THE green turtles of Sydney Harbour tell a tale of two cities.
That these endangered reptiles regularly visit to graze on seagrass meadows in the middle of a big city, and some have even made the harbour their home, attests to the improving health of Sydney's main waterway.
This week, the Herald delves into the state of our harbour, and its changing life both above and below the waterline.
The turtles - with the mussels that crowd on to channel markers and the seahorses that wrap themselves around shark nets - are a good sign the estuary is becoming cleaner.
Yet for a 13-year-old sea scout, Julia Spragg, her first encounter with a green turtle was far from a good experience.
With one flipper entangled in fishing line that had cut to the bone, another flipper severed, and deep tackle wounds to its neck, the animal had little chance of survival.
When fellow members of the 1st Sailors Bay Sea Scouts found its mangled body while kayaking in the beautiful reaches of Middle Harbour, they were sad to see how much it had suffered.
''It was not nice,'' said Julia.
This young guardian of the harbour wishes more people, particularly those fishing, could see the devastating results of leaving bottles, bait bags and tackle around.
''If you see rubbish, just pick it up. It's not a big job,'' she said.
Geoff Ross, a wildlife management officer with NSW National Parks, said the entangled turtle might have been a long-time harbour resident, and its recent death was a concern.
''The loss of just one breeding-sized individual can have an impact on the species,'' he said.
Although remedies such as waste-retention traps on stormwater outlets had significantly decreased the amount of debris entering the harbour, individuals could do more, he said.
The pollution we can see in the harbour is just one of the many man-made threats to the estuary. These range from an industrial legacy of dumping toxic metals in its sediments to the future effects of global warming.
An increase in sightings of sea turtles, which prefer warmer climes, could be a sign conservation strategies were having an effect but it could also be linked to climate change, Mr Ross said.
Archaeological material from middens at Balmoral Beach and Cammeray suggests Aborigines might have eaten turtles, although Val Attenbrow, of the Australian Museum, said the evidence was not conclusive, with only some bone fragments found. The East Australian Current, a conveyor belt from the tropics on which the turtles ride, is strengthening, with warmer, saltier water found 350 kilometres further south than 60 years ago.
An influx of tropical fish has made the harbour even more of a wonderland for underwater photographers, bringing rare species such as a pair of ornate ghost pipefish that were recently spotted in Chowder Bay.
While more than 570 kinds of fish have been identified - many more than the 350 types found in the whole of Europe - for fishing guide, Craig McGill, it is not the fish, or the the visiting dolphins and whales, or even the fairy penguins, that epitomise the big improvements he has seen in water quality in the past 20 years.
It is the filter feeders on the marinas, pylons and piers. Middle Harbour has an abundance of oysters, he said.
''And the channel markers in the main harbour have had a growth of mussels we've never seen before.''
A ban on anti-fouling paint containing tributyltin was a large contributor. And improvements in the management of sewage and stormwater have reduced other microscopic pollutants.
Twenty-four of 28 swimming sites in the harbour complied 100 per cent of the time with bacterial guidelines between October 2008 and April 2009.
But February this year was a very different story, as the sea scouts of Sailors Bay well know. When any of them fell into the water, they were quickly ushered out to have a shower.
The heavy rains washed debris and road run-off into the harbour, said scout leader, Adrian Spragg . ''It smelt and it was oily.''
In February, only three swimming sites - Redleaf Pool, Nielsen Park and Watsons Bay - passed safety tests, according to Harbourwatch.
This run-off is why the sediments in Sydney Harbour remain some of the most contaminated in the world.
Stuart Taylor, an expert on the harbour bed, said there are almost 21 million tonnes of contaminated sediments, containing thousands of tonnes of copper, lead and zinc, as well as pesticides and other chemicals.
''This is where the detritus of civilisation ends up. Everyone living in the catchment contributes,'' Dr Taylor said.
When there is low rainfall, the contaminants settle quickly and when the sediments are disturbed, they tend to fall back in much the same area, rather than spread.
About 1.5 centimetres of sediments are deposited each year, but rather than providing a fresh top layer, worms and shrimps burrow into the mud and mix it up, he said.
More than 90 per cent of the harbour contains contaminants in surface sediments that exceed guidelines based on US studies. This could be having adverse ecological effects, said Dr Taylor of Geochemical Assessments, who carried out his studies with Associate Professor Gavin Birch at the University of Sydney's School of Geosciences.
Some hot spots, like Homebush Bay, have undergone remediation, but others, where there is no industrial culprit to pay for a risk assessment and clean-up, and where any evidence of effects on human health is lacking, remain untouched.
One of the first studies on the effects of the sediments has been done by Nathan Knott and Emma Johnston of the University of NSW and Sydney Institute of Marine Science.
Surprisingly, being repeatedly doused in heavily contaminated sediments from Rozelle Bay for 10 days had no effect on a range of creatures, including sea squirts and sponges.
But ''further research is required to assess the potential impacts of long-term exposure,'' the scientists said.