A white-faced storm petrel skims above the surface of Hauraki Gulf near Burgess Island. Photo / File
A white-faced storm petrel skims above the surface of Hauraki Gulf near Burgess Island. Photo / File
In the last of a five-part series looking at Auckland University's research in the region's blue backyard, science reporter Jamie Morton discusses how far our ocean ecosystems can be pushed with the head of the university's Institute of Marine Science, Professor Simon Thrush.
Your research focuses on what are called ecosystem "tipping points". These sound dramatic. What are they?
Tipping points represent a rapid transformation that occurs when an ecosystem loses its resilience, or its capacity to cope with change.
This could mean a change in some valuable resource - fish or biodiversity - or a change in the way the ecosystem supports our wider values for our coasts and oceans, or what we call "ecosystem services".
The recent earthquakes in the Kaikoura region lifted the shoreline up, stranding plants and animals above the high-tide mark.
This single powerful force changed the nature of the coastline, but tipping points occur through much more subtle changes in the interactions between organisms and their physical environment.
Tipping points occur because the connections between components of the ecosystem change and changes to these connections can occur without a major driving force like an earthquake.
Often, we focus on the negative changes when we lose a valuable resource, but we can improve and restore our marine ecosystems - this often relies on the science of tipping points.
One of the most well-used marine examples of a tipping point is the collapse of the cod in north-eastern Canada.
Crayfish in the marine reserve at Leigh, north of Auckland. Photo / File
Fisheries managers had no idea the collapse was coming and when it happened it not only changed the way the coastal ocean worked but closed the fishery with major impacts on local communities.
One critical feature of tipping points is that, once the system has changed, internal mechanisms and feedbacks in the new system work to slow recovery.
This certainly happened with the cod fishery, which did not bounce back once the fishing pressure was removed.
Many other examples have been reported associated with changes in ocean currents, the quantity of waste we pour into the oceans, land runoff or the removal of organisms to the extent that their abundance is so low they cannot play their ecological roles.
Tipping points, then, are a property of how complex systems work and thus they require a more integrated and holistic view to understand what is causing them.
This makes gathering and interpreting information on change difficult - but it is possible.
Marine scientists have been reporting and debating tipping points since the 1970s, but this has been a major and growing area of marine research in the last 20 years.
New theories and technologies have helped us understand these changes.
But the profound connection between nature and society, as illustrated in the Canadian Cod fishery, add a sense of urgency to change the way we manage our impacts and restoration efforts in the marine environment.
So how might you describe the way a marine ecosystem actually works?
We often take for granted the way things work until they are broken, and marine ecosystems are no different.
Here's an example of why understanding how ecosystems work is important: if you walk around one of our harbours, you'll see signs of cockles, pipi and other shellfish that live in the sand.
These animals feed by pumping seawater over their gills to filter seawater and this increases the supply of organic matter to the seafloor.
The over-fishing of some predator species had put others down the food chain under pressure. Photo / File
As part of this process, the shellfish generate pressure gradients in the sediment that move the water sitting between the grains of sand.
This effect can make the whole seafloor breathe, pumping in oxygen-rich seawater and pumping out water rich in nutrients.
This pumping profoundly influences the nature and speed at which microbes in the sediment break down and transform organic matter.
The dissolved nutrients that result from this process are pushed out of the sediment by pressure gradients where they are intercepted by the microscopic plants that live at the sediment-water interface.
These plants are the most important base of the food chain for many of our harbours and they also help to stick muddy sediments to the seabed.
Some of the microbes in the sediment convert dissolved nutrients into inert nitrogen gas which is lost to the atmosphere.
This process is particularly important as we face increasing concerns about nitrogen pollution to our coasts and estuaries.
It's one of the few ways to effectively remove nitrogen and lower the risk of what's called eutrophication.
In a nutshell, shellfish interact with microbes, sediments and water flow to affect the productivity, water clarity and risk of eutrophication in our harbours.
These animals really are at the heart of the hidden infrastructure that supports many ecosystem services.
They help to process waste, but only up to a limit and that limit is likely affected by multiple factors, among them climate change, ocean acidification, fishing pressure, sedimentation and pollution.
In the Hauraki Gulf, particularly, how have we seen the balance affected?
When snorkelling around the shallow reefs in the gulf, it's quite common to see cleared rock surfaces with high numbers of kina, or sea urchins.
From the research conducted in the marine reserve at Leigh, we now know that these areas, known as urchin barrens, are often generated by fishing of top predators.
BEFORE: An urchin barren in the Hauraki Gulf. Photo / Nick Shears
AFTER: This kelp forest shows a spot in the Hauraki Gulf that has been rejuvenated. Photo / Nick Shears
As the large fish and rock lobsters started to return to the Leigh marine reserve they cropped down the numbers of sea urchins, allowing kelp plants to re-establish and a kelp forest ecosystem to develop.
It's not just the type and number of fish that are important here; size matters, and bigger fish with bigger mouths can effectively feed on bigger urchins.
"Trophic cascades" like this, where predators affect herbivores that affect plants are typical of the kinds of ecological change that occurs when food webs go pear shaped - distorted by fishing down the foodweb.
Changes in the Leigh marine reserve show that the system can come back into balance when the stressor - in this case fishing - is removed.
But for highly mobile species that move outside the protection of the reserve, we need other forms of management.
As a conservation tool we need to consider what is happening around the reserve, especially in regions of very high fishing pressure such as the Hauraki Gulf.
Other factors as well as fishing, impacts from land like sediments and nutrients, and climate change are important too.
In our own ecosystems, is it possible to predict tipping points?
To date we do not have a reliable suite of indicators that forewarn of a tipping point.
Research is underway around the world to develop this and here in New Zealand we are developing ways to improve our ability to assess the risk of a tipping point associated with cumulative and multiple stressors.
We have been able to demonstrate tipping points with long-term monitoring data and we are now investigating data collected by multiple agencies from the marine environment to see just how good it might be in providing clues to tipping points.
All of this is "work in progress" but this does not mean we can not think about and improve marine management right now.
The very nature of a tipping point means that we will not be able to predict the edge of the cliff with certainty - so imagine what environmental management might be like if we acknowledge a risk of surprising outcomes.
This bare seabed in the Mahurangi Harbour leaves little reminder of a horse mussel habitat once found here. Photo / Jenny Hillman
This could mean we are much more focused on integrated and ecosystem-based management, that we take seriously the risk of cumulative stressors and the legacy of historical events, and that we take out some insurance so that if the system does change we have a chance of saving the ecosystem functions that underpin many of our values associated with marine ecosystems.
So how bad would conditions have to get for the environment to reach a tipping point?
The massive amount of sediment that has and continues to enter the Hauraki Gulf, Manukau and Kaipara Harbours as a result of early land clearance, forestry, farming and urban development, have certainly changed our coast.
Loss of shellfish beds associated with disturbance to the seafloor and decreased water clarity is also apparent.
Urchin barrens that result from overfishing of predators are also signs of stress.
Not all of these changes are in the distant past and often you only need to go back one or two generations to hear stories about how our coast was very different from what you might be able to see today.
We know more change to our coastal ecosystems is inevitable and we need to pay more attention to opportunities to improve our marine ecosystems, not just document decline.
This will challenge the "business-as-usual" approach, but critical will be developing techniques to actively restore marine ecosystems.
On land we replant stream margins and restore coastal vegetation - but on the seafloor we are just beginning to trial techniques to restore features like shellfish beds.
How exactly will climate change fit into the picture? Is it something of a wild card?
There is now strong scientific evidence that our planet is warming and our oceans play a major role in how these climate-change effects will manifest.
For our coasts many forms of physical change could occur - sea level rise, ocean acidification, changes in the intensity and frequency of storms, changes in ocean currents, and changes in the thickness of the surface layer of ocean water where phytoplankton will photosynthesise.
To date, these efforts have largely focused on restoring greenshell mussels with the help of the mussel industry.
This enthusiasm for making the place better is wonderful, but scientific research is needed to help provide the tools needed for successful planning and implementation of restoration activities.
We are starting this work with research looking at the survivorship of transplanted mussels, ways to enhance settlement of juvenile mussels into restored beds and, importantly, analysing the ecosystem service benefits provided by mussel beds.
This is essential work if we are to show the value of restoration and also learn where to restore beds of shellfish when we are trying to achieve specific goals like better shoreline protection, removing contaminants, providing juvenile fish habitats and improving biodiversity.
In some places, restoration may be fast, and in others it will be slow, but this is one of the few direct positive things we can do to improve our coastal environment.
