Diurnal fluctuations in seawater pH influence the response of a calcifying macroalga to ocean acidification

Abstract

Coastal ecosystems that are characterized by kelp forests encounter daily pH fluctuations, driven by photosynthesis and respiration, which are larger than pH changes owing to ocean acidification (OA) projected for surface ocean waters by 2100. We investigated whether mimicry of biologically mediated diurnal shifts in pH-based for the first time on pH time-series measurements within a kelp forest-would offset or amplify the negative effects of OA on calcifiers. In a 40-day laboratory experiment, the calcifying coralline macroalga, Arthrocardia corymbosa, was exposed to two mean pH treatments (8.05 or 7.65). For each mean, two experimental pH manipulations were applied. In one treatment, pH was held constant. In the second treatment, pH was manipulated around the mean (as a step-function), 0.4 pH units higher during daylight and 0.4 units lower during darkness to approximate diurnal fluctuations in a kelp forest. In all cases, growth rates were lower at a reduced mean pH, and fluctuations in pH acted additively to further reduce growth. Photosynthesis, recruitment and elemental composition did not change with pH, but δ(13)C increased at lower mean pH. Including environmental heterogeneity in experimental design will assist with a more accurate assessment of the responses of calcifiers to OA.

Keywords: coralline macroalgae; environmental heterogeneity; natural pH fluctuations; ocean acidification; temperate rocky reefs.

Figure 1.

Summertime pH_T measurements over 5 days within a kelp bed located in 1–2 m water depth. Grey bars indicate periods of darkness.

Figure 2.

Mean daytime and night-time pH within the mixing tanks in the laboratory experiment showing the pH treatments contrasted against the conditions in the field. (a) Mean seawater pH_T supplied to the ambient pH 8.05 static (±0.01) (solid line) and fluctuating (±0.40) (dotted line) treatments during the day and the night. (b) Mean seawater pH_T supplied to the OA scenario pH 7.65 static (±0.01) and fluctuating (±0.40) treatments during the day and the night. The standard error for each mean = 0.01. (c) pH values within the kelp bed habitat (1–2 m depth) over 20 days between April 2010 and May 2011.

Figure 3.

Biotic responses of the articulated coralline macroalga A. corymbosa exposed to one of four pH treatments for 40 days (see table 1 and figure 3 for pH conditions). (a) Relative growth rates and (b) recruitment of A. corymbosa. Treatments sharing the same letter in (a) are not statistically different (Tukey's HSD p ≤ 0.05), whereas in (b) there were no treatment effects. Error bars represent standard error, n = 6.