Mysteries of the deep

By Andrew Stevenson

Is Sydney Harbour a friend or foe in the fight against climate change? Oceanographer Edwina Tanner shares her passion about finding the answer.
Image of Edwina Tanner

Edwina Tanner at her Chowder Bay ‘wet lab’. Credit: Ted Sealey

Sydney Harbour is the birthplace of modern Australia and the postcard that defines a global city. For Edwina Tanner, it is also the most beautiful petri dish in the world, a living laboratory where she and her colleagues can investigate whether estuaries around the world are a help or hindrance to global warming.

The climate change story is the compelling backdrop to the work being undertaken by Edwina Tanner, a physical oceanographer in the School of Geosciences at the University of Sydney, and others such as her research partner Professor Ian Jones, from their base at the Sydney Institute of Marine Science at Chowder Bay, near Mosman.

From the window, through the trees, the harbour’s sparkle is not a distraction. For Tanner it is the main event. She knows the ocean beyond the heads is buffering the world from far more extreme climate change by absorbing enormous quantities of carbon dioxide. But does the harbour help or hinder that process? Is it source or carbon sink?

Before she can answer this Tanner needs to explore the water beneath and measure its composition. What’s the temperature? Salinity and acidity? What are the nutrient levels? And, crucially, what are the levels of oxygen and carbon dioxide and how do they change as you travel from Parramatta weir to the heads; how do they change by day and night, over the year and with inflows of rain and run-off?

“What we are looking at is how this little chemistry lab, which is the harbour, processes the carbon released by the breakdown of soil, trees and plant life and how it then gets into the ocean,” Tanner explains. “Is it doing a job of actually processing the carbon and providing a sink before it gets into the ocean or is it producing more carbon for the ocean to deal with?”

“You have fresh – and saltwater meeting, high organic content water and low content water and the question is how do all the chemical and biological processes play out along the length of the estuary and where is this happening? We want to know how much carbon dioxide is being absorbed or released into the atmosphere and how much organic carbon is being flushed out into the ocean.”

"I was drawn to the oceans because ... I thought ‘this is where you can probably do some good; where you can really make a difference’."

Ultimately, the ocean will prove the main game but the harbour is a great place to start, says Tanner. “The ocean is bigger but we need to know what is happening here, which is where all the terrestrial carbon is entering the water system, in order to quantify what is happening in the overall carbon budget. It might only be a small piece but we still need to know that piece.

“And, while it may only be a small percentage of the world’s water, there’s a whole bunch of estuaries around and they all add up. If you look at every estuary around the world you actually have a sizeable contribution. If there is something we can do in estuaries to reduce the impact of carbon and other pollutants it is worth looking at. Also, you can get to understand the processes quite well by looking at the estuary: it’s a fast-action environment.”

Tanner’s passion for her research has been intertwined with the rise of our awareness of global warming. “I have always been interested in the climate story and I guess as I was going through my degree the climate story became louder. When I first started there were about five papers published a year; now there are thousands of papers published on climate science each year. When I started in physical oceanography it wasn’t a big issue,” she says.

Before that, however, Tanner, who enrolled at university as a “bit of a greenie”, flirted with agriculture and forestry, although neither inspired her passion. “But I was drawn to the oceans because I could see them having this major impact on the climate and I thought ‘this is where you can probably do some good; where you can really make a difference’.’’

Seeding the ocean

Image of Edwina Tanner

Like many – but much later than most –Tanner had been deeply affected by Rachel Carson’s pioneering works on environmental damage, The Sea Around Us and Silent Spring. “They were really quite a revelation and I decided I have to do something to help the planet heal,” she recalls.

The oceans are already doing their bit, explains Tanner. “The ocean is central to the climate story because it absorbs between one-third and one-half of the anthropogenic (or human-induced) carbon dioxide. If it wasn’t doing that job we would be in dire straits already.”

Specifically, the heroes of the story are phytoplankton, microscopic organisms that use photosynthesis to consume carbon dioxide and release oxygen that are doing the work. The process is most active in nutrient-rich areas of the ocean. Often they are the result of upwellings, when water rises up from the depths of the ocean, bringing nutrients to the surface which interact with sunlight, allowing phytoplankton to grow.

The notion that this process could be enhanced by seeding the ocean – which is characterised by large desert-like areas low in nutrients – to cause phytoplankton to bloom and process carbon dioxide is a compelling one for Tanner and Jones, and could be a fertile area for future research.

The process is at work in the harbour, too, and as Tanner cruises the waters collecting samples she can actually see it happening. “It’s really quite amazing,” she says. “You can actually see the response of the CO2 in wet conditions with patches of phytoplankton bringing down the level of CO2 using the nutrients that have been washed into the harbour.

“That response is actually due to the phytoplankton blooming, producing oxygen in response to a rain event which brings in more nutrients. That’s why it is so interesting to study. You can see these large-scale processes happening here in the harbour in this short period of time.”

"We are looking at how the harbour processes the carbon released by the breakdown of soil, trees and plant life, and how it gets into the ocean."

The harbour is a discrete entity but it includes diverse conditions. At the weir at Parramatta the water is comparatively stagnant, low in oxygen and with little processing of carbon dioxide happening. The fresh water carries carbon in the form of particulate matter such as leaf litter and soil which is eventually processed, some of it in the form of CO2.

The groundwater, seeping in underneath, brings more carbon into the system. Then, as you go down the river and you meet with the fresh ocean water, more oxygen enters the system, the phytoplankton come and do their bit and the fish come and feed. The cycle of life, this interplay of ocean and river, so richly supportive of life, goes on and on.

Tanner feels she is nibbling away at one part of a much bigger story. “Everyone has got to do what they can,” she says. “I liken it to my sister who did a degree in health science and she would always nag us about a healthy lifestyle. We would say ‘go away’ but she came back with really good examples and we would start to change, one thing at a time. I guess climate science is like that. You’ve got to bite off a little chunk, do what you can and maybe move onto the next chunk. That’s how you build a holistic approach: everyone is doing their bit and hopefully working together to contribute to understanding the bigger picture.”

And solve it we will, believes Tanner. “My personality is to be the eternal optimist. That’s just who I am. I think humans are smart. We can get over it,” she says. “I am really quite encouraged by this generation of children. They have grown up with climate change, knowing it is a big problem. They know about environmental pollution and I’m hoping they have a much better attitude towards these global problems than our generation had. We’re only just getting it now.”

As a scientist who understands the data, however, Tanner accepts the scale of the challenge. “We’ve got a big job ahead,” she says. “As scientists we have answers but there’s so much politics we have to get past to get the answers heard. Humans will persist. And life will persist: look at all the things life has gone through, with massive climate change events in the past. Humans are quite resilient. But I think we will need to be.

A model of harbour nutrition

partnering with Edwina Tanner to build a computer model of the magnificent estuary that sits at the heart of Sydney. The aim of the Sydney Harbour Environment Data (SHED) model is to collect and collate an enormous array of data about what is happening within the waters of the harbour and crunch it into forms that can be used for advanced scientific research, as well as providing a useful tool for the interested layperson.

Jones says the model will be available free to any researcher, providing vital infrastructure for a suite of harbour studies that are either planned or underway. It builds on earlier work by Gavin Birch and Serena Lee from the University of Sydney, which modelled the movement of contaminants in the harbour. But its reach is much broader, both geographically and in the range of variables being collected – including temperature, nutrients, salinity, carbon dioxide and oxygen levels.

Image of salinity map

The image shows how the Sydney Harbour Observatory models the millions of litres of freshwater that flow into the harbour. The freshwater forms a discrete layer above the saltwater and the plume shoots across the top of the harbour from the river tributaries towards the heads. As the rain stops and the plume slows, it starts to break down and mix with the saline water.

Tanner and Jones want to use the data, some of which is collected from set points and some taken from vessels as they traverse the harbour, to answer questions about nutrient flows and their relationship to carbon dioxide. The studies will also allow researchers to begin assessing how climate change will alter the functioning of the harbour: how recreation, commercial activities, primary productivity and ecology will be affected by changes in the flow of nutrients and other contaminants.

The research will produce an accessible display of variables such as currents, storm surge, water temperature and pollution levels – available almost as it happens. Jones hopes the model will eventually be taken over by others. “When it is operationally sound we would like to convince an organisation like the Bureau of Meteorology to take it on.”

In fact, the bureau is already involved in the project, along with the Sydney Institute of Marine Science, the CSIRO, Professor Bradley Eyre, a biogeochemist from Southern Cross University, and researchers from Macquarie University.

Jones says SHED promises to be another important analytical tool. “What is unusual is that we are getting real-time inputs of rain rates, wind speed and tide heights and we are able to give, continuously, a picture of the properties throughout the whole harbour,” he explains. “But you will be able to come back in a year’s time, ask ‘what was happening in June that affected my mussels?’, look back at currents, temperatures and salinity and come up with the answer.’’