Menindee fish at risk of Hypoxic blackwaters

Hypoxic Blackwater spills into Menindee Lakes as High Floodwaters continue to flow down the Barwon-Darling River (satellite image taken 10/02/2022). PICTURE: SENTINEL HUB

Low oxygen levels could again be a strain for Darling River fish, as high floodwaters move downstream into the Menindee Lakes. Hypoxic blackwater, which occurs naturally through the breakdown of organic matter, can create a deficient level of oxygen in the water during flooding events. In combination with any sudden temperature increase could create an inhospitable environment for fish.

NSW Department of Planning and Environment are monitoring the Hypoxic or low oxygen blackwater to mitigate the potential impacts of the flows.

Director of Water Planning Implementation, Allan Raine, said that these blackwater events are a by-product of the welcome arrival of water to the Barwon-Darling River.

“This is typical of the Barwon-Darling River, which has one of the highest flow variabilities in Australia.”

“There are reports of blackwater events as far back as the 1870s for this ‘boom and bust’ river system”. Mr Raine said that a group of agencies, including WaterNSW, will be working together to monitor and help minimise the impacts of the flood events.

“The two major risks over the coming weeks are the floodwaters that are low in dissolved oxygen levels moving into the Menindee Lakes and the potential for floodwaters on the lower darling floodplain to flow into the lower Darling River.”

According to the Australian Government Initiative, Water Quality Australia the importance of regular flows and reinstating natural wetting and drying cycles is critical in ensuring flows are well oxygenated. The natural breakdown of matter causes blackwater events; agricultural practices, increased and extended dry periods, and the resulting extensive build-up of debris can cause excessive amounts of bacteria developing in the water. This reduces the amount of dissolved oxygen available to fish and crustaceans. More regular flooding allows floodplains to clean out the debris. Extended dry periods mean that when a flood occurs, there is a massive build-up of organic matter that finds its way into the river.

In an initial biopsy of a recent fish kill on the Paramatta River, the Environmental Protection Authority (EPA) cited blackwater as the likely cause of the thousands of fish that washed up along the shoreline.

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Executive Manager of Systems and Operations for WaterNSW, Adrian Langdon said the department will ensure that releases from Menindee Lakes will promptly be reduced to allow the blackwater front to come down the Barwon-Darling mix and become diluted before it is released from Lake Wetherell and continues downstream.

“Releases from the other lakes will continue into the lower Darling via Weir 32 to help dilute the water being released from Lake Wetherell.”

Mr Langdon said, “The smaller lakes will act as a refuge for fish, particularly Lake Tandure, allowing fish to move into these areas to escape low dissolved oxygen water in lake Wetherell”.

Unlike the Menindee fish kills of 2019 in which aquatic life was suffocated from the lack of oxygen in stagnant blue-green algae pools, NSW Department of Planning, Industry and Environment have emphasised the importance of flooding events and that fish are often able to escape even the most badly affected blackwater areas by swimming up or down stream.

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