Background
The authors write that "most aquatic organisms have an optimal pH for growth and survival," and, consequently, that "there has been much recent interest in how CO2-induced ocean acidification will influence [marine] biological systems." On the other hand, they note that "runoff from acid sulphate soils (ASS), which represents a much more immediate threat to biologically productive estuaries, has by contrast received relatively little attention." And, therefore, they proceed to partially atone for this prior neglect.

What was done
Working within three estuaries of New South Wales (Australia), all within the same transitional subtropical/temperate climatic region, Amaral et al. sampled wild oyster, gastropod and crab populations at sites close to and farther away from drains that discharged ASS runoff into them. And what did they find?

What was learned
The three researchers report that wild macro-invertebrate populations were indeed affected by runoff from ASS, "but to a lesser extent than as predicted from laboratory and field experiments with naïve organisms." Their sampling confirmed, for example, that "the oyster Saccostrea glomerata and gastropods (primarily Bembicium auratum) were less abundant at ASS-affected than reference sites," but they found that "crab abundances did not differ between the acidified and reference sites," and that "impacts to bivalves and gastropods were far smaller than predicted." In addition, they say that "although at ASS-affected sites gastropod populations were dominated by smaller individuals than at reference sites, oyster populations were skewed towards larger individuals," and that "even at ASS-affected sites, oyster and gastropod abundances were within the range encountered in estuaries that are not influenced by ASS runoff."

What it means
Amaral et al. say their study suggests that "at the population level, calcifying organisms display a certain degree of natural resistance to recurrent disturbance from ASS runoff." And although they indicate that "the mechanisms by which populations of oysters, gastropods and crabs may persist in ASS runoff were not examined," they conclude that "the commonality of response across functional groups, suggests that natural behavior, long-term physiological acclimation and genetic selection may be responsible," and they opine that "these mechanisms may not only operate to influence organisms' susceptibility to ASS runoff but also, potentially, CO2-induced ocean acidification and other stressors."