Abstract
There is a growing recognition for the need to understand how seawater carbonate chemistry over coral reef environments will change in a high-CO2 world to better assess the impacts of ocean acidification on these valuable ecosystems. Coral reefs modify overlying water column chemistry through biogeochemical processes reflected in thesuch as net community organic carbon production (NCP) and calcification (NCC). However, the relative importance and influence of these processes on seawater carbonate chemistry vary across multiple functional scales (defined here as space, time, and benthic community composition), and have not been fully constrained. Here, we use Bermuda as a case study to assess 1) spatiotemporal variability in physical and chemical parameters along a depth gradient at a rim reef location, 2) the spatial variability of total alkalinity (TA) and dissolved inorganic carbon (DIC) over distinct benthic habitats to infer NCC:NCP ratios (< several km2; rim reef vs seagrass and calcium carbonate (CaCO3) sediments) on diel timescales, and 3) compare how TA-DIC relationships and NCC:NCP vary as we expand functional scales from local habitats to the entire reef platform (10’s of km2) on seasonal to interannual timescales. Our results demonstrate that TA-DIC relationships were strongly driven by local benthic metabolism and community composition over diel cycles. However, as the spatial scale expanded to the reef platform, the TA-DIC relationship reflected processes that were integrated over larger spatiotemporal scales, with effects of NCC becoming increasingly more important over NCP. This study demonstrates the importance of considering drivers across multiple functional scales to constrain carbonate chemistry variability over coral reefs.
Highlights
Coral reefs provide ecosystem services worth trillions of dollars (Costanza et al, 2014) that are threatened by local and global anthropogenic stressors
Coral reefs are thought to be vulnerable to ocean acidification, largely because the ecosystem foundation is built from biogenically precipitated calcium carbonate (CaCO3) (Kleypas et al, 1999; Kleypas and Yates, 2009)
At the community-scale [several km2 over day(s)], total alkalinity (TA)-dissolved inorganic carbon (DIC) relationships were driven by local metabolism and community composition, where higher slopes of TA-DIC were observed at Hog Reef relative to Bailey’s Bay
Summary
Coral reefs provide ecosystem services worth trillions of dollars (Costanza et al, 2014) that are threatened by local (e.g., overfishing, eutrophication, sedimentation) and global anthropogenic stressors (e.g., climate change and ocean acidification; Hoegh-Guldberg et al, 2007; Wilkinson, 2008; Hughes et al, 2017). Numerous experimental studies have demonstrated decreases in coral calcification rates (Chan and Connolly, 2013) and Chemical Variability in Coral Reefs increases in rates of CaCO3 substrate and sediment dissolution (Andersson et al, 2009; Cyronak et al, 2013) under increasing seawater CO2 concentrations and decreasing pH. One of the challenges when making projections on how coral reefs will respond to ocean acidification on an ecosystem scale is the tightly coupled feedback between water column chemistry and benthic metabolic processes (Anthony et al, 2011; Kleypas et al, 2011; Albright et al, 2013; Andersson and Gledhill, 2013; Shaw et al, 2015; Takeshita, 2017). Coral reefs modulate the overlying seawater chemistry through two main metabolic processes: net community production (NCP) and net community calcification (NCC). The relative importance of these drivers and how they influence seawater carbonate chemistry variability over multiple functional scales, defined here as space, time, and benthic community composition, in the natural environment is not fully constrained
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