By Alison Ballance
Underwater ‘crop circles’ that have been made in kelp forest around the Hauraki Gulf are part of a global experiment, and marine biologist Nick Shears hopes they will also show us how resilient – or not - the dominant kelp species around New Zealand might be in the face of climate change.
“The idea is that kelp forest researchers all around the world are removing kelp in different temperature regimes and we’re going to look at how the recovery rates vary with temperature,” says Nick. “As well, here in the Hauraki Gulf we have a big turbidity gradient, so we’re also interested in how temperature changes might interact with light. Obviously when it’s warmer the plant respires more, so it also needs to photosynthesis more to make up for that – and to do that it needs more light.”
One of the expected consequences of climate change will be an increase in severe weather events, such as the recent heavy rainfalls experienced in Dunedin and the Kapiti Coast (check out this week’s Extreme Weather and Climate Change story for more about that). While most people are focused on what the impacts of this will be on land, marine biologists are concerned about the flow-on effects to neighbouring coastal ecosystems. Nick explains:
“One of the expectations of climate change will be reductions in light due to more storms, more runoff and more sediment coming in. But also with more sea level rise you’re going to have more coastal erosion bringing in more sediment and causing lower light.”
Ecklonia radiata is northern New Zealand’s most common kelp species, and it is also found in Australia, in much warmer conditions than it lives in here. It is considered to be a keystone species, which underpins the coastal ecosystem. If the recruitment and survival of the kelp is affected by warming temperature and increasing sedimentation, it could have a significant impact on the healthy functioning of our marine ecosystem.
While long term sea temperature records collected from the Leigh Marine Laboratory do not yet indicate any increase in temperature, Nick says that the ocean near the Portobello Marine Laboratory in Dunedin has begun to warm, as has water around Tasmania. These increases are due to increases in the intensity of the South Pacific sub-tropical gyre.
A major part of Nick’s research is involved in studying marine reserves and monitoring the impact of protection. He carried out PhD research in the Leigh marine reserve where, prior to protection, large numbers of sea urchins, Evechinus chloroticus, had created significant areas of kina barrens. Full protection 40 years ago has led to an increase in the numbers of big predators such as crayfish and snapper, and a subsequent improvement in the kelp forest as the predators eat the kina that were eating the kelp.
Nick has recently been involved in monitoring at the Poor Knights Islands marine reserve. While there are good numbers of snapper at the Poor Knights, there have always been very few crayfish. While the number of kina has declined since protection, there has been a big increase in the numbers of a large black subtropical long-spined urchin called Centrostephanus rodgersii.
The same species has relatively recently established in Tasmanian waters, where it is overgrazing seaweeds and invertebrates on rocky reefs and creating urchin barrens. It has established there due to the same changes in the temperature and strength of East Australia Current that are increasing sea temperatures in southern New Zealand, and it is having a detrimental effect on lobster and abalone fisheries. Nick is not sure why they’re increasing here since there is no evidence of warming sea temperatures, and he speculates that as they are larger than kina snapper alone may not be able to control them in the absence of crayfish
"They’re a more intense grazer than the regular kina and will graze right down to bare coralline. So they’ve had a big impact in Tasmania, and we’ll just need to keep an eye on what’s happening with them here."
Nick holds joint positions in the Institute of Marine Science and the Department of Statistics at the University of Auckland. In 2011 he was awarded a Rutherford Discovery Fellowship from the Royal Society of New Zealand to support his research into kelp forests and rocky reef ecosystems.
You can listen to the audio or download a podcast of Nick's interview here: