Implications of declining rhodolith growth in a warming Arctic: a century-scale perspective

Crustose coralline algae (CCA) precipitate high-Mg calcite, contributing to carbonate sediment budgets and benthic habitat complexity. Rhodoliths formed by CCA are key bioconstructors in high-latitude photic zones, playing a role analogous to coral reefs in tropical systems, albeit with markedly slower accretion rates—on the order of micrometers per year. Their sensitivity to environmental change makes them valuable archives for assessing long-term climatic and oceanographic variability, but also challenges their future role as calcifying ecosystem engineers.

This study integrates historical temperature records with a temporally explicit model to reconstruct rhodolith growth trends over ~90 years in Arctic waters. Our results reveal a significant inverse relationship between summer seawater temperatures and rhodolith accretion at 11 and 27 m depths, with an average decline in growth of 8.9 μm per °C (95% CI: 1.32–16.60 μm °C⁻¹; p < 0.05). At 46 m depth and under mesophotic conditions, no significant trend was observed. These findings suggest that increasing ocean temperatures—potentially compounded by acidification and turbidity—are suppressing the calcification rates of CCA beyond their thermal optimum, thus reducing their net carbonate production.

Given the global distribution and ecological importance of CCA-generated rhodolith beds, our findings highlight a geologically significant reduction in biogenic carbonate formation in polar regions. This decline not only alters present-day benthic systems but also has implications for interpreting past climate signals recorded in high-latitude carbonate deposits.