Environmental and compositional correlates of growth rate in deep-water bamboo corals (Gorgonacea; Isididae)

Abstract

Little is known about the factors that affect the growth rates of deep-water corals. Analysis of 23 specimens of bamboo corals, a ubiquitous deep-water group, that were reliably aged using radiometric techniques or validated increment counts indicates that the radial growth rate of the coral's calcite internode declines with depth and increases logistically with ambient temperature. Temperature alone accounts for about half of the variance among specimens in lifetime mean growth rates. Growth rates plateau below 2°C and above 5°C, which likely reflects physiological constraints on growth and calcification. Thus far, the only other effect on growth rate identified is a tendency for colonies from the NE Pacific to grow faster relative to ambient temperature than colonies in other regions. Analysis of elemental composition of the calcite internode, based on data available for 16 of the 23 specimens, shows that, contrary to suggestions in the literature, growth rate does not correlate with Sr/Ca ratios, neither among or within colonies. Rather, within-colony ontogenetic variability in increment width, an index of short-term growth rate, correlates positively with calcium weight-frac- tions and negatively with the ratio between P and S, an index of organic content. The underlying mechanism is likely to be a link between crystal ultrastructure, which varies with growth rate, and the ratio between CaCO3 and organic matrix in different crystal types. Between colonies, composi- tion correlates poorly with growth rate. This could reflect spatial and temporal heterogeneity in other environmental factors that determine internode composition, as well as difficulties in standardising P measurements across specimens.