It has been shown that most bryozoans precipitate carbonate in isotopic equilibrium with sea water (Smith et al. 2004, 809), although bryozoans with internal symbiotic hydroids may not (Smith et al. 2004, 813). Bryozoan mineralogy is a complex mix of calcite and aragonite, each of which has naturally different ratios of oxygen and carbon isotopes, meaning that simple isotopic corrections are difficult (Smith et al. 2004, 819). Smith et al. (2004, 819) found, however, that in modern New Zealand bryozoans, there is a strong trend towards higher isotopic ratios (δ13C and δ18O) in deeper water and at higher latitudes, both related to temperature. Smith et al. (2004, 819) further found within a single branch of a colony of Cinctipora elegans Hutton 1873, that endozone and exozone carbonate can have different carbon stable isotope ratios, perhaps due to different rates of calcification.
In a three-year study of colonies of Cellaria sinuosa (Hassall 1840) off the coast of Roscoff, France, Bader and Schäfer (2005) found that δ18O is incorporated into the skeleton at close to isotopic equilibrium with sea water and documents seasonal changes in temperature, although δ13C is characterised by lower relative values than the predicted equilibrium and does not show a seasonal cycle.
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