5% around New Zealand by the end of the century, in response to a decline in nutrients. 

Phytoplankton need nutrients like nitrate, phosphate, and silicate, which are primarily supplied from waters below the surface layer. However, in a warming ocean the density step between the surface and deep ocean will become more intense, and so reduce the supply of nutrients to the surface. This will limit phytoplankton growth and have a flow-on effect for open ocean food webs.

Large changes are forecast in areas like the eastern Chatham Rise, a significant region for New Zealand’s major fisheries and biodiversity. It is currently the most productive fishery region in Aotearoa's waters, but the models project a decline in phytoplankton and associated particle flux reaching 12.6% by the end of the century in this region.

Places like the central Tasman Sea are projected to experience the smallest amount of changes, which mean these areas could be a potential climate refuge for organisms fleeing rising temperatures.

The authors state that the regional variation of the oceans in response to climate change should be carefully considered when making policy decisions and informing the public.

The article ‘Climate change projections for the surface ocean around New Zealand’ authored by Cliff Law, Graham Rickard, Sara Mikaloff-Fletcher, Matt Pinkerton, Erik Behrens, Steve Chiswell & Kim Currie is available for access at Taylor & Francis Online in the latest issue of the New Zealand Journal of Marine and Freshwater Research Vol 52.3