Studies of long-term exposure to multiple stressors on predator–prey interactions are necessary to determine the effect of coastal degradation on organisms that have had generations to adapt and acclimate to change. In New South Wales, Australia, a natural gradient of multiple stressors produced by acid sulfate soil effluent was used to determine the impact of exposure to multiple stressors on predator–prey dynamics between mud crabs Scylla serrata and Sydney rock oysters Saccostrea glomerata. Wild oysters were collected from two polluted and two reference sites that varied in their distance away from a flood gate that acted as a point source of water with low salinity, low pH and low alkalinity. Oysters from sites affected by multiple stressors and those from reference sites were offered to mud crabs in 48-h laboratory no-choice feeding trials. Oysters from affected sites had lower mortality than those from a reference site that was farthest from the source of polluted water. Linear models containing distance from flood gate best explained oyster mortality. Differences in rates of mortality were due to the decreased time crabs spent foraging on affected oysters. Long-term exposure to acid sulfate soil effluent alters trophic dynamics between predators and prey, which may have consequences for coastal food webs.