Impact of environmental seasonality on stable isotope composition of skeletons of the temperate bryozoan Cellaria sinuosa

Stable oxygen and carbon isotopes of skeletons of the anascan bryozoan Cellaria sinuosa (Hassall) from waters off Roscoff (outer British Channel) were studied to obtain information on seasonal changes of the environments monitored in the skeleton of the bryozoan. The isotope signals found in the skeletons cover a time span of three years (1993–1995). δ18Oc is incorporated into the skeleton close to isotopic equilibrium, whereas δ13C is characterised by distinctly lower values relative to the predicted equilibrium indicating a strong component of respiratory CO2 incorporated into the bryozoan skeleton. δ18Oc values document the annual cycle of temperature while δ13C shows no clear seasonal cyclicity of primary productivity. The lack of correlation between δ18Oc and δ13C values in individual profiles, however, monitors the offset between seasonal temperature and phytoplankton cycles. Correspondence of δ13C values with the bryozoan life cycle indicates reduced metabolism during reproduction and refers toward a switch of used carbon reservoirs from respiratory CO2 to DIC pool. ia sinuosa shows an age of 1.5 years with an annual growth rate of approximately 32 mm/yr. Carbonate production by C. sinuosa on the shelf off Roscoff varies between 12 gr/m2/yr and 57 gr/m2/yr and lies within the range of cold-water carbonates.