Abstract
Abstract. We use observations of dissolved inorganic carbon (DIC) and total alkalinity (TA) to assess the impact of ecosystem metabolic processes on coastal waters of the eastern Red Sea. A simple, single-end-member mixing model is used to account for the influence of mixing with offshore waters and evaporation–precipitation and to model ecosystem-driven perturbations on the carbonate system chemistry of coral reefs, seagrass meadows and mangrove forests. We find that (1) along-shelf changes in TA and DIC exhibit strong linear relationships that are consistent with basin-scale net calcium carbonate precipitation; (2) ecosystem-driven changes in TA and DIC are larger than offshore variations in >70 % of sampled seagrass meadows and mangrove forests, changes which are influenced by a combination of longer water residence times and community metabolic rates; and (3) the sampled mangrove forests show strong and consistent contributions from both organic respiration and other sedimentary processes (carbonate dissolution and secondary redox processes), while seagrass meadows display more variability in the relative contributions of photosynthesis and other sedimentary processes (carbonate precipitation and oxidative processes). The results of this study highlight the importance of resolving the influences of water residence times, mixing and upstream habitats on mediating the carbonate system and coastal air–sea carbon dioxide fluxes over coastal habitats in the Red Sea.
Highlights
IntroductionSeagrass meadows and mangrove forests are sites of intense metabolic processes
Coral reefs, seagrass meadows and mangrove forests are sites of intense metabolic processes
We examine concentrations of total alkalinity (TA) and dissolved inorganic carbon (DIC) over and around coral reefs, seagrass meadows and mangrove forests, and we compare these to the same properties measured in offshore Red Sea surface waters
Summary
Seagrass meadows and mangrove forests are sites of intense metabolic processes. These habitats are distributed heterogeneously in the coastal zone, at shallow depths where perturbations in the carbonate system by metabolic processes can have the greatest influence on water chemistry and air– sea carbon dioxide (CO2) exchange (Bauer et al, 2013; Camp et al, 2016; Cyronak et al, 2018; Gattuso et al, 1998; Guannel et al, 2016; Unsworth et al, 2012). The open ocean is geographically separated from the benthos and land so their respective influences on the carbonate system can often be ignored over short timescales. Terrestrial and freshwater inputs (dissolved and particulate), sediment exchanges, biological processes, and changes in circulation and water residence time must all be considered when studying perturbations in the carbonate system of the coastal zone (Doney, 2010; Duarte et al, 2013; Giraud et al, 2008; Jiang et al, 2014; IPCC, 2014)
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