In situ changes of tropical crustose coralline algae along carbon dioxide gradients

Abstract

Crustose coralline algae (CCA) fulfill important ecosystem functions in coral reefs, including reef framework stabilization and induction of larval settlement. To investigate in situ the effects of high carbon dioxide on CCA communities, we deployed settlement tiles at three tropical volcanic CO2 seeps in Papua New Guinea along gradients spanning from 8.1 to 7.4 pH. After 5 and 13 months deployment, there was a steep transition from CCA presence to absence around pH 7.8 (660 μatm pCO2): 98% of tiles had CCA at pH > 7.8, whereas only 20% of tiles had CCA at pH ≤ 7.8. As pH declined from 8.0 to 7.8, the least and most sensitive CCA species lost 43% and 85% of cover, respectively. Communities on upward facing surfaces exposed to high light and high grazing pressure showed less steep losses than those on shaded surfaces with low grazing. Direct CO2 effects on early life stages were the main mechanisms determining CCA cover, rather than competitive interactions with other benthic groups. Importantly, declines were steepest at near-ambient pH, suggesting that CCA may have already declined in abundance due to the recent seawater pH decline of 0.1 units, and that future severe losses are likely with increasing ocean acidification.

Figure 1. Seawater chemistry over the Control and High CO₂ tiles.

Black horizontal lines indicate medians; boxes enclose the upper and lower quartiles of the data, whiskers mark the maximum and minimum values excluding outliers, while round circles show outliers. The plots show the median values of 45 Control and 71 High CO₂ tiles, with each of these 45 and 71 median values composed of 4 to 23 measurements taken directly over each of the tiles during four two-week long visits in the period 2011–2012 (data are listed in the Supplementary Tables S1 and S2, online). pH is at total scale, Omega calcite and aragonite = saturation states of calcite and aragonite, TA = total alkalinity, DIC = dissolved inorganic carbon.

Figure 2. Log ratios of the cover of crustose coralline algae (CCA) at High CO₂ over Control sites, on the three reefs (Reef Communities, N = 80 transects), and on the top- and bottom-sides of the settlement tiles after 5 and 13 months of deployment (5.M, 13.M; N = 120 and 116, respectively).

Squares indicate back-transformed means, the error bars are 95% confidence intervals (Supplementary Table S3). For example, the ratio 0.3 indicates the mean cover at High CO₂ is 30% of that at the Controls. Differences are all significant at the 5% level (error bars do not include the value 1.0).

Figure 3. CCA settlement tiles at Control sites (left) and High CO₂ sites (right) at volcanic CO₂ seeps in Papua New Guinea.

Tiles in situ after 13 months deployment (a, b). Top sides of tiles after 5 months deployment (c, d), and after 13 months (e, f). Bottom sides of the tiles after 5 months (g, h), and after 13 months (i, j).

Figure 4. Changes in the cover of various taxonomic groups of CCA along the pH gradient (Supplementary Table S4).

Top row: total CCA cover (all CCA taxa combined), on the top- and bottom-sides of the tiles after 5 and 13 months deployment (5 M., 13 M.; N = 120 and 116, respectively). Middle and bottom row: changes in cover of specific CCA taxa on the bottom-sides of the tiles after 13 months. The red solid lines show the estimated cover as a function of pH, dashed lines show upper and lower 95% confidence intervals. The grey dots and vertical bars show mean cover at a pH level of 8.0 and 7.8, and the 95% CI of these estimates. The ‘x’ symbols (jittered vertically for clarity) show the pH of the individual tiles on which the specific CCA taxa were present (blue) or absent (red).

Figure 5. Violin plot showing changes in the frequency of settlement tiles with CCA present on their bottom-sides, along the pH gradient.

The dark and light grey areas indicate the densities of observations of tiles with and without CCA, respectively, both after 5 month (5 M.: N = 120) and after 13 month of deployment (13 M.: N = 116). The rugs indicate the median pH for each tile, i.e. the pH to which the tiles were exposed to in the field. The transition from CCA presence to absence is steep at a pH of ~7.8.