Stable isotope profiles of skeletal carbonate validate annually-produced growth checks in the bryozoan Melicerita chathamensis from Snares Platform, New Zealand

Abstract

The bryozoan Melicerita chathamensis Uttley and Bullivant, 1972 produces colonies that exhibit visible growth segments defined by narrow growth checks. If these growth checks are annual, then colony age, and seawater variations among seasons and years can be quantified. The purpose of the present study was to use stable isotope profiling to evaluate whether the segments between growth checks represent annual temperature cycles in this species. We applied three independent methods to determine colony age of six colonies from 168 m depth on the Snares Platform located south of New Zealand. First, each colony was X-rayed to determine the location of the growth checks based on skeletal density. Second, branch width was measured for each zooid generation along the growth axis to locate the growth checks. Third, we measured stable C and O isotope values along the colony axis. Branch width patterns corresponded broadly with X-ray patterns, suggesting colony ages of 1.5–7.5 yrs (mean: 4.0 yrs). δ18O profiles suggested colony ages of 4.0–6.5 yrs (mean: 5.3 yrs). This species does precipitate its skeleton in isotopic equilibrium with seawater, such that annual growth checks correlate with cooler winter temperatures. A conceptual model is proposed for the annual growth cycle in this species. In the most complete colony, the δ 18 O-derived temperatures correlated with inter-annual variations related to the El Nino–Southern Oscillation Index.