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MEPS prepress abstract   -  DOI:

High growth resilience of subarctic rhodoliths Lithothamnion glaciale to ocean warming and chronic low irradiance

David BĂ©langer*, Patrick Gagnon

*Corresponding author:

ABSTRACT: We paired a 361-d laboratory mesocosm experiment and a 383-d field experiment with Newfoundland rhodoliths (Lithothamnion glaciale) to test the overall hypothesis that growth in subarctic rhodoliths is chiefly controlled by irradiance. Rhodoliths in the laboratory were exposed to one of five seawater temperatures (ambient, 2, 4, 7 and 10°C) and either of three irradiances (low [0.02], intermediate [0.11], and high [0.27 mol photons m-2 d-1]). Rhodoliths in the field were held in cages at three depths (8, 15, and 25 m). Laboratory results demonstrated that growth is unaffected by temperature between ~1 and 16°C. Field results suggested that growth ceases at temperatures near or below 0.5°C and showed that L. glaciale’s annual growth profile comprises three distinct phases — two of positive growth separated by one of arrested growth — and that the switch from one phase to the next coincides with seasonal shifts in sea temperature and light regimes. Rhodoliths at 25 m appeared to utilize light nearly twice as efficiently as rhodoliths at 15 m, which enabled similar growth at both depths despite the ~60% lower irradiance at 25 m. We conclude that growth is chiefly controlled by irradiance and that temperature effects may override, but not interact with, those of irradiance during the coldest months of the year. Subarctic L. glaciale rhodoliths are resilient to changes in sea temperature over a relatively broad thermal range, with sustained growth even at temperatures above those normally observed during most of the year in Newfoundland coastal waters and northwards.