Taking the metabolic pulse of the world’s coral reefs

Abstract

Worldwide, coral reef ecosystems are experiencing increasing pressure from a variety of anthropogenic perturbations including ocean warming and acidification, increased sedimentation, eutrophication, and overfishing, which could shift reefs to a condition of net calcium carbonate (CaCO3) dissolution and erosion. Herein, we determine the net calcification potential and the relative balance of net organic carbon metabolism (net community production; NCP) and net inorganic carbon metabolism (net community calcification; NCC) within 23 coral reef locations across the globe. In light of these results, we consider the suitability of using these two metrics developed from total alkalinity (TA) and dissolved inorganic carbon (DIC) measurements collected on different spatiotemporal scales to monitor coral reef biogeochemistry under anthropogenic change. All reefs in this study were net calcifying for the majority of observations as inferred from alkalinity depletion relative to offshore, although occasional observations of net dissolution occurred at most locations. However, reefs with lower net calcification potential (i.e., lower TA depletion) could shift towards net dissolution sooner than reefs with a higher potential. The percent influence of organic carbon fluxes on total changes in dissolved inorganic carbon (DIC) (i.e., NCP compared to the sum of NCP and NCC) ranged from 32% to 88% and reflected inherent biogeochemical differences between reefs. Reefs with the largest relative percentage of NCP experienced the largest variability in seawater pH for a given change in DIC, which is directly related to the reefs ability to elevate or suppress local pH relative to the open ocean. This work highlights the value of measuring coral reef carbonate chemistry when evaluating their susceptibility to ongoing global environmental change and offers a baseline from which to guide future conservation efforts aimed at preserving these valuable ecosystems.

Coral Reef Carbonate Chemistry Variability at Different Functional Scales

“…The TA-DIC slope on the platform-scale was significantly higher than at Hog Reef and Bailey's Bay, consistent with patterns observed from reefs globally, where TA-DIC slopes were typically higher when collected over larger spatial scales (Cyronak et al, 2018b). The larger slope is most likely driven by the longer residence time of seawater, i.e., the integration length for NCC and NCP.…”

“…The TA-DIC relationship had a significantly higher slope at the platform-scale, where the chemical variability reflected processes that were integrated over longer periods. This is most likely because the contribution from NCC on TA-DIC changes becomes increasingly important over NCP over longer integration timescales (Falter et al, 2013;Cyronak et al, 2018b).…”

“…The relationship between TA and DIC has been proposed as an approach to calculate NCC:NCP over benthic communities, as NCC and NCP alter TA and DIC in known ratios (Deffeyes, 1965;Suzuki and Kawahata, 2003;Andersson and Gledhill, 2013;Cyronak et al, 2018b). Consequently, TA-DIC relationships reflect the relative balance of the inorganic to organic carbon cycles, which are functions of the community composition and metabolic rates.…”

“…There is a growing recognition of the importance of examining biogeochemical processes across different functional scales (i.e., across time, space, and different benthic communities) on coral reefs to get a comprehensive understanding of the natural variability of seawater carbonate chemistry, the metabolic function of different habitats (e.g., net calcifying vs. net dissolving), and their ability to alleviate or exacerbate ocean acidification (Cyronak et al, 2018b). To place the current results into a broader context, we compared the TA-DIC relationships from Hog Reef with TA-DIC relationships from Bailey's Bay (similar spatiotemporal scales but different benthic communities) and across the entire Bermuda reef platform.…”

Carbonate system parameters of an algal-dominated reef along West Maui

“…The carbonate chemistry system is sensitive to changes in photosynthesis, respiration, calcification, and calcium carbonate (CaCO 3 ) dissolution, and can be characterized by measuring total alkalinity (TA), dissolved inorganic carbon (DIC), pH, pCO 2 , nutrients, salinity, and temperature. Analysis of these parameters yields valuable information on ratios of net community calcification and production, and can be used to identify biological and physical drivers of reef health and ecosystem function (Silverman et al, 2007;Shamberger et al, 2011;Lantz et al, 2014;Albright et al, 2015;Muehllehner et al, 2016;DeCarlo et al, 2017;Richardson et al, 2017;Cyronak et al, 2018). This is particularly important given growing concern that coastal and ocean acidification may shift reef ecosystems from calcification to dissolution by the middle to end of the century (Silverman et al, 2009;Andersson and Gledhill, 2013) with an overall reduction in calcification rates and increase in dissolution rates (Shamberger et al, 2011;Shaw et al, 2012;Bernstein et al, 2016) that can contribute to reef collapse (Yates et al, 2017).…”