Ecological advantage of polarized light pollution: positive effect of a dark lake patch at a canal inflow on habitat of non-biting midges

Abstract Dark artificial surfaces reflecting highly and horizontally polarized light usually have negative effects on polarotactic aquatic insects detecting their habitats by the horizontal polarization of water-reflected light. This ecologically disadvantageous phenomenon is called polarized light pollution. We have observed that the water between the concrete walls of a harbour of the Hungarian Lake Balaton is continuously dark from autumn to spring due to the inflow of a canal rich in dissolved humic substances. Using ground-born imaging polarimetry, we demonstrated that this dark water patch reflects light with higher degrees of polarization than the brighter lake water. Our hypothesis was that the stronger horizontally polarized light reflected from the dark water patch is more attractive to swarming, water-seeking and egg-laying non-biting midges (Chironomidae) than the surrounding brighter lake water. With larval samplings, we showed that both the density and the average size of chironomid larvae were significantly larger in the harbour than in the surrounding lake. This finding may represent an ecological advantage of polarized light pollution: polarotactic chironomids are intensely attracted to a strongly and horizontally polarizing, seasonally dark water patch at the canal inflow, where the abundance of larvae increases. It should be taken into consideration that increased larval abundance might result in increased swarming intensity which could affect humans by causing considerable nuisance.