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.

Ocean acidification is expected to alter community composition on coral reefs, but its effects on reef community metabolism are poorly understood. Here we document how early successional benthic coral reef communities change in situ along gradients of carbon dioxide (CO2), and the consequences of these changes on rates of community photosynthesis, respiration, and light and dark calcification. Ninety standardised benthic communities were grown on PVC tiles deployed at two shallow-water volcanic CO2 seeps and two adjacent control sites in Papua New Guinea. Along the CO2 gradient, both the upward facing phototrophic and the downward facing cryptic communities changed in their composition. Under ambient CO2, both communities were dominated by calcifying algae, but with increasing CO2 they were gradually replaced by non-calcifying algae (predominantly green filamentous algae, cyanobacteria and macroalgae, which increased from ~30% to ~80% cover). Responses were weaker in the invertebrate communities, however ascidians and tube-forming polychaetes declined with increasing CO2. Differences in the carbonate chemistry explained a far greater amount of change in communities than differences between the two reefs and successional changes from five to 13 months, suggesting community successions are established early and are under strong chemical control. As pH declined from 8.0 to 7.8, rates of gross photosynthesis and dark respiration of the 13-month old reef communities (upper and cryptic surfaces combined) significantly increased by 10% and 20%, respectively, in response to altered community composition. As a consequence, net production remained constant. Light and dark calcification rates both gradually declined by 20%, and low or negative daily net calcification rates were observed at an aragonite saturation state of <2.3. The study demonstrates that ocean acidification as predicted for the end of this century will strongly alter reef communities, and will significantly change rates of community metabolism.

Larval settlement is a key process in the lifecycle of benthic marine organisms; however, little is known on how it could change in reduced seawater pH and carbonate saturation states under future ocean acidification (OA). This is important, as settlement ensures species occur in optimal environments and, for commercially important species such as abalone, reduced settlement could decrease future population success. We investigated how OA could affect settlement success in the New Zealand abalone Haliotis iris by examining: (1) direct effects of seawater at ambient (pHT 8.05) and reduced pHT (7.65) at the time of settlement, (2) indirect effects of settlement substrates (crustose coralline algae, CCA) preconditioned at ambient and reduced pHT for 171 days, and (3) carry-over effects, by examining settlement in larvae reared to competency at ambient and reduced pHT (7.80). We found no effects of seawater pH or CCA incubation on larval settlement success. OA-induced carry-over effects were evident, with lower settlement in larvae reared at reduced pH. Understanding the mechanisms behind these responses is key to fully comprehend the extent to which OA will affect marine organisms and the industries that rely on them.

Investigations of the strong environmental gradients within intertidal and subtidal rocky reefs have contributed significantly to our understanding of ecological processes, but studies exploring how algal community structure responds to the extreme environmental transition of the intertidal-subtidal interface are rare. Our objective was to examine patterns in macroalgal distribution and species richness with depth on temperate rocky reefs. Standing algal biomass and richness were measured on 6 representative reefs in southern New Zealand, across 5 depth strata from the high intertidal zone, 1.5 m above mean low water (MLW), to the subtidal zone, 10 m below MLW. We found a unimodal relationship between algal richness and biomass across the depths, where maximum species richness occurred at intermediate levels of biomass. These results are consistent with many terrestrial plant studies across strong environmental gradients. Biomass decreased down the shoreline, with the exception of the high intertidal where the lowest biomass was recorded, whilst species richness increased down the shoreline. Additionally, strong patterns of dominance were observed, with a single species (not always the same species) contributing >56% of the total biomass across all depth strata examined. This dominance could have important implications for ecosystem provisioning across this system, particularly if dominant species are found to be vulnerable to the impacts of local and/or global change. The strong environmental gradients that characterise the intertidal-subtidal transition on rocky reefs over relatively small and experimentally tractable spatial scales enable opportunities to further advance our understanding of the mechanisms controlling the distribution of biodiversity.