Inland waterbodies are often naturally acidic but are these ecosystems pre-adapted to inorganic acidification e.g., by acid sulfate soils (ASS)? We conducted a controlled mesocosm experiment with inorganically acidified wetland water and wetland sediment replicates to pH 3 from a naturally acidic (pH 3.9, conductivity=74microScm(-1)) wetland in south-western Australia. Following acidification, dissolved organic carbon and nitrogen declined, and chlorophyll a dropped to zero. Inorganic acidification mobilised metals from sediment sods with increased water concentrations of Cu, Fe, Mn, Ca, Mg and Al. Acidification showed no significant effect on diatom assemblage. Nonetheless, greatly reduced abundance and diversity of grazing zooplankton was observed. Macroinvertebrates generally showed abundance decreases, although filterer-collector taxa increased. Decreased primary production reduced functional diversity and consumer biomasses. These results suggest likely impact to ecosystem functioning of low pH, weakly-buffered and stained wetlands if exposed to inorganic acidification.