SEASONAL ISOTOPE PROFILING TO DETERMINE GROWTH RATES IN AN EXTANT GIANT BRYOZOAN FROM THE ADRIATIC SEA, CROATIA

Seasonal isotope profiling can be an effective tool for determining the ages of extant and extinct carbonate producing organisms. It works by documenting seasonal water temperature fluctuations as recorded in the oxygen isotope values from the organism's carbonate skeleton. The annual temperature cycles are then counted to determine organism age. Seasonal isotope profiling was compared to known mark and recapture data from SCUBA and zooid density profiling for two giant (42 cm tall) extant bryozoan colonies from the Adriatic Sea, Croatia. A total of almost 250 oxygen isotope analyses were performed on closely spaced skeletal samples from four replicate pathways on each colony. Annual cycles in oxygen isotope values indicate these colonies were four years old. Incremental and cumulative growth rates were calculated from the mark and recapture and isotope profiling. The results indicate a mean growth rate of 11.2 cm/year from mark and recapture and 10.6 cm/year from isotope profiling. These are the fastest growing bryozoans ever documented. The results are also compared to zooid density profiling. This technique uses seasonally-induced morphological changes in zooid size and thus zooid density to determine seasonal cycles. The correlation between the mean mark and recapture growth rate and the mean seasonal isotope profiling growth rate confirms the effectiveness of using seasonal isotope profiling to calculate changes in seasonality over short time intervals. Seasonal isotope profiling and zooid density profiling should be applicable to fossil bryozoans as well.