Variability of Mg-calcite in Antarctic bryozoan skeletons across spatial scales

Bryozoans exhibit a highly variable chemistry within their calcium carbonate skeletons. Previous studies have shown that the level of Mg-calcite in skeletons increases with increasing seawater temperature. For high-latitude regions such as the Antarctic, which have a low range of annual sea-temperature variation, there have been no studies on bryozoan skeletons with replicated sampling approaches suitable for statistical testing. Our aim was to conduct high-replicate, multi-site sampling to determine the variability in skeletal mineralogy of bryozoans from a site in Antarctica. During an expedition in January 2012, a total of 584 specimens representing 4 bryozoan species were collected from 8 sites at Adelaide Island, West Antarctic Peninsula, by SCUBA diving. All specimens were sampled within a 3 wk period and were selected to be of similar size, age and breeding status. We compared the variability in the wt% MgCO3 in calcite of skeletons among species and investigated the relative influence of environmental and biological factors on skeleton chemistry. The results of X-ray diffraction analysis showed that the wt% MgCO3 in calcite in bryozoan skeletons was statistically different among sites for all study species. The difference in wt% MgCO3 among sites may be explained by habitat fragmentation driving directional adaptation of isolated populations to local environmental conditions. The relationship between Mg-calcite and temperature was inconsistent among species, and the predicted positive correlation between seawater temperature and Mg-calcite was not exhibited in any of the species examined.
On this basis, we suggest that Antarctic bryozoan Mg-calcite should not be considered a reliable indicator of paleo-temperature.