GROWTH RESILIENCE OF SUBARCTIC RHODOLITHS (LITHOTHAMNION GLACIATE, RHODOPHYTA) TO CHRONIC LOW SEA TEMPERATURE AND IRRADIANCE

Rhodolith beds are pervasive marine biological systems in the subarctic North Atlantic. Limited knowledge about effects of temperature and irradiance on rhodolith growth limits the ability to anticipate the response of rhodolith beds to this ocean's chronic low, yet changing sea temperature and irradiance regimes. We carried out a 149-d laboratory experiment with Newfoundland Lithothamnion glaciate rhodoliths to test the predictions that growth (i) is inhibited at temperatures of similar to 0.5 degrees C and (ii) resumes as temperature increases above 0.5 degrees C, albeit at a higher rate under high than low irradiances. Rhodoliths were grown in experimental tanks at near-zero (similar to 0.7 degrees C) seawater temperatures during the first 85 d and at temperatures increasing naturally to similar to 6 degrees C for the remaining 64 d. Rhodoliths in those tanks were exposed to either low (0.02 mol photons.m(-2).d(-1) ) or high (0.78 mol photons.m(-2).d(-1)) irradiances during the entire experiment. Rhodoliths grew at a linear rate of similar to 281 mu m year(-1) (0.77 mu m.d(-1)) throughout the experiment under both irradiance treatments despite daily seawater temperature variation of up to 3 degrees C. Near-zero temperatures of similar to 0.5 to 1.0 degrees C did not inhibit rhodolith growth. Model selection showed that PAR-day (a cumulative irradiance index) was a better predictor of growth variation than Degree-day (a cumulative thermal index). Our findings extend to similar to 0.5 degrees C the lower limit of the known temperature range (similar to 1 to at least 16 degrees C) over which growth in L. glaciate rhodoliths remains unaffected, while suggesting that the growth-irradiance relationship in low-light environments at temperatures below 6 degrees C is less irradiance-driven than recently proposed.

Automated summary

The study confirmed that growth of Lithothamnion glaciate rhodoliths is not inhibited at temperatures near 0.5 degrees Celsius, and that cumulative irradiance is a better predictor of growth variation than cumulative thermal index.