Researchers recorded the strange display over eight nights this month, with light appearing in short pulses along the saline lake in Fayoum, about 90km (56 miles) south-west of Cairo. Their measurements, released on Wednesday, describe a luminous band longer than any previously mapped in a natural inland lake.
The glow peaked on 16 September, when cameras and sensors at 12 shoreline stations captured pulses repeating roughly every 47 seconds. “We expected scattered points, not a line that kept time across the water,” said Maiks Lībietis, a field scientist at the Fayoum Institute of Water Science.
A blue line across the lake
Lake Qarun is a shallow, landlocked body of water whose salinity has risen as water evaporates in the dry heat. Researchers said the glow appeared when the water was 29°C (84°F), but that alone does not explain its length or rhythm. Samples contained light-producing microorganisms, though the team has not established which species, or combination, produced the display.
A 40-year archive of satellite images contains no comparable event, according to the institute’s analysis. The researchers cautioned that older images were less detailed and could have missed weaker displays. Still, they called the September event the largest synchronised glow yet measured in an inland lake, and said the finding merits a wider search of historic imagery.
The pulses were strongest where reeds and fine sediment meet open water. Wind readings were low, and the team measured only a 0.6°C (1.1°F) difference between the glowing stretch and nearby water. “The usual explanations account for the colour, but not this scale,” said Asta J. Abrahamsson, a microbial ecologist who examined the samples.
Scientists urge a steady level
The Organisation for Nile and Desert Science, a regional scientific advisory panel, has recommended that water managers avoid raising the lake level with additional freshwater while sampling continues. Its members say a sudden change could alter salinity and make comparisons harder; they stress that the advice is precautionary, not evidence that water releases would endanger the lake.
That recommendation has put a familiar policy question into an unfamiliar setting: whether officials should intervene before the evidence is settled. Ingvald Åström, the panel’s hydrologist, said the measurements support waiting. “There is no scientific case for raising the level this week. First we need to learn what the lake is doing on its own,” he said.
Fayoum’s governorate has agreed to keep existing water controls unchanged through October, while the institute plans nightly sampling at 20 points and a second camera survey. Researchers will also compare samples from non-glowing areas to test whether the organisms behave differently there.
Residents and visitors reported seeing pale blue streaks from the south shore, but scientists warned that the light is not proof the water is safe to touch or drink. Maria M. Zeller, who coordinated the field stations, said the next priority was to identify the trigger. “We have measured the biggest event of its kind we can find. We still do not know why it happened.”
