Editing Murray cod (section)

====Blackwater events====
Blackwater events are emerging as a very serious threat to wild Murray cod stocks in lowland river reaches. Blackwater events occur when floodplains and ephemeral channels accumulate large quantities of [[leaf litter]] over a number of years and are then finally inundated in a flood event. The leaf litter releases large quantities of dissolved organic carbon, turning the water a characteristic black colour and inducing a temporary explosion in bacterial numbers and activity, which in turn consume dissolved oxygen, reducing them to levels harmful or fatal to fish. (Fish essentially asphyxiate.) Water temperature is a critical regulator of blackwater events as warmer water temperatures increase bacterial activity and markedly reduce the intrinsic oxygen carrying capacity of water; events that may be tolerable for fish in winter or early spring may be catastrophic in late spring or summer due to the increase in water temperature.<ref name="Whitworth et al. 2012">
{{Cite journal
  | last = Whitworth | first = K. L. |author2=Baldwin, D. S. |author3=Kerr, J. L.
  | title = Drought, floods and water quality: Drivers of a severe hypoxic blackwater event in a major river system (the southern Murray–Darling Basin, Australia)
  | journal = Journal of Hydrology
  | volume = 450–451
  | pages = 190–198
  | year = 2012
  | doi = 10.1016/j.jhydrol.2012.04.057| bibcode = 2012JHyd..450..190W }}
</ref>

Blackwater events are often described as "natural" events—while there are some historical records of relatively severe events in smaller, more ephemeral systems (e.g. lower Lachlan, upper Darling), there is no record of severe events in the Murray River and its largest southern tributaries before water extraction and river regulation. In the Murray and large southern tributaries, very severe large-scale blackwater events are a relatively new but recurring phenomenon and appear to be an effect of river regulation curtailing the winter/spring flood events that formerly swept [[leaf litter]] away annually, exacerbated by long-term declines in rainfall and recurring prolonged drought events.<ref name="Hladyz et al. 2011">
{{Cite journal
  | last = Hladyz | first = S. |author2=Watkins, S. C. |author3=Whitworth, K. L. |author4=Baldwin, D. S.
  | title = Flows and hypoxic blackwater events in managed ephemeral river channels
  | journal = Journal of Hydrology
  | volume = 401
  | pages = 117–125
  | year = 2011
  | doi = 10.1016/j.jhydrol.2011.02.014
  | issue = 1–2| bibcode = 2011JHyd..401..117H }}
</ref>

Flood events in 2010 and 2012 following the prolonged Millennium Drought (1997–2009) induced very severe blackwater events; while formal studies of these events were limited due to the relatively rapid response times required and logistical difficulties,<ref name="King et al. 2012">
{{Cite journal
  | last = King | first = A. J. |author2=Tonkin, A. |author3=Lieschke, J.
  | title = Short-term effects of a prolonged blackwater event on aquatic fauna in the Murray River, Australia: considerations for future events
  | journal = Marine and Freshwater Research
  | volume = 63
  | pages = 576–586
  | year = 2012
  | doi = 10.1071/MF11275
  | issue = 7 }}
</ref>
angler photographs and observations of extraordinary numbers of dead Murray cod during these events and plunging catch rates after these events show they induced extremely heavy Murray cod mortalities along extensive tracts of the Murray River.