Diel variability in seawater pH relates to calcification and benthic community structure on coral reefs

Abstract

Community structure and assembly are determined in part by environmental heterogeneity. While reef-building corals respond negatively to warming (i.e. bleaching events) and ocean acidification (OA), the extent of present-day natural variability in pH on shallow reefs and ecological consequences for benthic assemblages is unknown. We documented high resolution temporal patterns in temperature and pH from three reefs in the central Pacific and examined how these data relate to community development and net accretion rates of early successional benthic organisms. These reefs experienced substantial diel fluctuations in temperature (0.78°C) and pH (>0.2) similar to the magnitude of 'warming' and 'acidification' expected over the next century. Where daily pH within the benthic boundary layer failed to exceed pelagic climatological seasonal lows, net accretion was slower and fleshy, non-calcifying benthic organisms dominated space. Thus, key aspects of coral reef ecosystem structure and function are presently related to natural diurnal variability in pH.

Figure 1. Site locations and continuously logged data (1 hr interval) using the SeaFET sensors.

Data are low-pass filtered (period = 2 hrs). The solid black lines in the pH and temperature plots represent a regional climatological mean (e.g. pH_cm), generated by combining the World Ocean Atlas 2009 (WOA09) data, the Takahashi pCO₂ climatology , and the A_T climatology of Lee et al. . In the pH plots, the dashed lines are the pH_csl values at the climatological seasonal low. FR = forereef, RT = reef terrace.

Figure 2. Results of image analyses on Calcification Acidification Units (CAUs).

a) Percent cover of the various ecological functional groups. Green bands = ‘fleshy’ invertebrate and algae species. The mineralogy for calcifying species is indicated in the figure legend. b) Original variable vectors of the CAP analysis overlaid as a bi-plot (Spearman Rank correlations >0.5) for both the biological and physio-chemical data from each site. The biological vector colors correspond to functional groups listed in (a).

Figure 3. Relationship between net calcification or percent cover analyses on CAUs and relevant pH metrics.

a) Means (± SE) of net reef calcification for total CaCO₃ and each polymorph of CaCO₃ against mean (± SE) daily ∑ pH·hrs above pH_csl. Both total net accretion (Y = 6.86x +0.96; F = 7.78, R = 0.813, P = 0.0444) and high Mg calcite calcification rate (Y = 7.75x +0.47; F = 12.26, R = 0.868, P = 0.0249) are related to mean daily ∑ pH·hrs above pH_csl. b) Percent cover of all calcifiers and for each polymorph of CaCO₃ against mean (± SE) daily ∑ pH·hrs above pH_csl. Only the percent cover of high Mg calcite is positively related to ∑ pH·hrs above pH_csl (Y = 83.81x +48.52; F = 12.747, R = 0.872, P = 0.0234). Percent cover of calcitic organisms is negatively related to ∑ pH·hrs above pH_csl (Y = 14.85–29.21x; F = 17.502, R = −0.902, P = 0.0139).