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 seawater temperature influencing the relative deposition of aragonite and calcite, and the extent of incorporation of magnesium into the calcite lattice. Previous to this study, however, the patterns and scale of this variability had not been examined in detail. The aim was to conduct a high-replicate, multi-site study on the skeletal mineralogy of temperate northern hemisphere bryozoans. In this study we investigate the range of skeletal aragonite and Mg-calcite variability between species and the relative influence of environmental and biological factors on skeletal biogeochemistry. The study was conducted in Scapa Flow, Orkney in the North East of Scotland. During a cruise in May 2012, a total of 480 specimens from three bryozoan species were collected by SCUBA diving. Samples were obtained from five study sites with similar depth 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 was statistically different between sites for all study species. This may be explained by differential population connectivity between sites influenced by the tidal regimes of Scapa Flow. Based on the evidence generated in this study, patterns of skeletal mineralogy did not fit expected trends with environmental conditions. 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.