Hamelin Pool, W.-Australia, is a natural laboratory to study stromatolite formation in a hypersaline lagoon. Stromatolitic aragonites sampled at supratidal, intertidal and subtidal environments show gradual increase in Ba isotopic compositions (d138Bacarb = -0.12-0.57‰) with decreasing Ba/Ca following conservative mixing between groundwater and seawater. We observe fingerprints of the groundwater end-member in a supratidal sample showing particular light d138Bacarb corresponding with elevated Mn/Sr and low d18Ocarb as indicator for discharge from the local Tamala Limestone aquifer. In contrast, stromatolitic carbonates formed in equilibrium with a saline end-member show heavy d138Bacarb. Lagoonal d138Bafluid ranges from 0.44-0.6‰ at increasing PSU (3.6-65.6). The calculated partition coefficient for Ba into stromatolites is lowest at the subtidal (logDBa=0.45) and highest at the groundwater discharge site (logDBa=0.86). Injection of groundwaters into Hamelin Pool likely contributes to enhanced aragonite precipitation at up to 0.32mm/yr, possibly catalysed by nucleation within or onto extra-polymeric substances of microbially diverse mats.