Ten years of Tridacna sclerochemistry at up to daily resolution from a controlled aquarium environment – Records of habitat change, induced seasonality and growth variability

Giant clams such as Tridacna sp., with their rapid shell accretion (mm-cm/year), decade-long lifespans and aragonitic shells, are invaluable (palaeo)environmental archives, potentially providing information at (sub-)seasonal timescales. Royal Burgers' Zoo aquarium in Arnhem, The Netherlands, opened a live coral reef eco-display in 2000 and introduced T. squamosa from Vietnam in 2001. One specimen (TS2) that died in 2011 facilitated a decade-long comparison of carefully monitored aquarium conditions with Tridacna sclerochemistry and growth, whose results we present herein. Spatially resolved El/Ca ratios by LA-ICPMS (at up to daily resolution) as well as micromilled δ¹³C and δ¹⁸O data were transferred onto a sclerochronological framework at a daily resolution, which enabled the detailed correlation of aquarium parameters with sclerochemistry. We show that environmental stresses such as transportation, introduction to a new aquarium environment, shifts in water change regimes and artificial seasonality from 2009 onwards have severe impacts on the organism's growth and sclerochemistry, and are particularly manifest in this sample in increased Mg/Ca and Sr/Ca ratios. Growth rates were reduced from $\sim$20 to $\sim$7 μm/day during transportation into the aquarium, and from $\sim$10 to $\sim$2 μm/day during shifts in water change regimes in the aquarium. A disruption of daily El/Ca cyclicity was marked during transportation-induced stress, but within three weeks of introduction the organism acclimatised to the aquarium and returned to natural growth rates and cyclicity. Three years of induced temperature seasonality (25.0–26.5 °C) most notably affected Na/Ca, while the resulting expected δ¹⁸O variability was not resolvable due to TS2's strongly decreased growth rate coupled with our large drill-step size for sampling. Despite a transient spike in aquarium [NO^-₃] from $\sim$0.02 to 2.4 mg/L, caused by accidental over-fertilisation, no stress effect was apparent in the organism's sclerochemistry or growth rate. Changes in the water-exchange regime appear to have far more sclerochemical impact, as these are reflected in the overall δ¹⁸O, Ba/Ca and associated growth rates. The detailed decade-long Tridacna record shows that even systems with comparatively little environmental variability can produce large degrees of shell heterogeneity, and also highlights the importance of establishing a detailed (counted) chronology for sclerochemical interpretations.