Simulated diurnal pH fluctuations radically increase variance in-but not the mean of-growth in the barnacle Balanus improvisus

Shallow coastal waters are characterized by substantial diurnal fluctuations in pH, especially in nearshore environments. The biological effects of ocean acidification in combination with these natural fluctuations have received relatively little attention to date. We exposed multiple batches (approximate to different genotypes) of newly settled barnacles, Balanus improvisus, to constant pH under control (pH = 8.1) or stable acidified (pH = 7.7) conditions, as well as a treatment that simulated the maximum diurnal pH fluctuations seen in the nearshore habitats where this barnacle lives (+/- 0.2 pH units), superimposed on the stable acidified treatment (fluctuating acidified; 7.5 <= pH <= 7.9). We found that fluctuating acidification had no effect on mean response in growth and shell mineralogy, but caused an similar to 20-fold increase in variance of responses, compared with stable acidification. In contrast to these results, we found no effect of fluctuating acidification on variances of response ratios for barnacle survival and shell strength. Similarly, mean survival did not vary significantly with pH. However, we observed a strong negative effect of stable and fluctuating acidification on mean shell strength. Our finding that barnacles respond differently to fluctuating pH than to stable low pH indicate the importance of including fluctuating acidification treatments when studying species that live in variable environments. Importantly, because phenotypic variance is the raw material for natural selection, and thus lays at the heart of evolutionary responses to environmental variability and change, our findings also highlight the need to study changes in variance of-as well as mean-responses to changing ocean climates.