TEMPERATURE RESPONSES LIMITING THE GEOGRAPHICAL-DISTRIBUTION OF 2 TEMPERATE SPECIES OF CLADOPHORA (CLADOPHORALES, CHLOROPHYTA) IN THE NORTH-ATLANTIC OCEAN

For Cladophora sericea and C. rupestris, northern boundaries are determined by summer temperatures that are too low for adequate growth. Lethal summer temperatures at the S boundaries of both species restrict the extension of their distribution into warmer waters on the N American coast and in the Mediterranean. On the African coast and island groups in the Atlantic, both species would be expected to occur further S than is suggested by the present records, as summer temperatures are not lethal. In accordance with their distribution from temperate to (sub)arctic regions, C. sericea and C. rupestris were still capable of slow growth at comparatively low temperatures (5-10°C). C. rupestris can extend further into cooler waters in Greenland and Siberia, since there is still significant growth over the critical 0-5°C range. Highest growth rates were found between 15-29°C, but C. sericea sporulated rapidly above 15°C, so that plants remained small in size. The upper survival limit in long days was 25-30°C for both species. -from Authors