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

Abstract

We paired a 361 d laboratory mesocosm experiment and a 383 d field experiment with rhodoliths (Lithothamnion glaciale) collected in Newfoundland (Canada) to test the overall hypothesis that growth in subarctic rhodoliths is chiefly controlled by irradiance. Rhodoliths in the laboratory were exposed to 1 of 5 seawater temperatures (ambient, 2, 4, 7, and 10°C) and 1 of 3 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 3 depths (8, 15, and 25 m). Laboratory results demonstrated that growth is unaffected by temperatures between ~1 and 16°C. Field results suggested that growth ceases at temperatures near or below 0.5°C and showed that the annual growth profile of L. glaciale comprises 3 distinct phases, namely 2 of positive growth separated by 1 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.