Community production modulates coral reef pH and the sensitivity of ecosystem calcification to ocean acidification

Abstract

Coral reefs are built of calcium carbonate (CaCO3) produced biogenically by a diversity of calcifying plants, animals, and microbes. As the ocean warms and acidifies, there is mounting concern that declining calcification rates could shift coral reef CaCO3 budgets from net accretion to net dissolution. We quantified net ecosystem calcification (NEC) and production (NEP) on Dongsha Atoll, northern South China Sea, over a 2 week period that included a transient bleaching event. Peak daytime pH on the wide, shallow reef flat during the nonbleaching period was ∼8.5, significantly elevated above that of the surrounding open ocean (∼8.0–8.1) as a consequence of daytime NEP (up to 112 mmol C m−2 h−1). Diurnal‐averaged NEC was 390 ± 90 mmol CaCO3 m−2 d−1, higher than any other coral reef studied to date despite comparable calcifier cover (25%) and relatively high fleshy algal cover (19%). Coral bleaching linked to elevated temperatures significantly reduced daytime NEP by 29 mmol C m−2 h−1. pH on the reef flat declined by 0.2 units, causing a 40% reduction in NEC in the absence of pH changes in the surrounding open ocean. Our findings highlight the interactive relationship between carbonate chemistry of coral reef ecosystems and ecosystem production and calcification rates, which are in turn impacted by ocean warming. As open‐ocean waters bathing coral reefs warm and acidify over the 21st century, the health and composition of reef benthic communities will play a major role in determining on‐reef conditions that will in turn dictate the ecosystem response to climate change.

Coral Reef Ecosystems under Climate Change and Ocean Acidification

“…Reefs remaining in negative carbonate budgets were those where massive coral loss was high and recovery of branched corals was low. The composition of reef benthic communities, which are sensitive to thermal stress, have an influence on the sensitivity of coral reef ecosystems to ocean acidification (DeCarlo et al, 2017). In long-term studies done in mesocosms, carbonate balance of reefs tips toward overall dissolution under concentrations of CO 2 of more than 450 ppm , which matches similar conclusions from previous experimental work (Anthony et al, 2008;Wild et al, 2011;Andersson and Gledhill, 2013) and from the geographical distribution of coral reefs in relation to the aragonite saturation state of seawater (Kleypas et al, 1999b;Hoegh-Guldberg et al, 2007).…”

“…Coral bleaching events driven by elevated temperatures has also been shown to shift carbonate budgets of coral reefs from net accretion to net erosion (DeCarlo et al, 2017;JanuchowskiHartley et al, 2017). Sixteen years later, a third of reefs that were considered ecologically recovering (Graham et al, 2015) did not show positive carbon budgets (Januchowski-Hartley et al, 2017).…”

Coral Reef Drag Coefficients – Water Depth Dependence

“…5c). The barrier reef included rubble and small (typically 20-30 cm) corals and seagrass patches between E2 and E4, and sand, seagrass, massive Porites colonies and Acropora thickets (both commonly .1 m height) between E4 and E6 (DeCarlo et al 2017). Depth-average cross-reef currents of 10-30 cm s 21 (at E5) are forced by tides and surface gravity waves.…”

The perfect calm: Reoccurring mass die-offs on a remote coral atoll

“…Neap tides occurred on May 22, 2014, and June 10, 2015, each preceding a die-off event by 1-2 days (figure 1b). Although the mortality events observed on June 1, 2014, and June 16, 2015 did not occur during neap tides, current speeds on the adjacent reef flat at both of these times were relatively slow (<5 cm s -1 ) due to weak winds and small waves [7] [8], whereas bleaching in the later half of June through July 2015 caused 40% coral mortality [7]. The coral bleaching events were separated from the die-offs that we report here by 1-2 km, and in 2015 the coral mortality occurred 1-2 months after our observations.…”

“…We suspect that the die-offs were caused by hypoxia because suffocating fish from Apogonidae and Pomacentridae families were swimming to the surface to gulp air. Oxygen saturation on the adjacent reef flat is typically <10% around dawn, even reaching anoxia for 30-60 min on some days, due to high rates of community metabolism [8]. This depletion of oxygen might be exacerbated in the lagoon during calm conditions [9].…”