Temperature responses limiting the geographical distribution of two temperate species of Cladophora (Cladophorales; Chlorophyta) in the North Atlantic Ocean

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Abstract
The existence of temperature limits at the distributional boundaries is examined for two temperate species of Cladophora in the North Atlantic Ocean: C. sericea (Hudson) Kuetzing and C. rupestris (L.) Kuetzing. Experimental evidence suggests that their northern boundaries are determined by summer temperatures that are too low for adequate growth. Lethal summer temperatures at the southern boundaries of both species restrict the extension of their distribution into warmer waters on the North American coast and in the Mediterranean. On the African coast and island groups in the Atlantic, both species would be expected to occur further south 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.degree. C). Cladophora rupestris is able to extend further into cooler waters in Greenland and Siberia, since there is still significant growth over the critical 0-5.degree. C range. The highest growth rates were found between 15-20.degree. C, but C. sericea sporulated rapidly above 15.degree. C, so that the plants remained small in size. The upper survival limit in long days was 25-30.degree. C for both species, but an isolate of C. sericea with a ''gracilis''-like morphology from Begen, Norway, was slightly more heat tolerant. Such ''gracilis'' specimens should be considered as another species, C. flexuosa (Mueller) Kuetzing, whose distribution overlaps that of C. sericea. This would account for records of C. sericea from areas with apparently lethal summer temperatures. Isolates with the typical C. sericea morphology from the European coast (Brittany), Iceland and Labrador showed similar temperature tolerances, but there was some difference in their growth rates.