Ocean acidification alters early successional coral reef communities and their rates of community metabolism

Abstract

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.

The Impacts of Ocean Acidification on Marine Ecosystems and Reliant Human Communities

“…Seagrass meadows: increase or no change in primary productivity (135,136), grazing by herbivores (102, [141][142][143], competition between seagrass and macroalgae (93,137,140). Coral reefs: bioerosion (118), macroalgal overgrowth (94,95,120,123), community structure (94,95,119,125), overall, net ecosystem calcification (115,116,118), habitat-forming calcifier recruitment (122)(123)(124), structural complexity (127,128), taxonomic diversity (94,95,128). Other carbonate reefs: carbonate dissolution (131), macroalgal overgrowth (134), overall, net ecosystem calcification (133,134), habitat-forming calcifier recruitment (132,133).…”

“…Declines in the percent cover of crustose coralline algae, which are often used as recruitment substrates by corals, may also contribute to reduced coral settlement in high-CO 2 conditions (122). High-CO 2 effects on early succession dynamics lead to higher abundance of micro-and macroalgae and lower coral recruitment, although the mechanisms attributed to these shifts differ among studies: altered competitive interactions (123) versus chemical control (124).…”

Greater Mitochondrial Energy Production Provides Resistance to Ocean Acidification in “Winning” Hermatypic Corals

“…Short experimental durations (days to weeks) do not allow corals to acquire complete acclimation to high pCO 2 conditions and are clearly insufficient for achieving adaptation (multi-generation evolution) or ecological adaptation (selection of resistant species and/or individuals) to these conditions (Riebesell et al, 2010). Natural analogues such as volcanic CO 2 seeps have increasingly been used to complement laboratory experiments (Hall-Spencer et al, 2008), allowing the physiological study of corals that have fully acclimatized to elevated pCO 2 throughout at least their entire post-settlement lives (Noonan et al, 2018). At such natural analogues for future conditions, ocean acidification has drastically decreased the coverage and diversity of corals (Fabricius et al, 2011;Inoue et al, 2013;Enochs et al, 2015;Agostini et al, 2018).…”

Biodiversity of coral reef cryptobiota shuffles but does not decline under the combined stressors of ocean warming and acidification

“…A similar result was obtained from an experimental warming study in Antarctica, which showed that over the course of 9 mo, a 1 to 2 °C rise in temperature favored divergent taxa and resulted in distinctive species assemblages on warmed settlement panels relative to the control ( 62 ). Shifts in competitive dominance have also been found on settlement plate assemblages across natural gradients in acidification such that calcifiers were consistently replaced by fleshy algae under increasing acidity ( 15 , 63 , 64 ). In contrast, this study found differing responses among diverse calcifying taxa under reduced pH.…”

“…In this century alone, record-breaking sea surface temperature anomalies have resulted in widespread coral mortality ( 7 , 8 ), leading to a reduction in topographic complexity ( 9 ) and a shift in community composition ( 10 , 11 ). Likewise, in situ observations of coral reefs along naturally occurring gradients of acidification have shown declines in habitat complexity ( 5 , 6 ) and diversity ( 12 , 13 ), as well as changes in community structure ( 14 , 15 ). The combination of both thermal stress and acidification stress over the coming decades is predicted to have synergistic negative effects on reef resilience ( 2 , 3 , 16 ) by eroding the reef framework ( 17 ), shifting the structural dominance away from calcifiers and severely diminishing the biodiversity of this iconic ecosystem ( 2 , 4 ).…”