Statistics key to measuring the health of New Zealand's kelp forests

06 June 2018
Dr Nick Shears, Institute of Marine Science and Department of Statistics at work in a marine reserve (credit Paul Caiger)
Dr Nick Shears at work in a marine reserve. Photo by Paul Caiger.

Coastal forests of kelp are important habitats for a wide variety of fish, crustaceans and other sea life – but they are at risk from human activities on land that wash sediment into the sea.

"This is a major threat", says Dr Nick Shears, a marine scientist who holds a joint position between the Institute of Marine Science and the Department of Statistics.

“Sediment in the water affects the light that kelp forests need to photosynthesise and stay healthy – and a number of species rely on these being healthy.”

He says that landowners and councils need to take more action to prevent run-off.

“There have been big improvements on how we do things on land, particularly with sedimentation from new coastal developments, but there is still a lot of room for improvement, particularly around farming and forestry.

"Better management of land-based activities now will mean coastal ecosystems are more resilient as climate change brings elements we can’t control – such as sea-level rise, more high-intensity storms, greater coastal erosion, and more intense rainfall."

Nick’s call comes as he wraps up a five-year research programme that explored the issue of sedimentation and climate change on kelp forests in Auckland’s coastal waters, funded by a prestigious, six-figure Rutherford Discovery Fellowship.

The grant allowed him to develop a productive research group spanning both the Institute of Marine Science and Department of Statistics, and also fund two PhD students, Blake Seers and Caitlin Blain. Having the Rutherford, says Nick, gave him “an awesome opportunity” to boost research capacity in marine science.

This high-powered team worked at nine sites throughout the Hauraki Gulf, from the clear waters of the Mokohinau Islands to the siltier waters surrounding Rangitoto and Motutapu. They made a number of major findings and developments, among them the following:

  • Despite stricter rules on coastal land management, coastal turbidity (“murkiness” from suspended sediment and phytoplankton) over the past 20 years has improved little in the Auckland region, and is likely to get worse with climate change.
  • Where there is high turbidity, kelp grows only in shallow water where it can get enough light.
  • Surveys and measurements across the Hauraki Gulf found that the more sediment in the water, the lower the productivity and reduced resilience of kelp.
  • Experimentally removing 80m2 patches of kelp forests in different areas showed that they recovered more slowly in high turbidity areas and other, less productive competitors such as fucoid seaweeds took advantage of this to establish themselves.
  • Long-term ocean warming trends were variable around New Zealand’s coast – but greatest in southern New Zealand, not the north as many might think. (Nick says this finding challenges widespread expectations of tropicalisation in temperate regions.)  
  • The invasive kelp Undaria pinnatifida has established itself in northern New Zealand, despite the fact that northern waters were historically considered too warm for it to thrive. (The team has identified what other parts of the world risk invasion.)

The team also developed methods to carry out tasks such as measuring rates of respiration and photosynthesis of kelp, estimating kelp productivity and mapping forest distribution using satellite images. Altogether the team has produced 16 papers so far, with many more in the pipeline.   

Nick's PhD research, completed in 2003, was the first to demonstrate how the effects of fishing on kelp forest ecosystems could be reversed in marine reserves, and he continues to explore the effects of fishing on our coast.

"Fishing is the most historic, widespread and direct impact humans have had on marine ecosytems, but the ecological effects are largely ignored and under-appreciated. There is a general tendency to sweep these impacts under the carpet and attribute observed changes to other possible changes or stressors in the marine environment,” he says.

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