Drivers of carbon isotopic fractionation in a coral reef lagoon: Predominance of demand over supply

Carbon Isotopic Fractionation in Organic Matter Production Consistent With Benthic Community Composition Across a Coral Reef Flat

“…C 3 photosynthesis fractionates at approximately -25‰ to -29‰ (O'Leary, 1988). Photosynthetic isotopic fractionation in corals can be quite variable at -10‰ to -30‰ (Hayes, 1993;Carvalho et al, 2015), but is, on average, more positive than the fractionation found in marine algae. Thus, we hypothesize that the greater relative abundance of algae resulted in greater isotopic fractionation in the front zone relative to the back zone.…”

“…Applying the range of ǫ CaCO 3 considered in this study (-5‰ to 5‰) to the Smith and Kroopnick (1981) ǫ would result in a range of ǫ OM between -23.6‰ to -13.6‰, which is lower, but still overlaps, with our ǫ OM . Carvalho et al (2015) similarly defined a community-scale DIC isotopic fractionation factor that aggregated photosynthesis and biogenic calcification, but excluded respiration and carbonate dissolution. They documented a much larger range in isotopic fractionation than we observed in our bootstrap analysis, however their mean isotope effect estimates tended to converge between -15‰ and -10‰.…”

“…ǫ OM aggregates the isotopic fractionation due to all photosynthesis, and their respective pathways, as well as all respiratory processes (including those of fish and other mobile organisms). Photosynthetic preference for either CO 2 or HCO − 3 will also affect estimates of ǫ OM because of the equilibrium fractionation between CO 2 and HCO − 3 (ǫ CO 2 −HCO 3 −~-10‰) (Zeebe and Wolf-Gladrow, 2001), as well as the different photosynthetic pathways and degrees of "leakage" between photosynthetic cells and the ambient seawater (Carvalho et al, 2015). Algal respiration may exhibit some small isotopic fractionation (~3‰), with resulting changes reflected in the δ 13 C DIC (Carvalho and Eyre, 2011).…”

“…For example, photosynthesis results in the preferential uptake of 12 C, leaving seawater DIC enriched in the heavier 13 C isotope. The degree of this fractionation, which is dependent upon the rate of uptake and the photosynthetic pathway involved (Hayes, 1993;Carvalho et al, 2015), can range from 10‰ to 30‰ in reef organisms with the organic tissue isotopically depleted relative to the seawater (Smith et al, 1985). Calcification typically involves far less carbon isotopic fractionation, which can either leave the carbonate skeletons of corals and calcifying algae isotopically enriched or depleted relative to the seawater DIC by~0‰ to 3‰ (Smith et al, 1985;Gischler et al, 2009).…”

Historical Trends in pH and Carbonate Biogeochemistry on the Belize Mesoamerican Barrier Reef System

“…More precise insights into the causes of such changes can, however, be identified provided the δ 13 C of primary production and respiration are known (vertical contours on Figure 3). Values for biological tissue in coral reef ecosystems range from À17 to À31‰ (Briand et al, 2015;Carvalho et al, 2015;Muscatine et al, 1989;Reynaud et al, 2004;Titlyanov et al, 2010) and here we use a value of À24‰, the median value from Briand et al (2015), allowing us to contour Figure 3 for the removal/addition of DIC alone (i.e., change in NEP; black lines), as well as for the removal/addition of ALK:DIC with a 2:1 ratio (i.e., change in NEC; gray lines).…”