Short-term experiments exposing calcifying organisms to acidification have revealed altered growth and strength of their exoskeletons. We tested the hypothesis that multi-generational exposure to sustained estuarine acidification from runoff from acid sulphate soils (ASS) would: (1) reduce the shell strength of sessile or relatively immobile wild benthic invertebrates and (2) as a consequence render these invertebrates that rely on armour for anti-predator defence more susceptible to generalist benthic predators. First, we compared the force required to break the exoskeletons of Saccostrea glomerata, Bembicium auratum, and Heloecius cordiformis between replicate south-east Australian mangrove forests close to (acidified) and away from (reference) major ASS outflow drains. Second, we assessed differences in the susceptibility of oysters from acidified and reference forests to predation by the generalist muricid gastropod Morula marginalba. Mollusc shells were significantly weaker at ASS-affected than at reference sites, but the strength of crab carapaces was not influenced by acidification. Oysters from acidified sites were consumed by M. marginalba at a faster rate than oysters from reference sites in choice and nochoice experiments because M. marginalba required less time to drill through weaker shells. Many other predators such as crabs are generalist feeders that consume prey at rates inversely proportional to their shell strength. Hence, in the absence of effects of acidification on the ability of these predators to consume prey, molluscs at acidified sites may also be more susceptible to other such predators. This study highlights how human stressors can rapidly alter predator-prey interactions that have evolved over many years.

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Oyster populations periodically exposed to runoff from acid sulfate soils (ASS) are of depressed abundance and have fewer smaller individuals than unaffected populations, despite having similar recruitment levels to unaffected sites during dry periods. We examined how the timing and duration of exposure to ASS runoff influences the growth and survival of successfully settled oysters. We predicted that among 6-month-old oysters, growth and survival would be (1) lower among individuals continuously exposed to ASS-acidified waters than those that are episodically exposed, and (2) most negatively affected during rainfall events, which enhance transport of ASS runoff to estuaries. Six-month-old Sydney rock oysters, Saccostrea glomerata, were deployed at ASS-affected and unaffected sites within each of two south-east Australian estuaries. After 10 weeks, oysters were transplanted within and across sites in an estuary and maintained in situ for another 10 weeks. Oysters that remained for 20 weeks at ASS-affected sites grew at just over half the rate of oysters at reference sites. Oysters transplanted from acidified to reference sites grew more than oysters transplanted from reference to acidified sites or oysters that remained at reference sites. Unexpectedly, overall oyster mortality was low. Greater rainfall, and hence a lower pH, is likely to have accounted for the greater impact of acidification on growth during the second 10 weeks. Where oysters recruit to a 6-month age cohort, they may be able to tolerate subsequent, moderate, acidification events. Reduced growth during acidification periods may be offset by positive growth during intervening dry periods.