Hypoxic blackwater events can have devastating effects on riverine ecosystems. Although arguably exacerbated by river regulation, hypoxic events can also be mitigated by regulation infrastructure. We report on a collaborative effort to reduce the severity of hypoxic blackwater in the Edward-Wakool river system, a heavily regulated anabranch of the Murray River in south-eastern Australia. In 2010, significant rainfall in the catchment caused widespread flooding of large areas of redgum forest for the first time in 10 years producing high dissolved organic carbon loads which combined with high water temperatures resulted in an extensive hypoxic blackwater event and subsequent fish kill.
In response, Murray Catchment Management Authority (now Murray Local Land Services) facilitated meetings of key stakeholders to coordinate releases of oxygenated water from irrigation channels into hypoxic affected areas to create refuges for fish. The Murray Dissolved Oxygen Group was formed and with the assistance of the community and river managers, a Hypoxic Blackwater Protocol was developed using trigger points in dissolved oxygen levels to inform agreed management actions. The community were also engaged to assist with water quality monitoring and restocking native fish into affected areas. Fish monitoring revealed system recovery particularly, in the vicinity of the refuges validating the effectiveness of the protocol and collaboration. The protocol was tested again in 2016-17, with the largest flood in 40 years inundating floodplains not wet since 2010 and in some places since the mid-1990s. A hypoxic blackwater event ensued but the effects were lessened in critical areas through the implementation of the protocol preserving much of the native fish communities in the system.
Thus, despite river regulation being linked to increasing the occurrence, duration and severity of hypoxic blackwater events in the Murray-Darling Basin, the associated infrastructure can also be an integral part of mitigation measures.