The party's environmental spokesperson, MLA Philip McGuigan, made the comments after an all-party meeting on the ecological crisis taking place at the lough.
Lough Neagh provides more than 40 per cent of the north's drinking water and contributes to Northern Ireland's environment, biodiversity and the local economies that surround it.
In a statement Mr McGuigan said: "There was a good discussion about the future of Lough Neagh. All parties agreed that an urgent action plan needs to be developed immediately to address the serious issue of toxic algae in the Lough and the surrounding area, and its impact on water quality and the local environment.
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"With public health, as well as both economic and environmental sustainability now major concerns on the Lough, we cannot afford to be unclear when it comes to responsibility and accountability.
"The need for a cross departmental response and for NI Water’s remit to be extended to cover Lough Neagh was also discussed. We need an Assembly up and running to begin this work.
"Sinn Féin has long advocated for Lough Neagh to be brought into public ownership so that a clear management structure can be developed and long-term plans made for dealing with the numerous issues that face the lough, to keep it safe and sustainable for future generations."
