Physical versus biological control of element incorporation into biogenic carbonate: an in situ experiment in a New Zealand fjord

We exploit the pronounced and persistent low salinity layer in Doubtful Sound, southwest New Zealand, as a natural experimental system with sharp environmental gradients in which to study physical versus biological control of element incorporation into biogenic carbonate. Mussels Mytilus galloprovincialis transplanted to cages in the low salinity layer and saline layer in inner, mid and outer fjord habitats were used as biological integrators of physico-chemical conditions in each water mass over a period of several months. We used solution inductively coupled plasma mass spectrometry (ICP-MS) to measure the concentrations of 15 elements (Li, B, Mg, P, S, Mn, Fe, Co, Cu, Zn, Rb, Sr, Cd, Ba, Pb) in the experimental shell growth and relate spatial trends to those in salinity and elemental sources. S concentrations were consistently below detection limits while Zn and Rb showed no spatial variability. The majority of elements were subject to environmental control, likely reflecting ambient concentrations and salinity. Sr incorporation showed evidence of strong biological mediation of environmental signals via the crystal shell growth rate. These findings are a valuable addition to the growing body of literature seeking to resolve physical versus biological controls of individual trace element incorporation into biogenic carbonates, and have important consequences for their use as environmental indicators or tracers for ecological studies.