Variability in the skeletal mineralogy of temperate bryozoans:the relative influence of environmental and biological factors

Bryozoans exhibit a highly variable geochemistry within their calcium carbonate skeletons. Previous studies have predominantly attributed this variability to differences in seawa- ter temperature influencing the relative deposition of aragonite and calcite, and the extent of mag- nesium incorporation into the calcite lattice. However, the patterns and scale of this variability have not been examined in detail. We conducted a high-replicate, multi-site study on the skeletal mineralogy of temperate Northern Hemisphere bryozoans to investigate the range of skeletal aragonite and Mg-calcite variability between species and the relative influence of environmental and biological factors on skeletal biogeochemistry. During a cruise in May 2012 in Scapa Flow, Orkney, Northeast Scotland, 480 specimens from 3 bryozoan species were collected by SCUBA diving. Samples were obtained from 5 study sites with similar depths and physical characteristics. All specimens were collected within the same week and were selected to be of similar size, age and breeding status. The results of X-ray diffraction analysis showed that wt% MgCO3 in calcite and wt% aragonite in total CaCO3 were statistically different between sites for all species. This may be explained by differential population connectivity between sites influenced by the tidal regimes of Scapa Flow. No temperate bryozoan species showed the expected positive trends of increasing wt% MgCO3 in calcite or wt% aragonite in total CaCO3 with seawater temperature. Based on the data generated in this study, we suggest that both environmental and biological factors are involved in the control of skeletal mineralogy in some temperate bryozoan species.