Depth patterns in Antarctic bryozoan skeletal Mg-calcite: Can they provide an analogue for future environmental changes?

Factors related to depth have the potential to provide an analogue for future changes in the skeletal mineralogy of calcifying marine organisms and communities, given that oceanic pH decreases with depth, with a minimum pH of <7.7, which corresponds to the predicted pH of shallow waters in the next 85 yr. Antarctic bryozoans are often characterized by surprisingly broad bathymetrical ranges, and thus have potential for the study of depth-related environmental changes. This study addressed depth-related changes in the levels of magnesium (Mg)-calcite in Antarctic bryozoan skeletons for the first time in order to facilitate predictions of ocean acidification effects. Specimens (n = 103) belonging to 4 bryozoan species (3 cheilostomes and 1 cyclostome) were collected at various depths in East Antarctica (Terre Adelie and George V Land) during the CEAMARC cruise (December 2007 to January 2008), and Mg-calcite contents from their calcareous skeletons were studied using X-ray diffraction. A dataset was compiled from existing environmental data for both sampling and neighboring sites. All 4 species were found to be entirely calcitic with low or intermediate Mg-levels. The predicted negative correlation between pH and Mg-calcite was not evident. Higher Mg levels were found in Fasciculipora ramosa from the George V Basin, suggesting that high salinity shelf water creates favorable conditions for this species, although alternative environmental and biological factors influencing Mg-calcite in skeletons are also discussed for this species.