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We report an unexpected variation in the positional offset distributionsbetween Herschel-ATLAS sub-millimetre (submm) sources and their opticalassociations, depending on both 250-{\mu}m signal-to-noise ratio and250/350-{\mu}m colour. We show that redder and brighter submm sources haveoptical associations with a broader distribution of positional offsets thanwould be expected if these offsets were due to random positional errors in thesource extraction. The observation can be explained by two possible effects:either red submm sources trace a more clustered population than blue ones, andtheir positional errors are increased by confusion; or red submm sources aregenerally at high redshifts and are frequently associated with low-redshiftlensing structures which are identified as false counterparts. We performvarious analyses of the data, including the multiplicity of opticalassociations, the redshift and magnitude distributions in H-ATLAS in comparisonto HerMES, and simulations of weak lensing, and we conclude that the effectsare most likely to be explained by widespread weak lensing of Herschel-SPIREsources by foreground structures. This has important consequences forcounterpart identification and derived redshift distributions and luminosityfunctions of submm surveys.

Colour matters: the effects of lensing on the positional offsets between optical and submillimetre galaxies in Herschel-ATLAS