The interaction of ocean acidification and carbonate chemistry on coral reef calcification: evaluating the carbonate chemistry Coral Reef Ecosystem Feedback (CREF) hypothesis on the Bermuda coral reef

Abstract

Abstract. Despite the potential impact of ocean acidification on ecosystems such as coral reefs, surprisingly, there is very limited field data on the relationships between calcification and carbonate chemistry. In this study, contemporaneous in situ datasets of carbonate chemistry and calcification rates from the high-latitude coral reef of Bermuda over annual timescales provide a framework for investigating the present and future potential impact of rising pCO2 and ocean acidification on coral reef ecosystems in their natural environment. A strong correlation was found between the in situ rates of calcification for the major framework building coral species Diploria labyrinthiformis and the seasonal variability of [CO32-] and Ωaragonite, rather than other environmental factors such as light and temperature. These field observations also provide sufficient data to hypothesize that there is a seasonal "Carbonate Chemistry Coral Reef Ecosystem Feedback" (CREF hypothesis) between the primary components of the reef ecosystem (i.e. scleractinian hard corals and macroalgae) and carbonate chemistry. In early summer, strong net autotrophy from benthic components of the reef system enhance [CO32-] and Ωaragonite conditions, and rates of coral calcification due to the photosynthetic uptake of CO2. In late summer, rates of coral calcification are suppressed by release of CO2 from reef metabolism during a period of strong net heterotrophy. It is likely that this seasonal CREF mechanism is present in other tropical reefs although attenuated compared to high-latitude reefs such as Bermuda. Due to lower annual mean surface seawater [CO32-] and Ωaragonite in Bermuda compared to tropical regions, we anticipate that Bermuda corals will experiences seasonal periods of zero net calcification within the next decade at [CO32-] and Ωaragonite thresholds of ~184 mmoles kg−1 and 2.65. The Bermuda coral reef is one of the first responders to the negative impacts of ocean acidification, and we estimate that calcification rates for D. labyrinthiformis have declined by >50% compared to pre-industrial times.

Twenty Years of Marine Carbon Cycle Observations at Devils Hole Bermuda Provide Insights into Seasonal Hypoxia, Coral Reef Calcification, and Ocean Acidification

“…Previous studies of Bermuda reef biogeochemistry have included observations of CO 2 -carbonate chemistry Bates, 2002), reef biogeochemical processes (Bates et al, 2010;Courtney et al, 2016) and longer-term proxy assessments of pH (Goodkin et al, 2015). For example, Bates et al (2010) reported a seasonal reduction in coral calcification (i.e., coral skeletal growth rates) with reduced carbonate ion [CO 2− 3 ] and aragonite , contributing further evidence to linkage of OA to declining coral calcification. They also highlighted the strong seasonal coupling and feedback between rates of coral calcification (i.e., coral skeletal growth and water assessment of net ecosystem calcification, NEC), other ecological components of the reef system (i.e., macroalgae) and reef chemistry.…”

“…In the milieu of environmental issues that beset coral reef health and future viability, ocean acidification comingles with variability of natural conditions to influence coral reef biogeochemical processes such as calcification, photosynthesis, respiration and CaCO 3 dissolution. A suite of in situ, in vitro, experimental and model studies have shown decreases in both mature colony and juvenile coral calcification with reduced carbonate ion concentration [CO 2− 3 ] or aragonite saturation state (i.e., aragonite ) under natural and/or perturbed conditions (e.g., Gattuso et al, 1996Gattuso et al, , 1998Langdon et al, 2000;Langdon and Atkinson, 2005;Schneider and Erez, 2006;Anthony et al, 2008;Bates et al, 2010;Kroeker et al, 2010;Erez et al, 2011;Pandolfi et al, 2011;de Putron et al, 2011;Chan and Connolly, 2013;Kroeker et al, 2013a;Venn et al, 2013;Evenhuis et al, 2015). In other studies, bicarbonate ion concentration has been shown to be the primary modifier of coral calcification (e.g., Jury et al, 2010).…”

“…One of the unique aspects for understanding coral reef biogeochemical processes is the contemporaneous collection of offshore ocean CO 2 -carbonate chemistry at two long-term time-series sites (Hydrostation S; Bermuda Atlantic Time-series Study, BATS) since 1983 (e.g., Bates et al, 1996a;Bates, 2001Bates, , 2012Bates et al, 2014). Previous studies of Bermuda reef biogeochemistry have included observations of CO 2 -carbonate chemistry Bates, 2002), reef biogeochemical processes (Bates et al, 2010;Courtney et al, 2016) and longer-term proxy assessments of pH (Goodkin et al, 2015). For example, Bates et al (2010) reported a seasonal reduction in coral calcification (i.e., coral skeletal growth rates) with reduced carbonate ion [CO 2− 3 ] and aragonite , contributing further evidence to linkage of OA to declining coral calcification.…”

“…The source of all Bermuda inshore waters is the offshore Sargasso Sea surrounding the island and reefs of Bermuda. Any biogeochemical modification of CO 2 -carbonate chemistry of Bermuda waters reflects reef biogeochemical processes (e.g., Bates, 2002;Bates et al, 2010;Courtney et al, 2016). Differences in onshore (i.e., Harrington Sound and other sites) and offshore (i.e., BATS) ocean chemistry are used to determine rates of net community calcification (NEC) and net ecosystem production (NEP) for the period 1996-2016.…”

“…These factors include the following: (1) water circulation of the reef system (e.g., Falter et al, 2012Falter et al, , 2013Comeau et al, 2014c); (2) habitat complexity, community dynamics and variable responses between coral species (e.g., Bates et al, 2010;Kroeker et al, 2013a,b,c;Fabricius et al, 2014;Comeau et al, 2014b;Hendricks et al, 2015) (3) corals modifying internal and external CO 2 -carbonate chemistry (e.g., Anthony et al, 2011;Jokiel, 2011aJokiel, ,b, 2013Kleypas et al, 2011); (4) variability of reef CO 2 from diurnal time-scales (e.g., Ohde and van Woesik, 1999;Jokiel et al, 2014;Comeau et al, 2014a;Koweek et al, 2015) to longer time-scales (e.g., Shamberger et al, 2011;Edmunds et al, 2012;Gray et al, 2012;Shamberger et al, 2011;Shaw et al, 2012;Albright et al, 2013); and, (5) coral reef environments with naturally lower pH such as CO 2 seeps (Crook et al, 2011(Crook et al, , 2013Shamberger et al, 2014).…”