Ocean acidification is predicted to impact all areas of the oceans and affect a diversity of marine organisms.
However, the diversity of responses among species prevents clear predictions about the impact of acidification at the ecosystem level.
The results of our analyses indicate that there is an absence of numerous taxa in extreme low p H, compensated for by increased abundances of acidification-tolerant taxa (primarily crustaceans).
However, we found a corresponding decrease in the aggregate invertebrate biomass and a simplification of the trophic structure in extreme low p H conditions, suggesting that density compensation among tolerant taxa does not fully buffer against changes in ecosystem properties and function.
We found fewer taxa, reduced taxonomic evenness, and lower biomass in the extreme low p H zones.
However, the number of individuals did not differ among p H zones, suggesting that there is density compensation through population blooms of small acidification-tolerant taxa.
This variability in carbonate chemistry allows for an examination of the ecosystem effects of decreasing p H conditions, such as those effects predicted at the close of this century (19).
Individuals were classified to the lowest taxonomic group feasible (59% were classified to species) (Table S1), hereafter termed the operational taxonomic unit (OTU).In the low p H zones in the north and south sites, 47% and 32% of the hourly p H measurements were below 7.8 (the predicted average global sea surface p H value for the year 2100) (20), respectively, whereas ∼91% and 93% of the measurements were below 7.8 in the extreme low p H zones, respectively.