When a swollen river breaches its banks and inundates a floodplain, it sweeps a vast amount of leaf litter and other organic material back into the river. Bacteria breaks down this organic matter and releases significant amounts of dissolved carbon into the water. The carbon removes dissolved oxygen in the water, making it hard for aquatic creatures to breathe.
The CEWH is working with landholders in the Murray irrigation scheme on the NSW-Victorian border, who manage a network of canals, to ensure the fish can dodge the deadly blackwater.
Johnson said: “We’re trying to create freshwater refuges for our native fish to move into as these hypoxic black water conditions move through the system.”
Flora and fauna have evolved to thrive in Australia’s harsh cycle of drought and flood, but they are not coping with the impacts of climate change and development.
Over-allocation to irrigators shrunk natural flows and deprived wetlands of natural flows and average rainfall across south-eastern Australia has declined about 10 per cent over the past 30 years.
Waterbird numbers have been dwindling in the Murray-Darling for the past 35 years. Researchers estimate a 70 per cent reduction to populations. Native fish stocks are also far below historical levels. The iconic Murray cod is among dozens of fish and frog species native to the region that feature on the national register of threatened wildlife.
But when rain and floodwaters come, it’s a paradise for birds including straw-necked, glossy and white ibis, egrets, herons, spoonbills, cormorants, pelicans, magpie geese and brolgas, and they’re breeding in their hundreds of thousands.
At Narran Lakes in outback NSW, between Bourke and Walgett, is one of a handful of sites across the basin where pelicans are nesting there in the thousands.
The Gwydir Wetlands, west of Moree, is another crucial bird breeding site. Prior to development, bird colonies there bred in seven out of every 10 years, but reduced flow has caused that rate to fall to three in 10 years.
However, last year 45,000 waterbird nests were counted, including 30,000 straw-necked ibis alone. This year, another large-scale breeding event is under way and the CEWH is boosting flows, so as the floodwaters recede there is water still under the nests to help ensure maximum breeding success.
Johnson said: “People might think with environmental water you just turn the tap on and it’s done.
“We have to put a lot of effort into planning, gathering the science, thinking through what the prevailing conditions are, what the forecast conditions, are working with our partner agencies (in state governments).”
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