The spectacular transformation of a stretch of Venice’s Grand Canal to fluorescent green was due to fluorescein, a non-toxic substance used for testing wastewater networks. Photo: AFP

The spectacular transformation of a stretch of Venice's Grand Canal to fluorescent green was due to fluorescein, a non-toxic substance used for testing wastewater networks, local authorities said on Monday.

Residents noticed a stretch of Venice's Grand Canal turned bright green Sunday, prompting police to investigate amid speculation it could be a stunt by environmentalists.

But analysis showed "the presence of fluorescein in samples taken," said the Regional Agency for Environmental Prevention and Protection of Veneto (Arpav).

The results "have not shown the presence of toxic elements in the samples analyzed", the statement said, without specifying the origin of the substance.

The change in color noticed by residents raised eyebrows, with police looking into whether Sunday's development could be a protest by climate change activists. It is not the first time the Grand Canal has turned green. In 1968, an artist dyed the waters of the canal green in a stunt to promote ecological awareness.