Feedbacks and responses of coral calcification on the Bermuda reef system to seasonal changes in biological processes and ocean acidification

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 seawater carbonate chemistry. In this study, contemporaneous in situ datasets of seawater 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 carbon dioxide (CO 2 ) levels 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 [CO 2− 3 ] and aragonite saturation state aragonite , rather than other environmental factors such as light and temperature. These field observations 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 seawater carbonate chemistry. In early summer, strong net autotrophy from benthic components of the reef system enhance [CO 2− 3 ] and aragonite conditions, and rates of coral calcification due to the photosynthetic uptake of CO 2 . In late summer, rates of coral calcification are suppressed by release of CO 2 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. and aragonite . 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.

Multiple driving factors explain spatial and temporal variability in coral calcification rates on the Bermuda platform

“…Bermuda lies on the northern limit of tropical coral reefs ecosystems and thus experiences threshold oceanic environmental and chemical conditions with respect to temperature and light, and possibly X arag and pH, for supporting this ecosystem. It has been hypothesized that Bermuda is likely to be affected by OA sooner than lower latitude reefs, making Bermuda an important location to study the effects of seawater inorganic carbon chemistry on coral calcification rates (Kleypas et al 1999(Kleypas et al , 2001Bates et al 2010). Bermuda is also relatively spared from major human influences, such as pollution, run-off, overfishing, and dredging common to many US and Caribbean reefs and has been protected relatively successfully by local legislation.…”

“…At seasonal timescales, the net reef community metabolism may also affect the reef's susceptibility to pressures from OA. Strong net autotrophy from benthic components of the reef system may enhance X arag due to photosynthetic uptake of CO 2 , while X arag may be suppressed by the release of CO 2 from reef metabolism during periods of strong net heterotrophy, as proposed by the coral reef ecosystem feedback (CREF) hypothesis (Bates et al 2010). …”

“…For example, seasonal differences in X arag of up to 1.2 units have been recorded on the northern reef platform of Bermuda (Bates et al 2010), while lower latitude reefs such as Davies Reef (Great Barrier Reef, GBR) exhibit smaller seasonal ranges of 0.15 (Albright et al 2013). On diurnal timescales, the same is true for shallow reef systems.…”

“…Though field data are limited, correlation between calcification rates and temporal variability in seawater temperature, light, and X arag have been observed on coral reef ecosystems at diurnal (e.g., Suzuki et al 1995;Halley 2003, 2006;Price et al 2012) and seasonal (e.g., Silverman et al 2007;Manzello et al 2008;Bates et al 2010;Shamberger et al 2011;Albright et al 2013) timescales. However, the interdependence of these parameters makes deciphering the relative importance and the principle driving mechanism(s) difficult.…”

Two centuries of limited variability in subtropical North Atlantic thermocline ventilation

“…2). The south-shore coral has approximately 50‰ less variability (results of F-test for variance, P = 0.06, n = 27) than the North Rock coral 11 , which shows abrupt negative excursions interpreted as times of increased ventilation 4 and Bates et al 13 and injection of older thermocline waters into the surface layer. The contrasting signals between the corals indicate that the two locations are experiencing different apparent rates of ocean mixing.…”