Ocean Acidification

Skeletal carbonate mineralogy of the
bryozoan Odontionella cyclops (Busk, 1854) (family Foveolariidae)
is extremely variable, with calcite:aragonite ratio ranging from 27 to
100 wt% calcite (mean = 57 wt% calcite, sd = 15, n = 118), and Mg
content in calcite varying from 3.6 to 8.8 wt% MgCO₃
(mean = 6.2 wt% MgCO₃, sd = 1.1, n = 118). This study
examines the sources of this wide variability and the possible effects
of ocean acidification on bimineral invertebrates. Variation in
calcite:aragonite ratio in O. cyclops is neither environmental
nor related to colonial growth form, but appears to be astogenetic.
Primary calcification of the zooecial ‘box’ is all calcite, followed by
progressive construction of a secondary aragonitic superstructure which
includes avicularia. Consequently, young parts of the colony are
dominated by calcite, with increasing amounts of aragonite with age.
Very old parts of the colony may have the aragonite eroded or chipped
away to become again entirely calcitic. In contrast with many other
bryozoans that are entirely calcitic or mainly aragonitic, this
bipartite structure may result in increased vulnerability to ocean
acidification. Given the southern-temperate shelf-to-slope distribution
of this species, O. cyclops (and others like it) will begin to be
subjected to decreasing pH in only a few decades. The consequence could
be a modern sediment assemblage similar to a diagenetically-altered
fossil assemblage – missing aragonitic skeletal parts and species.