The trust wants to buy up thousands of acres of farmland to create a new wetland north of Cambridge.
But a scoping document from the Environment Agency says water levels in a number of lodes may need to be lowered.
This would transform the lodes into little more than ditches, destroying the unique character of historically-important fisheries.
Almost 600 people have now signed a petition to save them. They include pike anglers, walkers, boaters, councillors and an MP.
Campaigner Geoofrey Woolard said today: "It is also clear that the E-Petition is being noticed by others, as I have received an invitation to meet representatives of The National Trust on August 7 to talk through the issues.
"I have accepted the invitation and shall attend at Wicken at 5 p.m. on that day."
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